• Your life and health are your own responsibility.
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The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility

Caution: contains SCIENCE!

Important note! For a more up-to-date exploration of this subject, I strongly recommend my 2013 AHS presentation “What Is Metabolic Flexibility, And Why Is It Important?”

Most of us who eat a low-carbohydrate diet—Paleo, Primal, Atkins, or otherwise—experience anywhere from a couple days to a couple weeks of low energy as we adjust to it, an experience known informally as the “low carb flu”. And some people never seem to adjust.

Here’s why—and here are some ideas that might help you if you’re having trouble adjusting!

Note that low-carb isn’t an objective of a paleo diet: it’s just the usual consequence of eliminating grains and sugars.

It’s certainly possible to eat a higher-carb paleo diet—and it’s a good idea if you’re doing frequent, intense workouts like HIIT, Crossfit, or team sports after school—but you’d have to eat a lot of potatoes and bananas to get anywhere near the same amount of carbohydrate you used to get from bread, pasta, cereal, and soda.

Burning Food For Energy: Glycolysis and Beta-Oxidation

Our bodies have several ways to turn stored or ingested energy into the metabolic energy required to move around and stay alive. This is called cellular respiration.

The two main types of cellular respiration are anaerobic (which does not require oxygen) and aerobic (which requires oxygen). Anaerobic metabolism, also known as fermentation, is nineteen times less efficient—and we can only maintain it for short periods, because its waste products build up very quickly. This is why we can’t sprint for long distances.

We spend most of our time in aerobic metabolism. Our two primary aerobic sources of energy are glycolysis, which converts glucose to energy, and beta-oxidation, which converts fat to energy.

A Short Metabolic Digression Explaining The Above (Optional)

Strictly speaking, glycolysis is the start of both the aerobic and anaerobic oxidation of glucose: it converts glucose to pyruvate.

In the aerobic oxidation of glucose, the pyruvate is transported into the mitochondria, whereupon it is converted to acetyl-CoA and fed into the TCA cycle (aka the citric acid cycle or Krebs cycle, depending on how long ago you took your biology course.) This produces 19 times more energy than the original glycolysis!

In the anaerobic oxidation of glucose (“lactic acid fermentation”), the pyruvate is instead converted to lactic acid, which produces no more energy.

In the aerobic oxidation of fat (“beta-oxidation”), the fat is transported into the mitochondria, whereupon it is sliced up into individual acetyl-CoA molecules, each of which enters the TCA cycle.

Humans have no way to anaerobically oxidize fat.

I put this here because pictures of molecules are scientific!

A graphic flowchart of glycolysis. You know it's SCIENCE! because there are a lot of molecules and arrows.

Lots of fine print = SCIENCE!

And here's a flowchart of beta-oxidation.

Glucose is the simple sugar all cells use for glycolysis, and it’s the most common. The other simple sugars we can digest are galactose (found primarily in milk), which we convert to glucose—and fructose (found primarily in fruit, table sugar, corn syrup, and honey), which our liver converts directly to glycogen or fat.

Starch is just a bunch of glucose molecules stuck together. In fact, “complex carbohydrates” in general are just sugars stuck together…and we can only absorb them through the intestine once they are broken down into individual simple sugars by our digestive system. In other words, all “carbohydrates” are just sugar.

There is a lot more to talk about here, including glycogen storage and retrieval, gluconeogenesis, and de novo lipogenesis…but explaining all the pathways of digestion, energy storage, and cellular respiration would be an entire book in itself!

Moving on: while it’s OK for fat to hang around in our bloodstream for a while, high blood sugar is actively toxic to our tissues. (The long-term consequences of untreated diabetes—heart, kidney, nerve, eye, and muscle damage, leading to numbness, blindness, amputations, strokes, and death—are basically just long-term glucose poisoning.) So after we eat something containing any amount of carbohydrate, insulin ensures that the glucose is immediately taken into our cells and either burned for energy, stored, or converted into palmitic acid—a saturated fat!

This is why a “low-fat, high-carb” diet is really a high-fat diet. Unless your “high-carb” diet involves an intravenous glucose drip carefully metered to keep your blood sugar constant, most of the ‘carbohydrates’ (sugars) you eat will be converted either to glycogen or to palmitic acid (again, a saturated fat) before you use them. “Soluble fiber” and other indigestible carbohydrates are fermented into short-chain saturated fats, like butyric acid, in your colon. Fructose, of course, is converted directly to liver glycogen or to palmitic acid. And if you’re losing weight by burning your own fat, keep in mind that human fat has roughly the same composition as lard—approximately 40% saturated!

You might ask yourself if it makes sense that natural selection would select us to store energy in the form of something directly harmful to us. If saturated fat is really so terrible, and polyunsaturated fat is really so healthy, why doesn’t our body store energy as linoleic acid, like grains do?

Metabolic Flexibility and the Respiratory Exchange Ratio

If we have excess glucose in our bloodstream, our muscles will burn it first, because it’s toxic. But eventually we run out of glucose, and that’s when our bodies need to switch over to beta-oxidation—burning fat. The ability to switch back and forth between the two processes is called “metabolic flexibility” in the scientific literature.

Metabolic flexibility varies dramatically from individual to individual, which we would expect based on the widely varying experiences people report with low-carb diets. So how do scientists figure out what fuel our bodies are burning?

It turns out that beta-oxidation (fat-burning) produces less carbon dioxide than glycolysis (sugar-burning)—and we can measure that in our breath. The ratio of CO2 to O2 is 0.7 for beta-oxidation and 1.0 for glycolysis…so an RER (Respiratory Exchange Ratio) of 0.7 indicates pure fat-burning, and 1.0 and above indicates pure sugar burning. (You can read more about the RER here.)

Typical healthy people have a resting, fasting RER of approximately 0.8. Therefore, we can easily see that the frequent vegetarian and vegan claims of “Nothing else can provide any energy without first being converted to carbs” and “You can get plenty of energy from fat, but you have to go into ketosis to do it” are—like most nutritional claims made by veg*ans—complete bunk.

Metabolic Flexibility: The “Low Carb Flu” Is Not Your Imagination

It shouldn’t be a surprise that the obese and diabetic tend to have higher resting RERs, and that higher RER is a significant predictor of future obesity. If our ability to burn fat for energy is impaired, we’re going to have a hard time losing weight, and we’ll become ravenously hungry when our blood sugar runs out no matter how much fat we have available to burn.

Is this sounding familiar to anyone?

Sounds like the “low carb flu”, doesn’t it? When we talk about our metabolic “set point”, part of what we’re talking about is metabolic flexibility. It does no good to have a huge store of fat if we can’t burn it for energy!

Int J Obes Relat Metab Disord. 1992 Sep;16(9):667-74.
Fasting respiratory exchange ratio and resting metabolic rate as predictors of weight gain: the Baltimore Longitudinal Study on Aging.
Seidell JC, Muller DC, Sorkin JD, Andres R.

“…The adjusted relative risk of gaining 5 kg or more in initially non-obese men with a fasting RER of 0.85 or more was calculated to be 2.42 (95% confidence interval: 1.10-5.32) compared to men with a fasting RER less than 0.76.”

Furthermore, it turns out that people with a family history of type II diabetes, but who don’t yet have it themselves, have higher RERs and impaired metabolic flexibility.

Diabetes August 2007 vol. 56 no. 8 2046-2053
Impaired Fat Oxidation After a Single High-Fat Meal in Insulin-Sensitive Nondiabetic Individuals With a Family History of Type 2 Diabetes
Leonie K. Heilbronn1, Søren Gregersen2, Deepali Shirkhedkar1, Dachun Hu1, Lesley V. Campbell1

“…An impaired ability to increase fatty acid oxidation precedes the development of insulin resistance in genetically susceptible individuals.”

Also see:

AJP – Endo November 1990 vol. 259 no. 5 E650-E657
Low ratio of fat to carbohydrate oxidation as predictor of weight gain: study of 24-h RQ
F. Zurlo, S. Lillioja, A. Esposito-Del Puente, B. L. Nyomba, I. Raz, M. F. Saad, B. A. Swinburn, W. C. Knowler, C. Bogardus, and E. Ravussin

This is very important: we can see that impaired fat oxidation must be related to the causes, not the consequences, of obesity and diabetes. So we’ve struck another blow to “calories in, calories out”, and the idea that you’re fat just because you’re lazy.

In support of this theory, I note this paper, which contains the following graph of several individuals’ RER in response to a high-fat diet vs. a moderate-fat diet. Keep in mind that the change was only from 37% to 50% fat, which is relatively minor, and the paper doesn’t tell us what fats were being consumed…but this graph is still instructive:

The top graph is the average, the bottom graph is for each individual. Note that some adapted right away, some took several days, and some were still not adapted on day 4!

Higher RER isn’t all bad. It’s associated with having more fast-twitch muscle fibers, which are associated with a greater ability to build muscle mass. This fits the anecdotal evidence that people who gain fat easily also tend to gain muscle easily, whereas skinny people have a much harder time bulking up.

So maybe you’re lucky, or already in good health, and you adapt relatively quickly to a low-carb diet. But what if you’re not? What if you’re stuck with the “low carb flu”?

Regaining Your Metabolic Flexibility

Obviously we’d like to regain our metabolic flexibility. But how? Here’s one way:

Journal of Applied Physiology September 2008 vol. 105 no. 3 825-831
Separate and combined effects of exercise training and weight loss on exercise efficiency and substrate oxidation
Francesca Amati,1 John J. Dubé,2 Chris Shay,3 and Bret H. Goodpaster1,2

“…Exercise training, either alone or in combination with weight loss, increases both exercise efficiency and the utilization of fat during moderate physical activity in previously sedentary, obese older adults. Weight loss alone, however, significantly improves neither efficiency nor utilization of fat during exercise.

Diabetes September 2003 vol. 52 no. 9 2191-2197
Enhanced Fat Oxidation Through Physical Activity Is Associated With Improvements in Insulin Sensitivity in Obesity
Bret H. Goodpaster, Andreas Katsiaras and David E. Kelley

“Rates of fat oxidation following an overnight fast increased (1.16 ± 0.06 to 1.36 ± 0.05 mg · min−1 · kg FFM−1; P < 0.05), and the proportion of energy derived from fat increased from 38 to 52%." October 15, 2009 The Journal of Physiology, 587, 4949-4961. Improved insulin sensitivity after weight loss and exercise training is mediated by a reduction in plasma fatty acid mobilization, not enhanced oxidative capacity
Simon Schenk1, Matthew P. Harber1, Cara R. Shrivastava1, Charles F. Burant1,2 and Jeffrey F. Horowitz1

“…Resting fatty acid oxidation was unchanged after the intervention in WL [weight loss]. Consistent with an increase in maximal oxidative capacity, resting whole-body fatty acid oxidation was increased more than 20% after WL + EX [weight loss + exercise].”

In other words, despite the title, weight loss plus exercise increased resting fat oxidation…but just losing weight did not!

AJP – Endo April 2008 vol. 294 no. 4 E726-E732
Skeletal muscle lipid oxidation and obesity: influence of weight loss and exercise
Jason R. Berggren,1,2 Kristen E. Boyle,1,2 William H. Chapman,4 and Joseph A. Houmard1,2,3

“10 consecutive days of exercise training increased (P ≤ 0.05) FAO [fatty acid oxidation] in the skeletal muscle of lean (+1.7-fold), obese (+1.8-fold), and previously extremely obese subjects after weight loss (+2.6-fold)…These data indicate that a defect in the ability to oxidize lipid in skeletal muscle is evident with obesity, which is corrected with exercise training but persists after weight loss.”

How about that? It turns out that exercise is important after all…not because of the calories you burn by exercising, which you usually replace right away because you’re hungry, but because it helps you regain metabolic flexibility. Exercise stimulates your body to burn more fat, both during exercise and at rest.

And that’s what health is about: we’re not interested in losing weight if it just means losing muscle. We’re interested in losing fat.

There are other benefits beyond fat loss, too: exercise tends to normalize broken metabolisms.

Diabetes March 2010 vol. 59 no. 3 572-579
Restoration of Muscle Mitochondrial Function and Metabolic Flexibility in Type 2 Diabetes by Exercise Training Is Paralleled by Increased Myocellular Fat Storage and Improved Insulin Sensitivity
Ruth C.R. Meex1, Vera B. Schrauwen-Hinderling2,3, Esther Moonen-Kornips1,2, Gert Schaart1, Marco Mensink4, Esther Phielix2, Tineke van de Weijer2, Jean-Pierre Sels5, Patrick Schrauwen2 and Matthijs K.C. Hesselink1

“Mitochondrial function was lower in type 2 diabetic compared with control subjects (P = 0.03), improved by training in control subjects (28% increase; P = 0.02), and restored to control values in type 2 diabetic subjects (48% increase; P < 0.01). Insulin sensitivity tended to improve in control subjects (delta Rd 8% increase; P = 0.08) and improved significantly in type 2 diabetic subjects (delta Rd 63% increase; P < 0.01). Suppression of insulin-stimulated endogenous glucose production improved in both groups (−64%; P < 0.01 in control subjects and −52% in diabetic subjects; P < 0.01). After training, metabolic flexibility in type 2 diabetic subjects was restored (delta respiratory exchange ratio 63% increase; P = 0.01) but was unchanged in control subjects (delta respiratory exchange ratio 7% increase; P = 0.22).”

Did you catch that? “Metabolic flexibility in type 2 diabetic subjects was restored”?

Unfortunately, this study didn’t measure resting fat oxidation, like the others—but it does suggest that there’s no need to kill yourself with “Biggest Loser”-style misery. 30 minutes of cycling at 55% of maximum effort twice a week, and one session of weight training once a week, was enough to restore metabolic flexibility. That doesn’t sound very intimidating, does it? (And there are many better and more entertaining ways to get half an hour of moderate aerobic exercise than sitting on a stationary bike.)

“Aerobic exercise was carried out on a cycling ergometer twice a week for 30 min at 55% of a previously determined maximal work load (Wmax). Resistance exercise was performed once a week and comprised one series of eight repetitions at 55% of subjects’ previously determined maximal voluntary contraction (MVC) and two series of eight repetitions at 75% MVC and focused on large muscle groups (Chest press, leg extension, lat pull down, leg press, triceps curls, biceps curls, abdominal crunches, and horizontal row).”

…Yet You Must Take Advantage Of Your Newfound Metabolic Flexibility

Of course, our newly-regained flexibility won’t help if we stuff ourselves with the government-recommended 7-11 servings of “heart-healthy whole grains” (= “carbs”, = sugar) per day, because we will be constantly burning sugar. Only when we’re done burning glucose can we use our newfound flexibility to burn some fat.

That’s one reason, among many, why I eat a paleo diet—and why I don’t snack. (For more on that subject, read “Why Snacking Makes You Weak, Not Just Fat”.)

A Short Digression: Please Stay Off The “Faileo Diet”

Some ‘paleo’ books still insist that saturated fat is bad for you and paleolithic people didn’t eat much of it, which is absolute nonsense. But your calories have to come from somewhere…if not fat, then from protein or carbohydrates. And since those same books also usually disallow potatoes and other convenient sources of starch, you’re basically stuck eating lots of lean protein.

As a result, you’ll eat very few calories, because of the satiating effect of protein—which is fine if you’re just trying to lose weight, but disastrous if you’re physically active, because you’ll be perpetually exhausted. This is why fat-phobic ‘paleo’ is sometimes called the “Faileo Diet”.

The Difference Between Beta-Oxidation and Ketosis

Here’s where I say something that might be controversial: I think going cold-turkey VLC (very low carb) or zero-carb makes the transition much harder, particularly for people who are already physically active.

Beta-oxidation (fat-burning) occurs nearly continually, and produces much of our energy at rest once insulin has cleared any sugar spike out of our system. However, our body does have some requirement for glucose, which it satisfies in the short-term primarily by having the liver make it—a process called gluconeogenesis.

If we eat zero carbs, or very few, over a period of time, our body enters a state called ketosis, in which some of our tissues that used to require glucose shift over to burning ketone bodies, which are alternative products of fat metabolism. And while it is true that our brains and hearts actually run more efficiently on ketones, it takes several weeks for our bodies to fully adapt. Meanwhile, we lack energy for high-effort activities, because our muscles are depleted of glycogen, which is made from glucose.

So you might not have the “low-carb flu”—you might be stuck in an unnecessary multi-week rut of keto-adaptation.

Interested in learning more about ketosis? Read Stephen Phinney’s “Ketogenic Diets and Physical Performance” (Nut Metab 2004, 1:2) for more information about ketosis and the process of keto-adaptation. Once you’ve read that, if you’re deeply interested in the ketotic state, try ketotic.org…and if you’re interested in a ketogenic diet that isn’t nutrient-deficient or disgusting, try these articles from the Drs. Jaminet (Ketogenic Diets I, Ketogenic Diets II).

There is a persistent myth that ketosis is dangerous: it’s not. People (including some doctors) commonly confuse it with ketoacidosis, a pathological state usually only found in uncontrolled diabetics and (rarely) raging alcoholics.

Even worse, you might be stuck in the state informally known as “Low Carb Limbo”—in which you’re eating too few carbohydrates to fuel high-effort, glycolytic activity, but too many carbohydrates to ever keto-adapt.

If you’re active and determined to keto-adapt, read this excellent article from Primal North, “Keto-Adaptation vs. Low Carb Limbo”.

Conclusion: Stay Out Of The Muddy Middle

In summary, it’s much easier and quicker to burn fat via beta-oxidation than it is to adapt to ketosis…so unless ketosis is your goal, you might be making your transition to a healthy diet much harder by keeping your carb intake too low.

I think that if we keep our carbohydrate intake near our body’s requirement while not in ketosis, which is perhaps 15-20% of total calories—and only eat those carbohydrates with meals involving complete protein and fat, not by themselves—most of us should be able to gain the fat-burning benefits of metabolic flexibility without suffering the pain of trying to adapt to ketosis. So if you’re new to Paleo or low-carb eating, you’re stuck with long-term “low-carb flu”, and especially if you’re already physically active, try adding some root starches like potatoes or sweet potatoes to your meals. (Or white rice, if you’re following the Perfect Health Diet.)

(And if you’re determined to keto-adapt, go fully ketogenic, as per “Keto-Adaptation vs. Low Carb Limbo.”)

Live in freedom, live in beauty.


For more information, continue reading my 2013 AHS presentation “What Is Metabolic Flexibility, And Why Is It Important?”

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Permalink: The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility
  • anand srivastava

    Very informative. Shared on my Facebook.

  • Seems quite a comprehensive post. A good read.

    I knew my fat-burning had improved (keto-adaption or beta-oxidation, I am not sure which), when I found IF to be a desirable activity. Skipping breakfast became the norm, without so much as a second thought.

  • HeMan

    After about 10 years of being paleo-ish, I’ve found my body has become quite adaptable, even when I get sloppy, or eat a tonne of carbs.

    I do the intermittent fasting out of preference and never seem to be hungry, even after I’ve had a high-carb day (sometimes gorge on potatoes/other root veggies/ice cream) or gone too hard on the wine (alcohol metabolism is a third pathway and takes precedence over all others)

    It drives people nuts that I can go the entire day and never need to eat.

  • Alyssa

    HOLY CRAP. That was LITERALLY my reaction to learning why exercise is important in this article, and also my reaction to learning about what happens when one’s body adapts to ketosis. This was an extremely informative article, & I’m looking forward to your future articles! 😀

  • Aaron Blaisdell

    Awesome post! I have similar experiences as HeMan mentioned in his comments. I typically IF by eating in an 8-hour feeding window during the day, have stable energy and mood regardless of for how long I’ve fasted or whether my previous day was a high-carb day (which I do occasionally just for the pure pleasure of it), or a low-carb day, or anywhere in between. I’ve gravitated towards the Perfect-Health Diet even before I discovered the website and book by the Drs. Jaminet.

  • Samantha Moore

    Excellent, clear post. Thank you!!

  • Anand: 

    Thank you! This was a long and difficult one to finish, because I kept getting hung up explaining things.  Really it should have been about three different posts.  I'm glad it's understandable, even though it's long.


    Absolutely! I feel the same: I'll often eat one big meal a day, in the evening…and I'm definitely not keto-adapted, so it's purely a function of metabolic flexibility and being able to slip seamlessly into beta-oxidation. That's why I think a lot of people are making the transition too hard on themselves.


    It's liberating to not have food run your life, isn't it?  One wonders how humans ever got anything done if we always got faint and weak three hours after we ate.  Answer: we didn't, because that's not how our metabolism is supposed to work.


    Thank you!  I learn a lot writing these articles, and I'm glad I can pass some of that knowledge on to you.


    This is fascinating: I think quite a few of us are converging on a similar set of eating strategies.  Don't eat breakfast unless you're hungry (which, if you eat big meals in the evening, you're probably not), eat about 1.5 meals per day in an 8-hour window (or less), eat plenty of animal fat, and so long as your carbs go along with your meal(s) you don't have to watch them so closely.  

    I'll probably write an article about this in the future.  Thank you, Asclepius, and HeMan for your insights!


    I appreciate the support: it helps to know that people find my articles valuable.  The best thing you can do in return is spread them around!


  • Katie @ Wellness Mam

    Another great article, as always. I’m passing this one on to my “have to eat granola bars to keep the blood sugar constant” friends!

    I’ve noticed a similar experience as you guys mentioned above, with not being hungry for all three meals a day, and usually skipping at least one (and this is while pregnant!). I’ve done the opposite though. I love breakfast, so I eat a big meat and veggie omelet and then often am not hungry until later in the day, if at all. If I do get hungry, I eat a big salad with some meat/homemade dressing in the early evening.
    It really surprises me how the ability to go without eating for more than a few hours really upsets some people!

    Also interesting to note… This is my first truly low-carb grain free pregnancy, and it has been carefully monitored by my doctor and midwife because of a high risk delivery last time. My blood test results are all better this pregnancy (iron, vitamin D, etc) and I just completed a 2 week monitoring of blood glucose for gestational diabetes (since I wouldn’t drink the glucose solution) and all levels are in excellent ranges. (I also did this with my last pregnancy, and my fasting and 2-hour readings were at least 10 points lower). So, despite the fact that I’m not eating unless I’m hungry (novel concept!) I seem to be healthier and baby is doing wonderful. Funny how that surprises the doctor…

    I’ve been working on a post about the “low carb flu” also, and will definitely link to yours for a more in depth explanation!

  • Dana

    It’s funny but even with ketoacidosis, people focus on the ketones as the cause of the problem but actually, in diabetics, what they really need to be worrying about is that uncontrolled blood sugar–the ketones are there to begin with because the body just can’t do anything with the sugar, for some reason. Ketosis occurs in the absence of sugar availability and, well, if you can’t respond to your own insulin anymore, that’s the same thing as having no access to sugar.

    Could be that the massive amount of ketones is causing the metabolic acidosis but if they don’t get the sugar thing solved they won’t solve the extreme ketosis thing either. But I wonder whether the glucose is not also contributing to the acidosis. I don’t know enough about this to say.

    I don’t mind being in ketosis and in fact appear to switch back and forth into it without much problem, not much more than a day or two of grouchiness. I have a tendency to migraine and/or cluster headaches, which ketosis seems to help on top of letting me burn off my excess body fat. I don’t want to have to run ten miles a day to lose the fat–that’s ridiculous. I should be able to do that while sitting down, it’d suck if I broke my leg on that ten-mile run and then couldn’t lose weight for six weeks while I healed up! People don’t think about that when they over-focus on exercise as a weight loss method. Exercise is vital for health, obviously, but people who are temporarily or permanently disabled need to be able to not use it as a calorie-burning crutch.

  • Asclepius

    One thing I thyink you might find intersting. Last year I wanted to test how far I could go fasted.

    I completed the ‘Welsh 14 Peaks’ completely fasted. This involves summiting all 14 of the mountains in Wales over 3000 ft within the space of 24 hours, without using any form of transport (bar walking/running). The total length is about 24 miles, but the walks to the start point and down from the finish point take it to over 30 miles in total. Total ascent is just over 12,500ft! By the time I had completed it I had fasted for over 30hrs (IIRC). You can read more here. The ability to cycle fat easily played a massive part in my success.

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  • julianne

    Before I discovered the paleo diet, I was a zoner and zone diet instructor. Having taught many people how to zone and watched them through the initial few weeks, I also observed nasty transition symptoms in about 20% of people. Yes they switched from high carb to lower carb 30% carbs with calorie reduction, however this is far above the ketogenic threshhold. I.e eating 90 – 150 g carbs day. The symptoms were identical to low carb flu. Barry Sears has a different take on this. He says it is a release of archidonic acid (AA) from fat cells, from a previous high carb + omega 6 diet. This diet leads to a large amount of AA sequestered in fat cells. The sudden release leads to production of the inflammatory eicosanoid hormones which give the following symptoms – headaches, constipation, fatigue, aches and pains and more interestingly an increase in any inflammatory condition the person suffers from. Increased allergies, eczema or asthma flare up or increased joint pain if arthritic etc. Al of these are linked to overproduction of AA derived eicosanoids. There are two ways to reduce it – 1. add more carbs – it stops the fat loss or slows it down, 2. add a high dose of omega 3 which counteracts the inflammatory hormones.

  • » Do Calories

    […] it is their mission to store fat and they don’t want to let it go.” In people with a healthy flexible metabolism, the collection of fat cells is more like a gas tank in a car, simply a reservoir of stored energy […]

  • Katie:

    That's very interesting.  My mother, who is eating paleo-ish, does exactly the same thing: she eats an omelet for breakfast, and then nothing until dinner.  

    Maybe this is a gender thing: can any of my other female readers offer insight?  What is your meal schedule?

    Congratulations on your soon-to-be larger family!  I'm glad to hear everything is going well.  And it sounds like you're not getting bizarre food cravings like many pregnant women, which indicates that you're probably nutritionally replete.


    Interesting point about ketoacidosis!  I don't know enough at this point to comment usefully, either…but it seems like ketoacidosis is just the ketone version of high blood sugar due to lack of insulin.

    Re: exercise, that's exactly my point.  Exercise isn't about burning calories: even if you do burn them, you get hungry and replace them.  It's about maintaining metabolic flexibility.  And I think this is why the more sedentary you are, the more you benefit from low-carb: you're forcing yourself to burn fat by simply not providing carbs instead of depending on metabolic flexibility to switch over.


    That's an excellent writeup…strong work!  I've done similar epics on my mountain bike (about 11K vertical feet and 60 miles, on rocky singletrack), but never on foot…and certainly not fasted.  I'll have to try a fasted epic sometime.

    I have some intriguing data that suggests high-fat diets might increase the threshold of beta-oxidation (in other words, you're able to burn fat more quickly), but to my knowledge, no long-term studies have been done on this.  Anecdotally, though, it does seem like we can raise our maximum fasted output via LC/HF dieting and fasted exercise.


    That's very interesting: thanks for sharing the data!  

    I wonder about the Sears theory, though: if it's purely a release of AA from fat, shouldn't it happen with every weight loss plan?

    My biggest question, though, is: let's say you're burning a few hundred calories of your own fat each day.  AFAIK it won't be any higher in LA or AA than whatever foods you were eating before that caused you to store the LA and AA in the first place!  Right?  Maybe if you were on an extreme low-fat diet before…but human fat is much like lard, perhaps 11% polyunsaturated and mostly LA, as compared to most “healthy” vegetable oils that are far higher in LA…

    I'll have to look into this some more, but right now I'm not sure I buy into that theory.  Any insight you have is appreciated.

    (Julianne writes the blog  Paleo Zone Nutrition, by the way.)


    Thank you all for interesting and constructive comments!  I'm glad to have attracted a constructive and erudite readership.


  • julianne

    Interesting thoughts. Well thought I’d google ‘new diet headaches’ and found that there were a number of people asking about their headaches on a lower cal diet, (not low carb, just standard low cal). And same kind of pattern, start off fine for 3 – 4 days then start feeling bad / headachy etc (hence the common notion of ‘detoxing’ when going on a diet)

    Also I saw a study some time back that showed toxins stored in fat cells were released when dieting. Can’t find it right now – but this could also cause problems.

    If not AA (or perhaps it is LA release) – why then do inflammatory symptoms increase like an eczema or asthma?

    Why do only about 25% people get these symptoms? Even though they had the same change in calorie / carb on the new diet?

    I’ll have to back and research this some more.

  • julianne

    About the eating – I often have a big breakfast then nothing until dinner.

    Oh and I just googled jenny craig and weightwatchers and headaches and found people reporting transition symptoms similar to “low carb flu”

  • I am NOT My Scale (A

    […] Thanks for the rant, it's a good reminder for me too! I just read this post yesterday, and it also helped immensely. Now I know I am working on my metabolic flexibility and […]

  • Julianne:

    I think you're on the trail of something.  Thank you for looking into this, and please let me know what you find out!  Though I'm skeptical for the reasons above, I'm open to a mechanism by which Sears is right after all. 

    I see reports of similar but more severe reactions from people who are fasting…but it seems to start more quickly and go away more quickly, usually in a couple days.  Whatever it is seems to be related to the start of fat loss, and it seems to also cease even though fat loss hasn't ceased (which is why I'm skeptical of “detox” theories).  Hmm…..


    Back to the mealtime subject: I'm not working on a large data set here, but it seems like the women who eat less than the traditional “3x a day plus snacks” are eating breakfast/dinner, whereas the men are eating lunch/dinner or afternoon/late dinner.  I'm filing this under “things that I don't understand yet”.


  • Beth@WeightMaven

    I made this comment over on perfecthealthdiet.com, but my desire is to moderate my carbs at a level that’s healthful, but flexible.

    It may be the addict in me, but I’d like to periodically have a higher carb meal without it being the equivalent of an OGTT on a VLC diet … with the kind of resulting BG spikes that will get you diagnosed as a diabetic!

    I’ve been tossing over the idea of paleo meets carb addict’s diet … you moderate carbs a la paleo, but you keep the bulk of the carbs/starches to one meal a day.

  • Beth@WeightMaven

    Re AA and stored body fat, a piece I think is interesting is that LA derived GLA is easily converted to AA via insulin’s upregulating delta 5 desaturase.

    So it’s possible that people losing weight who aren’t insulin resistance experience weight loss differently from those who are (who presumably have higher serum insulin).

    I’ve been losing a little over 2lbs/week for a while, and while I don’t still have low-carb flu or headaches, I most definitely am dealing with joint pain (my age doesn’t help matters). I’ve recently begun doing fish oil again. And because 6 months of paleo only got my fasting insulin down to 12 (from 25), I got my sedentary butt off the couch and began doing some Body by Science workouts. Will recheck the insulin in a couple months.

  • » Do Calories

    […] it is their mission to store fat and they don’t want to let it go.” In people with a healthy flexible metabolism, the collection of fat cells is more like a gas tank in a car, simply a reservoir of stored energy […]

  • Paleo Newbie

    GREAT post — I have been searching the web for how to deal with this horrible, horrible low-carb flu/withdrawal. Although I was gluten-free before, I was eating just insane amounts of sugar. Often breakfast AND lunch were a candy bar and 20 oz. Coke. Insane. I’m now 8 days off all sugar & artificial sugar, but the withdrawal is awful. I’m not doing low-carb — I’m having sweet potatoes with lunch and dinner, and 1-2 pieces fruit a day. I had headaches for the first few days. I haven’t lost any weight, and the fatigue is just ridiculous. Any advice? Go more low-carb and try to suck it out? I’m not sure how I could eat *more* carbs.

  • Peggy

    I pretty much always eat 3 high fat, super low carb meals a day, and then fairly often skip one or two of them for no reason other than it feels right. I’ve been doing low carb paleo for six years, occasionally giving in to large amounts of fruit. This never makes me feel good so I always have to give it up eventually. (I was pretty insulin resistant before.) When I maintain the high carb relapse for longer than a couple of weeks, I lose my metabolic flexibility and end up struggling through one or two very tired weeks. It never fails, our author makes a very solid point.

    Thanks for the work! This is a great article.

    I particularly loved this bit of insight “You might ask yourself if it makes sense that natural selection would select us to store energy in the form of something directly harmful to us.” Thanks again!

  • julianne

    @ Paleo Newbie:
    Try large doses of omega 3. Personally I am only a fan of good quality fish oil that I know has been tested for oxidation, as that is the biggest issue. I recommend people with transition symptoms take about 10 – 15 caps day, You should also take 500iu vit E and 1000mg vit C, to keep it from oxidising in cell membranes. The high omega 3 is only short term to try to counteract the inflammation. Try it and let us know how you go – in my experience it helps. The transition symptoms usually diminish after 3 weeks.

  • Paleo Newbie:

    If you were drinking that much Coke, you're in caffeine withdrawal!  That would also explain the headaches.  You're trying to solve two problems at once: caffeine addiction and carb addiction, each of which is difficult by itself.

    See: http://en.wikipedia.org/wiki/Caffeine#Tolerance_an…..withdrawal

    The good news is that you're over the hump as far as the headaches.  Don't expect those to come back.  But since you were dependent on the caffeine/sugar hit for your energy levels, that's going to take longer to deal with.  

    As the article states, moderate aerobic exercise and weight training is effective in regaining metabolic flexibility.  Are you exercising at all, and if so, how?  

    And are you eating the fruit in between meals, or with them?  If you're eating the fruit as a snack, you're sabotaging yourself as far as trying to regain metabolic flexibility.  Fruit is fine, but eat it with a meal.  Same with any other “snack food”: either eat a meal or suck it up.  If you keep feeding your body little carb hits, it'll use those and keep you out of beta-oxidation, which is the only way you are going to lose any fat.


    I think that sugar/”carbs” eaten with a meal (i.e. containing plenty of complete protein and saturated fat) is far less harmful than sugar consumed as part of a snack.  It's the snacking that does you in: see “Why Snacking Makes You Weak, Not Just Fat”.  

    So if you want to eat more carbs, experiment with adding them to one of your meals.  My inclination would be breakfast, as it's usually closer to lunch than lunch is to dinner, but that sounds like you should just try it and see what works for you.

    Have you considered that you might be self-medicating with simple carbs to raise your serotonin levels?  Women do this much more frequently than men.  5-HTP (or even just tryptophan) can help, but check the contraindications before starting it.


    Thank you for contributing your observations!

    I've never cheated for long enough to lose my metabolic flexibility, but I remember exactly what it was like having to eat every three hours.  And I appreciate the support: thank you.


  • Paleo Newbie

    @J Stanton, thanks for such a quick reply and for really making me feel like I’m not crazy. Forgot to mention the exercise bit. I was completely sedentary because I have had bad foot pain for a long time. I’m working on it by transitioning to barefoot, but I can’t do much walking or cycling. I did do some weight training today. The cravings seem to be ending and food is actually satiating again, which are good signs. I just feel constant brain fog, though — like I’m partially asleep — all day long. But I’m coming from a place that is about as unhealthy of a diet as you could eat. Have you heard any other stories from others about how long these feelings lasted?

  • Paleo Newbie:

    Definitely some form of exercise is necessary: as the research shows, weight loss doesn't do it by itself.  Weight training is good if you can't walk or ride yet. To get aerobic exercise, you can do push-ups and other upper-body stuff even if you don't go to the gym.  Can you do air squats, or does that mess up your feet?  Because air squats will definitely get your blood moving and your heart rate up.

    If you're in a permanent foot damage situation, you can find one of those hand-bikes for people with paralyzed legs…but I hope that by slowly building up your barefoot walking you can rehabilitate your feet enough to walk and ride.

    Between being completely sedentary, eating a terrible diet, and being addicted to stimulants, I think it's going to take some time to work through all of your issues.  So long as you're making forward progress, which you are, I wouldn't be too concerned.

    Does a little bit of caffeine instantly clear the brain fog?  It might be interesting to drink a small black coffee, or a cup of tea, to find out if you're just in caffeine withdrawal.


  • Jay Dean

    I’m a Type II diabetic and what you describe is *exactly* my experience, after some years of experimentation with diets and exercise regimens. I have found it vital to get some sort of aerobic exercise every day, and I do mean “every”. With that discipline in place I can eat a satisfying (and very balanced) diet without hunger or the “low-carb flu” and maintain both weight and glucose levels.

    Of course this is just a single point of anecdotal evidence, but I’m living it and convinced. I’m coming to realize that the regular frequency of exercise is more important than the intensity of the workout. You cannot skip a few days and make it up on the weekend.

  • Paleo Newbie

    @JStanton, thanks for your thoughts. I’m going to keep plugging along. 2 weeks into this and I’m still having digestive trouble, so there may be something else going on here.

    BTW, I thought you’d get a kick out of the fact that your website is blocked where I work. The reason? “Illegal drugs.” ha ha.

  • Humpday Paleo Push T

    […] talks about metabolic flexability – your body’s ability to burn both fat and glucose and its […]

  • Jay Dean:

    Thanks for sharing your experience!  I also need to get some exercise each day or I don't feel right.

    Interestingly, most of the research I find on metabolic flexibility is on the ability to switch the other direction, i.e. to sugar-burning, which is also impaired with Type II diabetics (as one might imagine).

    Paleo Newbie:

    If you're having digestive trouble, you are likely to also have nutrient absorption issues.  If you've been on a high-sugar diet for such a long time you might have SIBO.  You might also have insufficient stomach acid (meat and fat requires more stomach acid to digest than carbs).  I'd look into those.

    Can you tell me which filtering software is blocking my site?  I can't imagine what I've said that would flag this place for illegal drugs…sheesh.


  • Mike T Nelson

    Hi there! LOVED the article on Metabolic Flexibility. Awesome stuff and very closely matches my thoughts. I am in the process of finishing up my PhD and my dissertation topic is Metabolic Flexibility. I am more than happy to help with any questions here!

    I can find the study, but one of my favs looked at high calories and very very low calories (attempting to do a fasting type comparison) in a double blinded manner for 48 hours. The subject ate this paste like food to attempt to make it a blinded experiment. What they found was that they handled the semi-fasting condition really well. Take home point–the mind is a factor in all of this too, not just the physiology.

    Fasting has been shown (I think it was Spark et al, but I would need to check) to increase Met Flex in people who were INflexible. Makes perfect sense—since you are are fasting, you either burn body or die since glycogen stores are very limited compared to fats; so fasting is one way to increase Met Flex.

    As to Dr Sears ideas, perhaps, but it still does not jive for me. Keep in mind that by adding more fat (even as fish oil), the body will adjust to burn more fat; just like adding more carbs it adjusts to carbs. This has been shown repeatedly on acute RER studies. Fish oil is great, but maybe it was the result of just eating more fat?

    Hope that helps a bit. Anything else I can do to help answer questions, please repost your questions.

    Mike T Nelson PhD(c)

  • DJSapp

    @Paleo Newbie: you’re describing a lot of the feelings that I had when I first started up on this. A quick question, how much water are you drinking a day?

    Whatever you answer, that’s probably not enough. Drink more water. I got dehydration headaches and sluggishness frequently when I was coming off the coffee/coke/mt. dew habit/monster energy drink habit. Burning fat and proper digestion takes a lot of water to keep it all working nicely.

  • CrossFit Tri-Cities

    […] The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility […]

  • Mike:

    Thanks for contributing!  I'm glad to see you here, and I'm glad to know that I've mostly got things right so far.  I'll have to look into the fasting connection, and into the low-calorie connection.  I'm a day and a half into a fast right now…and yes, I'd obviously be dead if I couldn't beta-oxidize for energy, because given all the skiing I did yesterday I can't have a lot of glycogen left.

    Incidentally, you can read Mike's blog here, at Extreme Human Performance.  


    That's a great point, and here's a hint: if you're hungry but the thought of eating any specific food makes you not want to eat it, you're probably dehydrated.  Drink a big glass of water and see how you feel.


  • Frustrated. | Mark&#

    […] the first week, so I could get a feel for what to eat. I also found this article really helpful: The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility &#45… I think I just needed some time to fix my metabolism, it's a big adjustment for your body to […]

  • Jeffrey of Troy

    Great site! Clicked through from Dr. Mike Eades..

    re: toxins from fat loss.
    Yes, we all have a ton of pollution in us, and the body puts it in fat to try to minimize the damage; so, supporting your body’s natural detoxification pathways is a must when reducing bodyfat percentage (fiber, water, walking, magnesium, probiotics, digestive enzymes).

  • Jeffrey:

    Thanks!  I wish Dr. Eades would post more often…but maybe that's why all his posts are so good.  I know that limiting myself to once a week makes my articles better.

    As far as fat loss = detox, do you have any references for that?  I keep hearing about it, but I haven't found any references that confirm that's what's actually happening.  It seems like autophagy (i.e. long-term fasting or protein restriction) would be more likely to result in such a reaction, since that involves breaking down tissues as opposed to just pulling some stored fat out of a cell.  I'm very interested in learning what's actually happening.


  • JMH

    This is also just my experience, one data point, etcetera…
    But I’ve been sick my entire life. I was given my first prescription for some kind of oral opiate at eight. (Attempting Paleo, but difficult with a college caf, assorted. Yesterday, I ate sushi rolls that I’d peeled the rice off of, because I’m so metabolically deranged, I’d really keep it as low carb as possible right now until my Crohn’s is under control.)
    Moral of my story… I know when I’m burning body fat, because I get vaguely stoned. If I have various flavours of opiates shoved in my fat cells, I can only imagine what else other people have hidden there. I would suspect, though have no evidence for this, that the people with the worst and longest lasting symptoms are the sickest, and thusly have taken the most medication.
    My $.02, YMMV.

  • JMH:

    Interesting!  I have a strong feeling of deja vu, like
    this subject has come up before and I'm forgetting some important piece
    of research.

    I see a lot of interesting anecdotal evidence, like yours, but I'm still looking for a mechanism by which things might actually end up being stored and recalled in this way.  Do opiates end up packaged into chylomicrons?  Etc.


  • Bruce Wilson

    Do you have suggestion for seniors to help restore body flexibility?

  • Bruce:

    Physical flexibility exercises are well outside my expertise. I can't even think of a good joke about it right now. Sorry about that.


  • lynn

    I’m there…thank you for the info. I have had achy legs, feet and arms for the last 4 weeks. Everything hurts. I quit working out…now it seems I should step up my workouts. I was thinking I had a virus. I guess the recumbent bike is my best friend right now.

  • » Do Calories

    […] it is their mission to store fat and they don’t want to let it go.” In people with a healthy flexible metabolism, the collection of fat cells is more like a gas tank in a car, simply a reservoir of stored energy […]

  • Nathan

    Thank you, that was quite informative.

    I did not find that my low-carb flu had ended, even after three weeks; so I did a carb refeed, which seemed to work. I was exercising a fair amount, but things got worse, not better (massive fatigue being the main symptom). So exercise is no doubt helpful, in general, but apparently it does not work for everyone.

    I really enjoy your site and look forward to reading more.

  • » Manic Monday

    […] has to switch on fat-burning again, and it takes a while. This feeling is what some call the “low-carb flu” (great post at that link, well worth a read). Once you’ve been eating a low-carb diet […]

  • Primal for female at

    […] […]

  • Nathan:

    Definitely if you're very active you'll need more carbs than the average person: beta-oxidation only works up to about 2/3 of your VO2max.  And I agree with your approach: I think that occasional refeeds are better than continual high-carb eating.

    I'll most likely write an article about carbohydrate metabolism and storage someday.  Once you understand those pathways, it's easy to understand how much carbohydrate you should be eating, and to know when you're eating too much or too little.


  • Hungry | Mark's

    […] […]

  • The Breakfast Myth,

    […] Remember my article about metabolic flexibility and the respiratory exchange ratio (RER)? The RER graphs from Young et. al. show that mice fed a high-fat breakfast maintained the flexibility to switch back to fat-burning after eating a high-carb, low-fat dinner…whereas mice fed a high-carb, low-fat breakfast were stuck burning carbs all day, even during the fasting period between breakfast and dinner. […]

  • Jeffrey of Troy

    Hi, sorry it took me so long to get back to you.

    Re: detox

    This subject is generally suppressed, as it it not biz-friendly, but here’s a decent piece on it

  • Adkins diet - Page 3

    […] Re: Adkins diet For anyone going into a low-carb diet, here's a great article you should read as you go face-down into the animal fat: The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility &#45… […]

  • » Manic Monday

    […] has to switch on fat-burning again, and it takes a while. This feeling is what some call the “low-carb flu” (great post at that link, well worth a read). Once you’ve been eating a low-carb diet […]

  • NY Times Article on

    […] If you're body is conditioned to burn fat for fuel, training fasted may be preferred. If you're a sugar burner, training fasted will suck, and your […]

  • Jeffery:

    It's a plausible mechanism, but I'd like to see some science.  Are you familiar with any experiments in which (for instance) people's blood levels of PCBs were measured during nutritional balance, and again during fasting (which should release them into the general circulation)?  


  • leah

    Love your evidence-based approach, with the easy to understand interpretation! Sign me up

  • leah:

    Welcome!  Glad you're here.


  • 3 Days in.... feelin

    […] This article is relevant to your interests: The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility &#45… Reply With Quote   + Reply to Thread « Previous Thread | […]

  • lynn

    I must be very insulin resistant. I have really gone VLC..I can see now why I was sore all the time. I work out hard. But in all that, I never have been able to lose weight even though I did 45 minutes of Weight training three days a week and 20 minute High intensity bike three days a week, ate 6 small kinda low carb meals per The Body for Life program.
    Because I was so sore, I slowed down my weight training. I stopped leg pressing 5 sets of 500 pounds and just began to do lighter sets. I have only lost 12 pounds. I started this Paleo low carb diet seriously at the beginning of June.I was gradually changing things during the month of May. I cleaned out my cabinets and fridge. I still have 40 pounds to lose. I am no longer aching. I have begun to get more energy. I find I am losing a pound to a pound and a half weekly. I can accept that. My waist has lost the most. 4 inches. I think that is a lot of inches for only losing 12 pounds. I also breathe better. No inhaler needed. What I love is that I no longer feel driven by constant hunger. I feel free. I never want to go back.

  • lynn

    I am my own guinea pig. When others ask me..Is that healthy? I just say..I am not healthy being fat. I am not healthy with 105 morning blood sugar..I am not healthy with high blood pressure. I was real strict about the diet and exercise program. In three years it did nothing. by the way…my blood pressure is now normal with out meds. It plummeted. Another reason why I will never go back. Our county fair is this week. I have not been tempted once. There is plenty of meat in the 4H farm booths.

  • lynn:

    I think it's much more important to focus on waist and belly circumference than on losing weight: the objective is to lose fat, not muscle.  And it sounds like you've turned the corner on metabolic adaptation, which is the biggest step.  Congratulations!


  • ~pjgh » Blog A

    […] his article The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility two key points stand out to […]

  • As always, an article generously littered with simple, distilled shots of good advice backed up by the biochemistry research placed in between.

    The summary, “if we keep our carbohydrate intake under our body’s requirement while not in ketosis, which is perhaps 20% of total calories—and only eat those carbohydrates with meals involving complete protein and fat, not by themselves—most of us should be able to gain the fat-burning benefits of metabolic flexibility without suffering the pain of trying to adapt to ketosis” is most useful.

    I have removed potatoes from my diet while fat loss is still a goal, as well as a healing my gut. I do include a good number of milder carbohydrates.

    Understanding that when well portioned, these will help keep me in a normal state and enable my body to use the remaining excess fat stores more efficiently and then residual fat stores when necessary really is the foundation for my next step now that the initial surge of fat loss from activity is slowing down.

    I don’t routinely weigh or measure myself or my intake. I have weighed myself and know my weight has dropped, but I am not focussed on weight loss; it’s fat loss for now and I fully appreciate that I might not lose as dramatically in terms of weight or size.

    Superb article, J! Thanks!

  • Paul:

    I'm glad you're finding it useful.  I try to explain the “why” as best I can, so that when I make a recommendation it'll make sense to you, and when you see conflicting advice you'll have some sane way to evaluate the conflict.


  • I’m Tired and

    […] If you are interested in understanding more about why the carb flu happens, check out this article at Mark’s Daily Apple  and this very detailed explanation from J. Stanton at Gnolls.org. […]

  • maría

    So, imagine how hellish life can be for someone with Chronic Fatige Syndrome and Multiple Chemical Sensitivity whose oxygen metabolism is kaputt, along with the ability to make energy out of glucose. One depends then on protein, and has to be very careful with the carbs one takes, because the diebetes-like symptoms can be very debilitating on a system that’s already extremely exhausted. Hell.

  • What Was It About To

    […] ride, maybe have a big fat (and protein) breakfast and skip lunch. The trick really is to become keto adapted, which may take 2-4 weeks. Most healthy people should have enough fat stores to use for at least a […]

  • María: 

    It is absolutely true that mitochondrial function is impaired in the obese, the future obese, and the past obese.  But I'm not sure how you can depend on protein if you can't make energy from glucose, because it's converted to glucose before being used as energy.  Can you explain?



  • Muscle Soreness and

    […] […]

  • I’ve just reading Jack Kruse re low carb flu. Interesting. Low carb diets lead to seretonin deficiency due to lack of absorption of tryptophan (the amino acid required for seretonin). Carbs are needed for tryptophan absorption in gut. If someone is already seretonin deficient, then this is more pronounced when going on a VLC diet. 5HTP will help as will increasing carbs.


  • Oops that should be serotonin

  • julianne:

    I just saw that article earlier today.  I've written before about the issues of empty simple carbs helping to get tryptophan/5-HTP across the BBB, but this is the first I've seen about gut absorption of tryptophan also being an issue.  I'd love to find some sources to learn more about it.


  • primordial

    Currently I am trying to go into ketosis. I went lc paleo and as long as I eat 50-200 gram carbs (veggies, fruits, nuts) a day everything is working wonderful.


    But I wanted to fight overeating on carbs because they don't make me satiated (I am skinny, though), so decided to go into ketosis (high-fat)


    For more than 4-5 weeks now I didn't eat more than 40 g carbs a day, in average less than 20, some days none at all. I feel like crap (That is, regarding my metabolism/circulatory), really weak.

    More important, my cognitive abilities seem to be at it's lowest since going off the carbs, I have even problems putting sentences together. And I thought a ketogenic diet improves cognitive performance.

    Normally the transition should long have been done, shouldn't it?

    Is there anything to do to help this process or should I just go on eating 20-40% carbs again?

  • primordial49:

    Two options.  First, 40g carbs may be bouncing you in and out of ketosis.  Try these articles for ways to make a healthy ketogenic diet that still tolerates some carbs:



    And if that doesn't work, it may simply be that ketosis is not for you.  Ketogenic diets usually only improve brain function in cases where it is impaired.

    Hope this helps!


  • primordial

    Thanks, J.!

    My hope was to heal my migraines, which are highly associated with an energy crisis in the brain due to hypoglycemia/glucose intolerance with similarities to epilepsy.

    1. Critchley M. Migraine Lancet 1933;1:123-6
    2. Wilkinson CF Jr. Recurrent migrainoid headaches associated with spontaneous hypoglycemia. Am J Med Sci 1949;218:209–12.
    3. Roberts HJ Migraine and Related Vascular Headaches Due to Diabetogenic Hyperinsulinism Headache 1967, July,41-62
    4. Dexter JD, Roberts J, Byer JA. The five hour glucose tolerance test and effect of low sucrose diet in migraine. Headache 1978;18:91–4.


    Roberts (1967)suggested , to change the name of migraine into “hypoglycemic headaches”:

    “The fundamental metabolic disturbance that usually triggers
    such headaches is recurrent hypoglycemia … [it is suggested that] the
    term migraine be replaced by … 'hypoglycemic headache.'”


    So it is likely a ketogenic diet can heal the energy crisis in the brain:

    Strahlman, R. Scott: Can Ketosis Help Migraine Sufferers? A Case
    Report. Headache: The Journal of Head and Face Pain. Volume 46 Page 182 –
    January 2006. doi:10.1111/j.1526-4610.2006.00321_5.x

    Apparantly also depression can be improved with a ketogenic diet: Murphy P, Likhodii S, Nylen K, Burnham WM: The antidepressant properties
    of the ketogenic diet. Biol Psychiatry. 2004;56: 981-983


    I improved my migraine vastly due to the last weeks. To see long term improvements and never suffer from it again I calculated tat I need to stay in ketosis for at least a few months to let my body recover.


    I read the articles. Seems it is harder to maintain a healthy ketotic state than I  thought previously. I suspect the cognitive impairment is due to the fact that not enough ketones are generated while gluconeogenesis is ineffective.


  • Debating whether or

    […] […]

  • Ted Hutchinson

    While there is much to be said for the role of exercise and training in improving mitochondrial biogenesis there are some readers for whom intensive exercise is not an option.
    The paper
    Mitochondrial energetics and therapeutics offers some other strategies that may be helpful. They suggest that a ketogenic diet is probably the best option although there are some supplements such as Resveratrol that may help.
    I think we also have to consider ways of protecting our mitochondria to hopefully prevent them becoming dysfunctional. Improving melatonin secretion and getting a good nights sleep may also be a useful strategy.
    Melatonin in Mitochondrial Dysfunction and Related Disorders

  • primordial49:

    I'm starting to see this a lot: the “bouncing in and out of ketosis” headaches, where you never fully keto-adapt but never build up your glycogen stores either.  I hope those articles help!


    Thank you for the references!  Do you have fulltext of Wallace et.al.?  If so, please contact me, as I'm very interested in reading it.

    (Note that your comment was caught in the spam filter, which accounts for the delay in posting it.)


  • Why Saturated fats o

    […] explain why saturated fats are superior to PUFAs and MUFAs? How does your own body store fat? The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility &#45… You might ask yourself if it makes sense that natural selection would select us to store energy […]

  • Dealing with carb cr

    […] am still transitioning. I get solid energy from fat now, I have regained my metabolic flexibility (The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility &#45…), the energy challenges took 2-3 months to fully shift for me. I ate very low-fat and very […]

  • Angel

    Thank you for this article! This explains really well my problems with low-carb.

    I’ve been on both Atkins and the Zone diet. Atkins was a miserable 3 week experience during which I didn’t even lose more than 1-2 pounds. The Zone diet worked a lot better, I lost about 15 pounds, but I always needed more carbs than what was specified. I was 26 years old then; already soon-to-be-obese, apparently.

    I started low-carb again in 2007, but I didn’t try going really low-carb, because I knew it wouldn’t work (although I assumed I was just a weakling for not being able to handle constant intense carb cravings). I did manage to lose some weight, but not nearly as much as I needed to lose. I wasn’t exercising then either, though.

    I think my metabolism is very gradually improving. I can now occasionally eat very low carb meals and not have carb cravings afterwards.

    And I very much appreciate the information on restoring metabolic flexibility. I started exercising again several weeks ago, and I have lost a little weight and am feeling better. I have noticed that I have a little more flexibility in when I need to eat, but not as much as I would like … fasting is still not a good idea for me.

    I have had to accept the fact that I have a broken metabolism, and will probably always carry around too much fat, but this article is the first real hope I’ve had that I might be able to restore most of my metabolic function. Thank you.

    I would be interested in knowing to what extent metabolic flexibility was restored in the T2 diabetics. Simply saying that it was restored implies that it was restored completely, which doesn’t sound realistic.

  • Angel:

    The thing with exercise and met flex is that the effect of each session only lasts a couple days.  Basically, exercise depletes muscle glycogen (if you exercise hard enough…short and intense is better than long and slow), which means your body has a place to store the carbs you're eating and it doesn't have to constantly burn them in order to keep your blood sugar down.  Once you fill your glycogen back up by eating carbs, the effect wears off.

    So be consistent about it: try to push yourself, even if it's only for a short time, at least every other day…and as often as you can reasonably manage.  It's more important to do a little bit every day or two than to do nothing all week and try to make it up on the weekend.  Remember, you're not exercising to burn calories, you're exercising to deplete glycogen.

    Another thing you might look at is the effect of low-carb on thyroid: the Jaminets at Perfect Health Diet have a recent series that may be helpful.


  • feel sluggish? | Mar

    […] […]

  • Constipation and Chr

    […] […]

  • PrimalNut

    Hi guys, I just want to add something.
    I know Milk isn’t exactly Paleo,Primal or Caveman’ish… but ever since I stopped consuming vegetables my belly has gone flatter.
    I get bloated like crazy from vegetables ( and never knew why until I read up here). Atm, I am trying to eat a somewhat carnivorous diet and I feel much better, gutwise. My only carbs come from 2-3 cups of RAW goats milk and perhaps a handful of berries or 1 green apple a day.
    I chose raw milk to be my ‘cheat’ simply because 1. it’s extremely nutritious and I need to grow new bone in my face (palatal expansion) 2. It kills ALL cravings and 3. the sugars from milk take a different path in the body if the live enzymes are present

    When the goats go on ‘strike’ end of fall I will try and not replace it with another farmers milk. I’ll suck it up and push through the craving (milk has opioids) and see what happens. Maybe I’ll be able to lose the last 5 lbs of chub around my waist 🙂

  • Glycogen: Storage En

    […] Diet or Low-carbohydrate diet reduces cancer riskPaleo VillageThe Blog of Michael R. Eades, M.D.The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility var analyticsFileTypes = ['']; var analyticsEventTracking = 'enabled'; var _gaq = _gaq || […]

  • Hey, 'Nut – milk is fine. Some areas of the world tolerate it much better than others – I'm northern European, Celtic by recent gene pool and very much a lover of dairy. Nothing wrong with dairy!

    I shudder to think you're missing out on your veggies! Which veggies don't you like? Which bloat you? Strong green veg with a nice slab of meat is absolutely sheer heaven! I mean … https://picasaweb.google.com/107179421315824659117/Cuisine#5623379282735605714

    Green veggues give a lot of the things that milk does, but more so … I really would encourage you to get more cabbage, sprouts, broccoli, asparagus, spinach and samphire into your diet. At the very least, green veg gives you such a lot more than other food sources can.

    Keep up your milk – you like it, you enjoy it and it does you well … no need to feel apologetic about dairy consumption.



  • Jon

    Interesting. I remember reading Atkins adamantly insisting that exercise, along with portion control, were irrelevant for LC weight loss. The exercise part didn’t make sense, given its role in glucose management for diabetics. There goes another excuse not to exercise.

  • PrimalNut:

    Some people simply don't digest veggies very well.  Peggy the Primal Parent is on a pure carnivore diet AFAIK and has lots to say on the subject.

    Since you're nearly zero-carb otherwise, I suspect the carbs from the milk are fine if you're lactose-tolerant.  You might consider trying a touch of starch if you cut the milk out entirely.  And heck, it's raw goat's milk!


    I suspect he'll be fine.  Cruciferous vegetables can be good, but as they were only domesticated and bred into their modern forms a few thousand years ago, I can't believe that they're necessary for our health.  And if they make him bloat up, then obviously he's not digesting them well.


    Yep. Exercise is important, but not for the reason everyone thinks.  You lose weight by having metabolic flexibility and burning fat whenever you're not eating…you don't lose it by trying to exercise off the weight, because that just makes you hungry.


  • Arbo

    J. Stanton — I follow a diet similar to Peggy from the Primal Parent and like most people on VLC I’m going to say that the problems people in your comment section are having are because not enough fat causing low blood sugar

    Even after you’re saying that primal diets are high fat, there’s still people who go keto and stay relatively low fat and think they’re on a ‘ketogenic’ diet. No brah, you’re on a low-blood sugar diet

    Julianne – carbs aren’t needed to produce serotonin. Our serotonin/prolactin levels increase as our insulin levels increases, they decrease as our insulin levels decrease — Someone who’s keto-adapted gets insulin from fat, so for them, fat produces serotonin

    Really, people need something new to talk about I know, but 99% of the anti-keto posts around the web are assuming that people on keto diets are in low insulin states. Vit C deficiency? Long term low insulin. Poor cognitive function? low insulin. Electrolyte imbalance? Low insulin.

    Seriously now, someone understand that you can have good insulin levels on a keto diet, the problem is that you have to eat lots of fat — the only modern meat that meets the criteria is fatty steaks, expense pork, and sausages. This is why the famous all-meat advocate Owsley ‘The Bear’ Stanley lived on fatty steaks as his definite source. You simply need a high fat-to-protein ratio if you’re going to make the low carb diet work

    Anyways, thanks for the work. This blog and Emily Deans are by far the best paleo blogs, both for how frequent you guys post and the good material — Hope you remind these unhealthy low-blood sugar ‘keto’ dieters to increase their fat before they end up with nutrition deficiencies from low insulin levels

  • Arbo:

    You're absolutely right that calories have to come from someplace.  If you're ketogenic and are strictly limited on carbs, then you'll need a lot of fat, because conversion of protein to energy is strictly limited.  And you can actually consume a surprising amount of carbs on keto if you're high in MCTs (e.g. coconut oil) and leucine.

    To be absolutely clear, I'm not anti-keto — I'm anti-almost-keto.  My opinion: either commit to keto, or bump your carbs up to the physiological ~15% of calories level.  To me, the low end of LC/VLC is very much a danger zone where many people end up never quite keto-adapting, so they get the pain of adaptation without the benefits of actually being in ketosis.  I see an alarming number of people stuck in this limbo.

    The Jaminets over at Perfect Health Diet have some excellent posts on the dangers of (and solutions for) ketogenic/VLC diets, as well as excellent suggestions for how to maintain a livable ketogenic diet.

    I appreciate the support, and I'm glad you find my articles valuable!  Note that the best way to say “thank you” is to pick up a copy of The Gnoll Credo…which I believe you'll find pleasingly carnivorous 🙂 


  • Greg


    Thanks for this great post. I’ve been doing keto (not almost-keto) for almost 2 years now, and it’s worked beautifully: lost 60 pounds, no hunger pangs, seemingly infinite energy on tap when I need it (to include exercising)… but I’m getting to the point where I’m weight-stable at about 21% body fat, and I’d like to drop a few percentage points. This article is really hitting home for me, as I have mostly discounted exercise. I knew about HGH and glucose/insulin sensitizing effects, but this puts another spin on things. In fact, I am going to commit myself to some moderate, non-hunger-inducing exercise in my schedule for 2 or 3 months to see if I can effect any sort of change in my homeostasis.

    So, thanks again, and especially thank you for being a paleo writer who doesn’t dump on ketosis – a valuable and, IMO, healthy metabolic state. (If you commit to it.)

  • Greg:

    I'm not actually sure how exercise affects met flex while in ketosis…I hope you'll report back and let us know what happens! 


  • Anna K

    Hi, I’m very confused on “The Difference Between Beta-Oxidation and Ketosis”. How are they different exactly? I thought that they happen in parallel and ketosis is a product of beta-oxidation. Are you saying that your body can get energy from beta-oxidation without depleting your glycogen stores first or depleting them slower so you never go into ketosis? Or does the quantity of ketones produced have to be large to be in ketosis?

    My understanding is that ketone bodies are formed by ketogenesis when the liver glycogen stores are depleted, which happens periodically to most people (who have a proper working metabolism) and who are either VLC, or just LC and haven’t eaten in a while, or normal carbs but do intermittent fasting, or work out a lot. Don’t you think you would be in ketosis after intermittent fasting and on top of it working out during a fast?


  • Anna K:

    There are actually three concepts here.

    Beta-oxidation is the mechanism by which your mitochondria burn fat for energy.  It happens all the time, even if you're not in ketosis: healthy people at rest in the fasting state are deriving nearly 90% of their energy from beta-oxidation regardless of how much glycogen is stored in their muscles, with the rest going to power tissues (like the brain) that require glucose.  Note that the body conserves muscle glycogen whenever possible, and only uses it during periods of intense activity (over 50-60% of VO2max).  

    Some ketones are generated from the backbones of triglycerides, but you're correct that most of them are generated by the liver when it runs out of glucose.  This triggers the state of ketosis.  As you mention, it's fairly easy to enter ketosis: most of us wake up in ketosis.

    Keto-adaptation is the process by which 1) normally glucose-dependent tissues, mainly the brain, switch from burning glucose to burning a combination of glucose and ketones, and 2) mitochondria proliferate and upregulate their ability to beta-oxidize for energy in the absence of muscle glycogen.

    The problem with keto-adaptation is that you quickly bump yourself out of it when you consume carbohydrate…so if you're eating LC/VLC but aren't maintaining continual ketosis, you never fully keto-adapt.

    Is this more clear?



  • JP

    I would be interested/puzzled on this comment on excercise:

    “…The thing with exercise and met flex is that the effect of each session only lasts a couple days. Basically, exercise depletes muscle glycogen (if you exercise hard enough…short and intense is better than long and slow)….”

    I’n in belief, that in general easy exercise is the one, which developes fat burn qualities (e.g L.S.D. runs for marathon to prepare fat burn to carry you the distance..). But is the above quote only related to getting to ketosis or some other connection I missed..?

  • Anna K

    yes, thanks!

  • What to eat before a

    […] Times Magazine Nutrition & Metabolism | Full text | Ketogenic diets and physical performance The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility &#45… Reply With Quote   + Reply to Thread « Previous Thread | […]

  • » “Eat T

    […] do think that low-carb diets can be great for initial fat loss, and for forcing mitochondrial fat adaption, but aren’t the optimal way to live for everyone everywhere as I once thought. I’m not […]

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  • Γιατί οι άνθρωποι λι

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  • Γιατί οι άνθρωποι λι

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  • Sample Paleo Meal Pl

    […] I discovered a real treatments for depression they only used in this list are pretty difficult to find these would like for our diet A details sample paleo meal plans for a […]

  • » “Eat T

    […] do think that low-carb diets can be great for initial fat loss, and for forcing mitochondrial fat adaption, but aren’t the optimal way to live for everyone everywhere as I once thought. I’m not […]

  • Mary T

    I’m on day 3 of an anti-fungal diet which is pretty much VLC. I’m taking some Glutamine in the mornings to keep my mind functioning at work. Are there any other supplements that can help me adapt to this new type of metabolism?

    Part of my reason for going on this is that I don’t seem to be absorbing Vit-D and Vit-K at needed levels even if I supplement. I’m definitely overweight and need to loose weight so if I can make this a sustainable practice, I hope to make this a long term lifestyle.

    FYI – Even when I consumed lots of carbs, I didn’t have high triglycerides or cholesterol. In fact it has only been the last few years where I moved into the “normal” cholesterol range… and no I’ve never fretted about a low fat diet.

  • training for wrestli

    I’ve cut cards to a bare minimum out of my diet and I’ve got to say the results are fabulous. Very informative stuff here, I never quite knew the science behind it before.

  • IF and muscle loss |

    […] […]

  • Carb overload? | Mar

    […] […]

  • » “Eat T

    […] do think that low-carb diets can be great for initial fat loss, and for forcing mitochondrial fat adaption, but aren’t the optimal way to live for everyone everywhere as I once thought. I’m not […]

  • Mary T:

    Sorry I missed your comment!  Anyway, if you're still around, I recommend using coconut oil as your cooking fat (the MCTs are very easily absorbed), and possibly some creatine if you're not eating much red meat.  Some people also find green tea helpful, particularly as a coffee substitute.

    Beyond that, the Perfect Health Diet supplement recommendations are pretty solid.  I wish you the best of success!


    It's reasonably straightforward stuff once you understand basic metabolism.  I'm glad you find it helpful!


  • Q&A: Affording P

    […] up to two weeks on a low carb diet – your energy should sky rocket as J. Stanton described here. (Richard Nikoley will be participating in a podcast on Monday in which they discuss low carb. I […]

  • One Year On «

    […] first week of going low-carb, I got the ‘low-carb’ flu. Sounds like more pseudo-science doesn’t it? Click the link and revel in the proper, […]

  • maggie

    I am a basketball player in high school… and about a week ago i went cold turkey paleo (still eating fruit though)… have had many of the symptoms listed including aches, pains, fatigue, muscle tiredness, headaches, dreams of eating carbs/sugar… Should I continue on this paleo? …I am a strong female athlete who weighs about 140 lbs… or any suggestions to ease the pain?

  • maggie:

    If you're an athlete and not looking to lose weight — especially if it's the season and you're practicing/training regularly — you'll definitely need to eat more carbs than a mostly-sedentary person trying to lose their gut.

    What do you mean by “cold turkey paleo”?  Whose book or version of paleo are you following, and what are you eating on a typical day?  Did you go paleo to try and solve any specific health problems?  What sports are you playing?

    Not knowing these answers yet, my initial advice would be to keep eating a strict protein+fat+veggies breakfast, but to add starch at your other meals.  Strict paleo pretty much only allows sweet potatoes, but that gets old very quickly.  White potatoes are fine IMO so long as you peel them, and white rice is relatively inoffensive even if it is a grain.  So long as you're avoiding all other grains (particularly wheat) and refined sugars, you'll be gaining the overwhelming majority of the benefits unless you have specific food intolerances.


  • An active person need not shy away from carbohydrate – it is a valid and vital macronutrient. Understanding how much you need does come from eating a good paleo diet – your body will speak to you more clearly.

    Much paleo dietary wisdom is shy on carbohydrates simply because so many people come to paleo fat, too fat. Pure starches in the form of white potatoes, plantain, taro, white rice and cassava are good for the active person. Eaten earlier in the day and just a little after rampant activity to replenish energy, it's all good.

    Paleo is a template, not a diet.

    Your template may well need lots more energy than an overweight person used to soda, fries and processed meat.

    Let's hear more about who you are and how you like to live to advise more on what you might like to consider as part of your template. Paleo eating is inclusive – it's what you can and should eat, not like a diet where you can't eat things.

    Eat real food is the key – meat, fish, shellfish and vegetables from good, raw, natural and organic sources. If your vegetables (and fruits) include more starchy foods and you're active, great!

    Meanwhile, understand that if you've been big on modern foods, you will have a period of detoxifying. Also, adjusting.

    Jump back with a little more about yourself … don't be scared of carbohydrate just because a paleo diet book says it's no good.

  • maggie

    Thank you for the quick response and helpful advice!

    What I meant by cold turkey paleo is- I maybe a switch immediately from my old eating habits (pretty much what is in the dining hall including sandwiches almost daily, pasta meals, whatever was being served, plenty of cereal, fruit, and sugary items). I am not sure exactly which version of paleo I am following, I was just guided by my mother and I eat only meat, eggs, vegetables (including sweet potatoes but no regular ones), fruit, milk (by my choice) and nuts (also I was eating peanut butter which I just recently found was legume and not a nut butter!). I usually eat an omelet or scrambled eggs (with bacon) in the morning with milk and a piece of fruit and for lunch/dinner I eat whatever meat is available in the dining hall with vegetables or over a salad! I play basketball and run track and also do crossfit. I have a basketball practice or game everyday, I shoot on my own everyday, and I crossfit on days that are not game or pregame days (I also go on runs when I can). I decided to go paleo because of the effects my parents saw; not just weight loss for them but the newfound well-being. After I was shown the negatives affects of sugar and constantly eating of carbs I decided to make the switch. I have also read much about how in order to improve as a runner (for track) I need nutrition and probably a smaller frame (I am 5’8” 140lbs and muscular and relatively skinny).
    I like your suggestions on the other starches that I can integrate some into my typical daily diet.
    I was definitely big on modern foods and I am definitely feeling the affects of the detoxifying! Although it does not feel good right now it is encouraging to know that I am sort of cleansing my body, as well it helps to hear from you folks about all of this! Thanks so much

  • maggie:

    Oh, good lord.  If you're practicing every day and Crossfitting frequently, you're going to have to eat MUCH more glucose than you're getting from fruit and the occasional glass of milk.  With that much activity, you might even be short on total calories — paleo foods are very satiating, and it's easy to not eat enough if you're very active. 

    As I say in “Eat Like A Predator”, “If you are active and not concerned with losing weight (or trying to gain it), you’ll want to eat more carbs than the average person trying to lose a few pounds.”  Your parents (older, probably relatively sedentary, trying to lose weight) are NOT a good guide to what you (young, extremely active, probably still growing) need to eat.

    I know that my athletic performance suffered for a couple months after going Paleo, because I didn't have a good feel for how much carbohydrate I needed to eat…I felt great mentally and for the first half-hour of exercise, but I bonked quickly because I was constantly glycogen-depleted.

    How to fix this?  My advice: EAT MORE STARCH.  Not sugary fruit — you don't want fructose, you want glucose.  Anything off the Jaminets' list of “safe starches” will do: sweet and regular potatoes (peeled), taro/sago/tapioca/cassava if you can get them, and white rice.

    In order to keep your consumption under control (so you don't lose control and binge), eat the starch at meals (preferably dinner and lunch, not breakfast), and eat it last.  Eat meat and veggies until you're satiated with them, and then start on the potatoes or rice.  Baked and mashed potatoes, simple white rice with butter and salt, potatoes and rice in scrambles

    Rule of thumb: eat more starch until your athletic performance stops increasing.  If you start gaining weight or binging, eat less.  But since you're in strong physical shape, your body will probably tell you how much you need.

    There are some advanced techniques you can get into later, but let's get the basics sorted first.


    (Annoying disclaimer: As always, this is simply my advice.  Your choice to take it, or not, is your own responsibility.)

  • […] do think that low-carb diets can be great for initial fat loss, and for forcing mitochondrial fat adaption, but aren’t the optimal way to live for everyone everywhere as I once thought. I’m not […]

  • Rob

    I just wanted to make the comment that “anaerobic” activities are not necessarily limited by waste product accumulation but by exceeding ATP production/repletion (supply) and likely central fatigue, resulting in keeping ATP high enough to prevent death. Great article.

  • Rob:

    AFAIK the limit on anaerobic metabolism is generally the buildup of lactic acid, which drops the cell's internal pH far enough that the chemical reactions stop working.  This is because, under anaerobic conditions, the pyruvic acid produced by glycolysis is converted directly to lactic acid — as opposed to aerobic conditions, where it can enter the mitochondria, be converted to acetyl CoA, and enter the Krebs cycle.

    If you have sources which demonstrate otherwise, I'd love to see them!


  • […] it is their mission to store fat and they don’t want to let it go.” In people with a healthy flexible metabolism, the collection of fat cells is more like a gas tank in a car, simply a reservoir of stored energy […]

  • Mags

    I have read and seen a lot about the importance of recovery food within a short span after working out. I have seen protein shakes, chocolate milk, fruit, etc. I am wondering what you believe is the best food/nutrients as recovery after workouts, especially rigorous ones?


  • Mags:

    It depends what you're doing!  For strength training of any kind, complete protein is most important.  In fact, I'm a big fan of protein after any workout: while muscles are indeed extra-sensitive to glucose immediately PWO, you can replete muscle glycogen anytime.  In contrast, your body has no way to store amino acids, so you'll need to provide them in order to allow protein synthesis that doesn't involve catabolizing existing tissue.  You shouldn't need a lot of fat PWO, since exercise frees it from your own adipose tissue.

    In conclusion, something high in complete protein, with carbs according to how much intense glycogen-depleting effort you've put out.  Strength training doesn't use much: that's mostly dependent on the phosphocreatine system.  Intense anaerobic effort is what'll deplete glycogen quickly, e.g. metcons, HIIT.

    Of course, real food is best, and you're not going to die if it takes you 30 minutes to get home and fix it.  But if you can't manage that, a can of tuna, some jerky, or a protein shake (and, optionally, a banana) will hold you until you can get to the real food.  In that case I use whey protein because it's absorbed very quickly.


  • […] I'll link this again, because many people seem to find it helpful: The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility &amp… tl;dr There's a difference between regaining the ability to burn fat (metabolic flexibility) and […]

  • […] the other? Whoa… there's all sorts of confusion with this question… Fatty Acid Oxidation The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility &amp… https://www.facebook.com/note.php?no…10112022391448 It isn’t necessarily ketosis that […]

  • […] improves metabolic flexibility. Fat burning ability improves (it has to), and insulin sensitivity increases. (1,2,3,4,5). […]

  • […] do think that low-carb diets can be great for initial fat loss, and for forcing mitochondrial fat adaption, but aren’t the optimal way to live for everyone everywhere as I once thought. I’m not […]

  • xaevir

    Need help. I am 2.5 weeks into the diet and I sleep terribly. I slept better the first week into the diet maybe bc of the low carb flu symptoms. The sleep I am getting is just not deep enough. I am so tired during the day, its killing my work. Should I try sleeping supplements like melatonin or 5htp? I am at my wits end. Thank you.

  • I went through a phase of this, waking up in the middle of the night and feeling like I couldn't get back to sleep. As odd as it sounds, go with it – the anxiety about not sleeping is probably what's keeping you awake. Apparently, we sleep in four hour blocks, so it is perfectly normal to wake up in the middle of the night after a short period of deeper sleep and then have a few hours of light sleep.

    Like the diet, it will normalise.

    Supplements might well be okay for a short-term gain, but I don't think they're good for the long term. How about changing what you do in the evening? Go for a walk before dinner, eat, sit outside, read. Ensure that your sleep room is blacked out with no digital interference, not even standby lights. Also, try not to engage in digital activity before bed – no computer or TV for a good hour beforehand; read a book to spark your creative brain.

    Stick with it and adapt your lifestyle to the positive goals.

    Good luck … it will normalise.

  • xaevir

    @Paul Holliday: Thank you for the help and the positive support. I need it right now. Right now I am wired and tired. I cant take a nap but also cant do any real mental work. This feeling kinda carries into my sleep. Its like I am never really awake and never really asleep. No rhythm. I hope it will normalise as you said. I am a programmer and my work has been horrible.

  • … then I can whole-heartedly recommend getting outside and having a walk. Walk wherever you can – urban, rural, wherever, just get outside and walk. Try to find some views where you can take in horizons. It will clear your head, set you up for a productive evening and hopefully bring some relieving sleep.

    Have you looked at herbal drinks? Chamomile is very soothing. Also, consider lavender for sleep – for me, it knocks me straight out! Some dried lavender in a bag under your pillow, or extract oil with a few drops on the pillow and turned over.

  • xaevir:

    It sounds like your circadian clock is screwed up.  Melatonin isn't magic, but it can help reset your cycle.  Take it soon before bed.

    “Wired and tired” means cortisol may well be messing up your sleep patterns.  I don't recommend skipping breakfast in that case: make sure you eat plenty of protein and fat for breakfast, even if you're not very hungry.  And don't eat late at night: no “bedtime snack”.  In your case, it's far better to go to bed a bit hungry and eat a big breakfast.

    For me, exercise is the most important factor in restful sleep.  If I don't walk, or bike, or work out, or otherwise get myself tired, I sleep poorly.  As Paul said, go for walks — during the day if at all possible.  Lift some heavy things.  Slam out some pushups, or even better, some burpees…again, during the day.

    Light-blocking curtains are a sound investment unless you want to wake up at dawn.

    Hope this helps!


  • […] week when I drop all grains and the carbohydrates associated with them. Apparently there’s a “low carb flu” that occurs in some people when they wean themselves off grains and sugars even though the Paleo […]

  • Susan

    Hi J.S.,

    Forgive this untimely response, but please count me as one of the females who does well eating breakfast & dinner, no lunch. (In the category of “things you don’t understand yet”, which I would guess is a very small category, indeed!) I tried to modify my routine for a Whole30 per ISWF recommendations: Eating 3 meals, and I found I was hungry and, therefore, snacking, much more often. The heck with that! Thanks for all the science.

  • Susan:

    I think there's something to the old glucostatic hypothesis: low blood sugar really does make you hungry, and not all of us have perfect blood sugar control.  Sometimes eating more often simply gives us more opportunities to experience a bit of reactive hypoglycemia!


  • Heather

    Hi, JS … Thanks muchly for the über-informative and useful site. I was pleased to find this page in particular. I have T1 diabetes and find that my blood sugar control is *much* better on a low-carb diet. If there were no other considerations (other than BG, I mean), I would probably fare best in ongoing ketosis. But I’m also quite physically active (rowing 3x/week, strength training, bicycle commuting), which creates a bit of a dilemma.

    About one month ago, I shifted from low-moderate carb to VLC/HF, and while my blood sugars have been generally great, I can’t say I’m feeling great while working out. Not totally crappy, but not at all 100% — even when I’m not working out (bit of fatigue). I’m also not, according to my blood ketone meter, in full ketosis — I don’t know why, as I’m following the dietary recommendations of Phinney/Volek etc.

    Things I’m wondering right now are whether I should reintroduce some good starches and try in my imperfect way to cover them appropriately with insulin, or if I should try more aggressively to keto-adapt (which might just mean waiting longer?), with the goal of maintaining excellent blood sugars.

    I realize you can’t possibly play endocrinologist to all of your web fans, but any thoughts you might have would be most welcome.


  • Heather – from the sidelines, get some carbs in there! VLC is admirable, for folks who are fat, have metabolic derangement or brain issues, but fit and healthy people, especially active people can and should eat carbier foods.

    Feed your fun!

    While it is seriously good fun to climb a mountain fasted, hike 20 miles with nothing but a vial of body warmed coconut oil, active people need to feed themselves. Roots do this well. If your endurance is decreasing, eat more carbs; if you get fat, eat less. That's my yardstick.

  • Heather

    Paul – thanks very much for the feedback. If it weren’t for my T1 diabetes (ie “metabolic derangement”), the choice would be easy. But I also need to think about the long-term consequences of high blood sugars, which are pretty difficult to avoid when trying to match exogenous insulin to carbs.

  • Heather:

    Keto-adaptation takes weeks, and is definitely a long-term commitment.  During the transition you'll be low on energy.  Even after you're done, you'll have excellent long-term endurance performance, but sprinting and shorter-term explosive performance will be impaired.  But it certainly seems like it would make blood sugar control easier!

    Really it depends on how systematic you can be about your insulin dosage and response.  See Paul Jaminet's excellent article How To Minimize Hyperglycemic Toxicity for several ways to keep your blood sugar response slower (and therefore easier to control with exogenous insulin). 

    And if you decide to go ketogenic, he's also written two great articles on how to make diets ketogenic but still nutritious (Part I, Part II).

    Note: this isn't medical advice, just my own thoughts.


  • Okay … I don't have the experience or qualification to advise further. That deeper understanding of blood sugar and achieved health is something beyond my experience.

    I can help with a name, though – Steve Cooksey. Diabetic, ketogenic, paleo and winning!

  • Heather

    Thanks very much, JS and Paul, for the info and references, all of which I will check out.

    And, absolutely, I know this isn’t medical advice.

    My plan is to give keto-adaptation a few more weeks. If it’s still not coming together for me at that point, I’ll try some cautious re-introducing of yams and whatnot — probably just one meal to begin with and see if I can establish some reasonably reliable insulin:carb ratios.

    Interestingly, a slower blood sugar response isn’t necessarily easier to cover with exo. insulin (though I do understand the advantage of lower GI foods, especially for T2 diabetics). The insulin delivery needs to be paced accordingly — which can be done with a pump, but I’ve found in the past that a glass of juice can be easier to cover than mashed potatoes with butter/gravy. Argh.

    Thanks again!

  • […] the topic: http://lewrockwell.com/sisson/sisson191.html and a more in depth scientific one here: http://www.gnolls.org/1984/the-science-behind-the-low-carb-flu-and-how-to-regain-your-metabolic-flex… This is supposed to go away after a week or two once our bodies adjust to the dietary differences […]

  • neopaleoman

    Just dropping a comment that I really find the site valuable and I’ll be checking out your book. Thanks.

  • Heather:

    Any updates?


    Let us know what you think…drop a note in the Talk forum.


  • Katherine

    I always figured that being unable to sleep at night when you’re losing weight was the natural response of the body, telling you to get up, go out, and kill something.

    I don’t have that issue since I gave up gluten almost 10 years ago. I sleep much better.

    I remember low-carb flu symptoms. I haven’t experienced it in many years. Being gluten-free and not eating substitute baked goods means I’m always pretty low carb. The past week I’ve been on a VLC diet and will be for at least another week or two. No symptoms.

  • Katherine:

    I suspect it's like gaining muscle: it's difficult to gain muscle mass for the first time, but it's easy to gain muscle mass you've previously lost.


  • alan2102

    JS: “Are you familiar with any experiments in which (for instance) people’s blood levels of PCBs were measured during nutritional balance, and again during fasting (which should release them into the general circulation)?”

    Here are some clippings and references that might be of interest:

    Weight Science: Evaluating the Evidence for a Paradigm Shift
    Linda Bacon1* and Lucy Aphramor2,3
    “there is emerging evidence that persistent organic pollutants (POPs), which bioaccumulate in adipose tissue and are released during its breakdown, can increase risk of various chronic diseases including type 2 diabetes [113,114], cardiovascular disease [115] and rheumatoid arthritis [116]; two studies document that people who have lost weight have higher concentration of POPs in their blood [117,118]. One review of the diabetes literature indicates ‘that obese persons that (sic) do not have elevated POPs are not at elevated risk of diabetes, suggesting that the POPs rather than the obesity per se is responsible for the association’ [114].”
    113 Lee DH, Lee IK, Song K, Steffes M, Toscano W, Baker BA, Jacobs DR Jr: A strong dose-response relation between serum concentrations of persistent organic pollutants and diabetes: results from the National Health and Examination Survey 1999-2002. Diabetes Care 2006, 29:1638-1644. PubMed Abstract | Publisher Full Text OpenURL
    114 Carpenter DO: Environmental contaminants as risk factors for developing diabetes. Rev Environ Health 2008, 23:59-74. PubMed Abstract OpenURL
    115 Ha MH, Lee DH, Jacobs DR: Association between serum concentrations of persistent organic pollutants and self-reported cardiovascular disease prevalence: results from the National Health and Nutrition Examination Survey, 1999-2002. Environ Health Perspect 2007, 115:1204-1209. PubMed Abstract | Publisher Full Text | PubMed Central Full Text
    116 Lee DH, Steffes M, Jacobs DR: Positive associations of serum concentration of polychlorinated biphenyls or organochlorine pesticides with self-reported arthritis, especially rheumatoid type, in women. Environ Health Perspect 2007, 115:883-888. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL
    117 Chevrier J, Dewailly E, Ayotte P, Mauriege P, Despres JP, Tremblay A: Body weight loss increases plasma and adipose tissue concentrations of potentially toxic pollutants in obese individuals. Int J Obes Relat Metab Disord 2000, 24:1272-1278. PubMed Abstract | Publisher Full Text OpenURL
    118 Lim JS, Son HK, Park SK, Jacobs DR Jr, Lee DH: Inverse associations between long-term weight change and serum concentrations of persistent organic pollutants. Int J Obes (Lond) 2010, in press. PubMed Abstract | Publisher Full Text



    Environ Health Perspect. 2009 Apr;117(4):568-73. Prevalence of metabolic syndrome associated with body burden levels of dioxin and related compounds among Japan’s general population. PMID: 19440495 PMCID: PMC2679600 Free PMC Article


    Diabetes Care. 2007 Mar;30(3):622-8. Association between serum concentrations of persistent organic pollutants and insulin resistance among nondiabetic adults: results from the National Health and Nutrition Examination Survey 1999-2002. PMID: 17327331 Free Article


    Obes Surg. 2006 Sep;16(9):1145-54. Increased plasma levels of toxic pollutants accompanying weight loss induced by hypocaloric diet or by bariatric surgery. PMID: 16989697


    Metabolism. 2002 Apr;51(4):482-6. Increase in plasma pollutant levels in response to weight loss is associated with the reduction of fasting insulin levels in men but not in women. PMID: 11912558


    Am J Physiol Endocrinol Metab. 2002 Mar;282(3):E574-9. Weight loss-induced rise in plasma pollutant is associated with reduced skeletal muscle oxidative capacity. PMID: 11832359 Free Article


    Int J Obes Relat Metab Disord. 2001 Nov;25(11):1585-91. Increase in plasma pollutant levels in response to weight loss in humans is related to in vitro subcutaneous adipocyte basal lipolysis. PMID: 11753575 Free Article


  • alan2102

    JS: “Keto-adaptation takes weeks, and is definitely a long-term commitment. During the transition you’ll be low on energy.”

    Note the title of the second-to-last reference in my post above: “Weight loss-induced rise in plasma pollutant is associated with reduced skeletal muscle oxidative capacity.”

    Hmmm. Is it possible that some of the problems of “keto-adaptation”, such as fatigue, are due to large quantities of POPs (PCBs and other) being released from adipose tissue and poisoning skeletal muscle mitochondria, and/or liver mitochondria?

    One of the problems with ketogenic and related diets can be very low fiber intake. Fiber is one of the few things that can carry fat-soluble POPs out of the body. (One of the others is the synthetic fat, Olestra — which has been used successfully for that purpose.) If the newly-released POPs are not excreted, they will be re-absorbed and recycled (enterohepatically) for quite a while, until dribs and drabs of them finally find their way out. But at the same time, the high animal fat content of these diets makes it almost inevitable that more (fat-soluble) POPs will be ingested, without any good way to be excreted.

    Maybe I’m overstating all this. Maybe it is not much of a problem. Or, maybe it IS a serious problem, for some people. Maybe individual variations in hepatic detoxicative processes, fecal flora, current dietary exposures to POPs (i.e. just how POP-laden is the food you’re now eating?), extant (historical) body burdens of POPs, liver functional reserve (i.e. have you been a drinker? just how much have you abused your liver?), etc., all add up to serious problems for SOME people, and not so much for others. Maybe.

    I think that fiber supplements, or even bile acid binding resins (which are fairly good at promoting the excretion POP-type crap), might be important for some people undertaking these very-low-fiber diets, particularly when large amounts of bodyfat are being mobilized. Apart from helping to sweep out POPs, fiber also has more obvious advantages, anyway.

  • alan2102


    I forgot (until now, this morning) that activated charcoal also works for detox and is better for this purpose, gram for gram, than fiber or resins (though probably not Olestra). Charcoal is wonderful for pulling fat-soluble toxins out of the body. It actually pulls them out of the blood, believe it or not, using the intestinal wall as though a dializing membrane. This is why it is used in emergency medicine in some intoxications. It absorbs stuff sitting in the GI tract, but also lowers blood levels of stuff already absorbed (literally pulls it out of the blood; yes, I know, this is amazing, but true!). The only problem is that it is messy — a pain in the ass to use. You have to be very careful not to spill it on anything that you care about, because it stains like ink, and the fine powder seems to *want* to end up where it does not belong. But beyond that hurdle, it is great. Only other thing is that it does not provide roughage for promoting bowel function; only fiber can do that. Charcoal, as a bolus, is physically inert, like so much dirt. When I take it, I combine it with psyllium powder, to provide the bulk that the bowel needs to move things along. As you might imagine, it turns the stools black, and green (green probably from the bile, which it absorbs). Also stains the tongue black for an hour or so.

    An alternative is Pringles Light potato chips made with Olestra (or just pure Olestra, if you can find it). More detail: bing for “pringles olestra PCBs” etc.; e.g.:
    Eurekalert link
    Yo-yo diet redistributes toxins in body tissue; Olestra+caloric cut boosts toxic excretion

    Why bother with charcoal, vs. Olestra? No reason, except (maybe) that charcoal has about 200X more science behind it, and is known to absorb a very wide variety of toxins and carry them out of the body. As a general detoxificant, it has a lead that will probably never be surpassed. I think the reason that there were so many pop articles about Olestra as a detoxifier is that it was such a novel and funny idea. Story writers could not resist: “[haha] Guess what? Pringles[tm] chips are a health food! [haha, chortle]”. Charcoal is not nearly as sexy.

    I still think supplemental fiber would be a good idea, to help physically propel this crud OUT.

  • […] and then one day I woke up sick.  Apparently when you go cold turkey on bad carbs you can get carb-flu.  Well I had it.  The only solution after this happening every other day for a week was to add in […]

  • Isabela

    Just from my own experience: I read in the newest Atkins book that the headaches associated with low-carb flu are actually due to a lack of salt. Glycogen burning eliminates a lot of water, washing out a lot of salt and other minerals in the process. Seems that an extra 1/2 teaspoon of natural salt (sea, Himalayan etc) a day would alleviate symptoms.

    And, a few weeks ago I went VLC, and for the first time I had a bad headache associated with it. But after a whole day of suffering I remembered the salt trick – ate the salt sprinkled on some cucumber, and consequently drank a bottle of water. The headache disappeared in a few minutes.

  • Isabela:

    I've seen the same point made by Dr. Eades, so I'm sure it's worth a try.


  • Max

    Great read! I was wondering two things:
    First, does it necessarily always take a long time to transition into ketosis, or is that just coming from a high-carb background? From what I understand via Lyle McDonald, it can be achieved in a few days, with moderate aerobic exercise (to not deplete muscle glycogen) to speed the process.
    Second, any thoughts on the cyclic ketogenic diet? I’m going to check it out soon, just trying to make sure I’m low-carb adjusted first (obviously ‘eating like a predator’ throughout the entire process). These may be interesting if you haven’t read them yet (first one is where I’m basing this comment off of…)
    Cyclical Ketogenic Diet and Exercise Performance
    Guide to Ketosis

  • Max:

    Ketosis happens fairly quickly: even during the overnight fast, if you're eating relatively low-carb to start with.  

    Keto-adaptation, the process by which previously glucose-dependent tissues (e.g. the brain) adapt to running primarily on ketones, is the process that takes several weeks.

    CKDs are a well-established method of losing weight.  The theory is that periodic carb refeeds maintain your insulin sensitivity (so your muscles are more sensitive to the effects of insulin, which drives protein into them as well as glucose and fat) and some amount of muscle glycogen (so your athletic performance doesn't suffer too badly).

    Obviously you will not keto-adapt on a CKD, but that's not the point.


  • Kratoklastes

    Maybe I’m being a bit hyper-sensitive, but I threw up in my mouth a little bit when I read

    ‘“You can get plenty of energy from fat, but you have to go into ketosis to do it” are—like most nutritional claims made by veg*ans—complete bunk.’

    I’m a vegetarian (I eat some eggs and some good [or great] cheese like Reggiano, Strong Cheddar and Roquefort), and I have never ever heard that particular nonsense claim promulgated by any vegetarian source that I respect.

    I eat a high fat, high protein diet (extra protein from 93% whey protein isolate – however if budget were no constraint I would switch to SunWarrior raw-food vegan protein), and can maintain a 225lb frame with a RHR of 58, a VO2Max of 46 and “1.3x BW for 10 bench” strength (think “Mike Mahler, but prettier” – MM is the ‘Aggressive Strength’ vegan kettlebell guy)… and I just turned 48, yo.

    (Yes, 225 is too fat at 6’1″: I’ve re-started IF to get my lard ratio back down to where it was when I was a teen – LeanGains style [“train fasted, train hard, no mercy SIR”]).

    Anyhow… basic point: trotting out some vegan or vegetarian straw-man is really irritating to us non-religiotard type vegetarians – particularly those of us who are veggie for reasons other than health (our household is veggie for reasons of animal welfare – the health benefits are secondary – and we both have strong research backgrounds so we have refined our diet to be varied, healthy, nutritionally complete and delicious).

    I am always amused by folks who think they’re onto something good while getting their protein from some piece of rotting carcase at 25-35% protein [max] – i.e., lower than Parmesan or Reggiano – and BV of about 80 and PDCAAS of 92… compared to my 93% WPI with BV of 104 and PDCAAS of 100, bitchez!

    Not all vegetarians are weak and skinny – and The Lovely (also vegetarian) is slim but not weak.

    How many of y’all have been with the same Lovely for 20 years… and she’s still the same size now as when you met her? For the record: she’s 5’7″, 108lb, RHR 52 and Vo2Max 48… and a barrister (which has nothing to do with coffee: outside the US it’s a type of legal Jedi-Ninja).

  • Kratoklastes:

    “That I respect” is the key phrase here. 

    I've had (vegetarian) authors of published reference books tell me with a straight face “Nothing can be burned for energy without first being converted to carbs.”  Dr. Oz still repeats the “Meat rots in your colon” myth on Oprah's website, last I checked (which was, admittedly, a while ago).  Then there's “[Glucose can be stored, converted, or burned,] but WITH FAT however, only ONE outcome is possible – fat storage”…which I just responded to earlier today in another thread!

    It sounds like you have a more realistic understanding of nutrition.  

    However, I'm seeing some fanaticism creeping into your language, e.g. “some piece of rotting carcase at 25-35% protein”

    First, as I've pointed out before, all living foods start rotting once you kill them in order to transport them to where they'll be eaten.  Vegetables and fruits rot, too, but that's certainly not a reason to stop eating them!  In fact, I view it as a positive: anything that bacteria won't eat probably isn't food, and we shouldn't eat it either.

    Second, the 25-35% protein in meat is because of water content.  Water is found in real foods.  Add some water or milk to that whey protein in order to actually consume it (I know you're not eating it dry, as it turns to glue in the mouth), and suddenly it's much less “protein-dense” than the meat.  It's also much less nutrition-dense, as fatty meat contains a host of essential fat-soluble vitamins and cofactors not found in purified ingredients like whey protein.

    I'm not against vegetarianism per se: it's possible, though difficult, to eat a nourishing vegetarian diet.  (Veganism is a different matter: it's evolutionarily incompatible and inherently malnourishing.)  So I'm sorry you were offended…but I'm frequently stuck dealing with bunk nutritional claims, and that spills over into my articles sometimes.


  • E Craig

    Kratoklastes said:

    How many of y’all have been with the same Lovely for 20 years

    18 years.  He and I are very happy together=)

  • Thomas


    thanks for that still great note. I undersant the ketosis state I call now “the bear diet” (meat, fish and berries). But i am wondering about reducing carb for stillness of mood and energy level for an average active thinking and training guy.

    I am confused by the words : If we have excess glucose in our bloodstream, our muscles will burn it first, because it’s toxic. But eventually we run out of glucose, and that’s when our bodies need to switch over to beta-oxidation—burning fat. The ability to switch back and forth between the two processes is called “metabolic flexibility” in the scientific literature.
    Because the word “switch” make me thin that beta_ox of fat and glycolis won’t occur in the same time. this would induce that as your brain require glucose, if you couldn’t have beta_ox while glucose in blood from carbs then you have to use gluconeogenesis to feed your brain.

    I want to burn Fat from food as a first source of energy when possible and feed my brain from carbs from food not from gluconeogenesis. Indeed i guess that gluconeogenes is expensive in terme of energy to get glucose. you confirm it is possible?

  • Jen W



    As far as I understand it, your brain ONLY requires glucose IF it's used to burning carbs.  The brain's glucose requirement is ONLY conditional.  The brain can run on ketones just fine.



  • […] If you are interested in understanding more about why the carb flu happens, check out this article at Mark’s Daily Apple  and this very detailed explanation from J. Stanton at Gnolls.org. […]

  • […] If you are interested in understanding more about why the carb flu happens, check out this article at Mark’s Daily Apple  and this very detailed explanation from J. Stanton at Gnolls.org. […]

  • Thomas:

    An individual cell will generally be using either fat or glucose for energy, because oxidation of one inhibits oxidation of the other.  However, your body is comprised of trillions of individual cells, so it's quite possible for some of them to be running on fat while some run on glucose.

    For instance, some of your muscles might be running on glucose (from stored glycogen) because they're putting out max effort, whereas others are running on fat because they're not putting out max effort.  And your brain runs on glucose (including products of glucose metabolism, like lactate), or ketones, no matter what you do, because fats don't seem to cross the BBB.  

    Furthermore, your muscles don't have to be running on glucose to absorb it.  If they're low on glycogen and insulin is present, they'll suck up blood glucose so they can synthesize more glycogen and build up their reserves.


    For a metabolically flexible person, fat is your source of energy whenever you don't have high blood sugar (from having eaten a bunch of carbohydrate).  So it's probably better to eat your carbs once a day, as opposed to snacking on them throughout the day (which will also make you hungry)…and they seem to help people sleep better when they're eaten at dinner.  

    Your individual need for carb will vary depending on your activity level and metabolism…some people do fine with lots, some people find that anything over ketosis levels makes them gain weight.  So experiment and find out what works best for you.


  • […] with no grain and then on the third or fourth day I would have one grain (to keep from developing carb flu) that day and then back to no grains for a few days.   See, I’m not supposed to eat gluten […]

  • Thomas

    Thanks a lot for your answer that explains things well.

    As I won’t go to a kestose adapted body, i have to experiment around the carbs doses i request as you suggest.

    Once i found the good path i would come and report here!


  • M. Schmidt

    Very enlightening ! Weigth loss is my main goal for now and I am still researching this whole healthy lifestyle deal (having eaten practically exclusively trash since 2005 and paying for it now). This has put me substantially closer to favoring paleo over vegan or any other lifestyle. I am deliberately using the phrase “lifestyle” here, because I don’t want a fix or a short-term solution. I want to redefine my body and my self (however cliche that sounds ;]).

    I have only one question though : How exactly would I measure my current RER level ? Is there an easy way ? I couldn’t find anything on this topic here and google also didn’t prove to be of use.

  • M. Schmidt:

    You have the right mental model.  When evaluating possible life changes, it's best to ask “Is this something I can sustain, and enjoy, for the rest of my life?”  

    RER is not a test that any health practitioner can prescribe or perform, to my knowledge.  AFAIK, it's typically performed in sports physiology labs at universities.


  • Danny J Albers

    Very good read J. Thank you

    This is surprisingly the first time i have seen this article from you.. not sure how I missed it.

    At Primal North we have coined at term “Low Carb Limbo” which describes the person eating to few carbs to support their daily activities, but to many carbs and protein to obligate the neccessary blood ketone level that will have your muscles using ketones preferentially over glycogen.

    This misguided approach will eventually cause all sorts of issues and it is why a blood keto meter is not optional but essential for anyone practicing athletics along with a ketogenic diet. It is a perpetual “low carb flu” as your energy demands are not met with carbs, and not met but having high enough blood ketones.

    You are also about the only person I have seen outside of Phinney and Volek who correctly makes a distinction between fatty acid oxidation and keto adaptation. We continually see the term “fat adapted” and people assume that means they are keto adapted. Of course they are not and this assumption hurts them as they simply shut off all carbs, up fat, and continue to exercise without adapation period.

    I submit in my “Low Carb Limbo vs Keto Adaptation” article that all humans are already “fat adapted” and able to oxidize fat when carbs are unavailable. That what we mean by “Fat adapted” is that we have actually worked on the ability of that system to ramp up as needed to much greater amounts than the average high carb eating member of the sedentary general masses.

    Anyway your article has me rambling, it was a worthy read. Thanks.

  • Danny:

    “Low Carb Limbo” is exactly the state I'm talking about, and is absolutely to be avoided. As you mention, many people are not eating enough carbs to fuel a glycolytic activity state, but too many to ever keto-adapt.

    And I've also been puzzled by the ongoing confusion between “fat adapted” and ketosis: “fat adaptation” is metabolic flexibility, whereas ketosis is…ketosis.  Met flex is a simple concept and key to understanding metabolic dysfunction — but with the exception of Mike T. Nelson (who is also presenting at the 2013 AHS), no one else seems to be familiar with the concept, let alone applying it to the observed reality that low-carb dieters face, whether paleo or otherwise.

    Note to readers: here's the Primal North page on Low Carb Limbo, which is definitely worth reading if you are physically active and want to stay on the ketosis side of the fence.


  • […] interested in the science behind the carb flu, J Stanton does a great job of breaking it down here:http://www.gnolls.org/1984/ Last edited by Paleobird; Today at 09:04 AM. Well-behaved women rarely make history : Laurel […]

  • […] (This is a great post for any of you nerdy science types who like to dig deep and read all the “why’s”). […]

  • […] post titled, “The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility”, is full of science and reasoning. The science was well presented, or at least the author tried to […]

  • […] have gone through it give to the general feeling of blech that attends switching over from being a glucose burner to a fat burner. Until your body makes this shift, you won’t feel your best. This takes anywhere from two […]

  • Dana

    Glucose is an anaerobic fuel, not aerobic. Its use as a cellular fuel predates the existence of multicellular organisms. (Insulin, in fact, is one of the oldest hormones on the evolutionary timeline, and can be found even in simple worms.) There are two types of fuel-burning that can be done by cells in the absence of oxygen: anaerobic respiration, and fermentation. I’m not a cell expert but from what I can tell from a cursory search, eukaryotic cells (cells with nuclei) tend to use fermentation when burning glucose. Glycolysis can happen in the presence of oxygen, but never *uses* oxygen. Cancer cells tend to favor glycolysis, in fact, because they tend to be unusually sensitive to free radicals and oxidative stress, so they do what they can to minimize contact with oxygen.

    This is a really useful post for me, though. One more piece of my personal puzzle because there is a hell of a lot of type 2 on my mother’s side and I had trouble with obesity early in my adult life. It seemed to be in response to hormonal changes, but still, certain dietary choices seem to have a better track record with reducing it.

  • Dana: 

    Strictly speaking, glycolysis is the first step in both aerobic and anaerobic respiration for humans: it produces pyruvate.  

    The second step for aerobic respiration is converting the pyruvate into acetyl-CoA and feeding it into the TCA cycle (aka the citric acid cycle or the Krebs cycle, depending on how long ago you took your biology course).  This happens in the mitochondria.

    The second step for anaerobic respiration is fermentation, in which the pyruvate is converted to lactate.

    Yes, it's complicated!  I've added an (optional) explanation to the article, which might help.  And I'll be covering this subject in more detail in my 2013 AHS presentation.


  • Lake Ingle

    Very informative. I’m assuming you suggest eating the required carbs with complete fats and proteins because of fats sugar spike dampening properties?

  • […] thats a good article… And even more info on the keto to beta-oxidation front. The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility &amp… You see ketosis is an entity unto itself. Whereas beta-oxidation (the most efficient source for […]

  • […] flu: symptoms and relief – Low carb food list The Low Carb Flu | Mark's Daily Apple The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility &amp… "I puked like a hero for the rest of the night," Anthony Bourdain, 2002. (After […]

  • Lake Ingle:

    Yes, the fats help slow down the spike, but it's also important to eat carbs with protein.  Insulin drives all nutrients into cells, not just carbs, so you'll at least do some good while you're shutting off lipolysis.


  • […] going to rely predominately on beta-oxidation for fuel. Maybe this one would have been better: The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility &amp… Reply With […]

  • […] to the typical 60% calories from carbs Standard America Diet, I suspect you will find such a change more […]

  • The Pooch

    Hello! I enjoy your articles a great deal! A couple of questions about this one, maybe you could offer your opinions:

    Doesn't moderate-to-intense exercise always burn sugar, due to the requirements of moving fast-twitch muscle fibers quickly? Can this type of exercise also stimulate beta-oxidation of fats during or after exercise?

    Will intense exercise (e.g. HIIT, CrossFit, etc) impair fat metabolism in any way?


  • The Pooch:

    Yes, the more intense you exercise, the more glucose you will burn…and as you approach VO2Max, fat-burning actually shuts off.  Reason: though fat is more efficient and energy-dense, and is the preferred fuel of the body, glucose contains a lot of oxygen in the molecule itself (C6-H12-O6), so fat requires more oxygen intake to burn.

    However, this is a bit of a red herring, because we don't spend very much time close to VO2Max.  Most fat burning occurs at rest — and an intense sprint session elevates resting metabolism for hours due to the demands of recovery, so you'll burn plenty of fat that way too.

    There are a few things to keep in mind when doing intense exercise:

    1. You'll need more carbs in your diet than a sedentary couch potato trying to lose weight.  It's very difficult to do a Whole 30 and Crossfit at the same time: you'll basically need to force-feed yourself bananas and sweet potatoes.

    2. You don't need to kill yourself for half an hour or more to get the benefits of elevated metabolic rate: even 30 seconds of sprinting will push up HGH levels and RMR for a while, and 6-8 minutes of HIIT will make you want to puke if you're doing it right.  After a while you're just trashing your ANS.

    3. If you do decide to go all-out and do a WOD, it takes at least three days for your ANS to recover.  Jamie Scott's 2013 AHS presentation talks about this in detail.  Low to medium-intensity cardio is fine, but trying to go all-out every day, or even every other day, is usually a recipe for burnout and failure.


  • Steve

    Different people are different.

    A Type 2 Diabetic friend of mine actually handles a modest amount of carbohydrates better than I do!

    Just a few steamed mushrooms or a bit of spinach is enough to knock me out of ketosis! That’s only about 6 grams of carbohydrate.

    Considering that Type 2 Diabetes runs in my family, I need to be careful.

    So, I’m aiming for hard-core ketosis to the point where I’m obsessing over the carbs in liver and eggs…

    But fear not for exercise – although my body seems to view carbohydrates as poison, my athletic performance is quite good (relatively speaking) on lots of fat with no carbohydates – even during extended fasting.

    I’ll never be competing at the elite level of athletics, but I am quite happy with my athletic performance fueled almost entirely by fat. As a bonus, my endurance is much better this way.

    The notion that people NEED carbs to train is simply incorrect. Carb-tolerant people, go ahead and load up, but understand this does not work for everyone.

  • Steve:

    Future installments of this series will talk about metabolic individuality — a concept well-understood in older research, but which fell by the wayside in the rush to solve the obesity crisis via government fiat.


  • Steve

    Looking forward to it!

    Your block rocks, by the way. I’ve got “The Gnoll Credo” on order from Amazon and have been spending the past couple months poring through your blog as well as those of Peter Attia, Dr Eades, and Hyperlipid.

  • Steve

    blog, not block, lol… meh, I guess it works either way 🙂

  • Steve:

    Thank you!  I greatly appreciate the support.  Feel free to discuss the book in the Talk forums once you've read it.


  • Steve

    I wonder if a major factor regarding individuation is the capacity and tendency of a specific person’s liver to produce glucose from protein.

    The unwritten assumption seems to be that this is similar among all people, but I’ve yet to see any evidence at all to support that position. Yet, it seems to be regarded as axiomatic.

    Actually, it is now known that many type 2 diabetics tend to produce larger quantities of glucose from protein than normal people, but this is usually written off as some sort of defect, perhaps due to insulin resistance.

    But what if it isn’t simply a symptom of disease? What if this is just how that person’s body is actually INTENDED to work? What if there’s not actually anything wrong with that person’s liver, or at least perhaps the state of insulin resistance is exaggerated?

    This is important to consider because a major adaptation of carnivorous animals is the ability and tendency to process large quantities of protein into glucose, while at the same time being extremely intolerant of dietary carbohydrates.

    A specific human being who has the biology more on the carnivorous side would tend to be viewed as “defective” and be instructed to continue consuming carbohydrates while then taking insulin and metformin to control blood sugar.

    From just a brief google search, I found this:
    “Nonavian carnivores such as house cats (Kettelhut and Migliorini 1980), rainbow trout (Palmer and Ryman 1972), white sturgeon (Hung 1991) and American alligators (Coulson and Hernandez 1983) are also glucose intolerant as indicated by prolonged glucose tolerance curves relative to omnivorous species such as chickens, rats and humans. In alligators, glucose intolerance is especially pronounced, and several days are required for normal blood glucose levels to be obtained after an intravenous glucose challenge. Rainbow trout are glucose intolerant due in part to the lack of hepatic glucokinase (GK;3 hexokinase type IV, EC, which is responsible for phosphorylating glucose and facilitating its uptake into hepatocytes (Palmer and Ryman 1972). Cats also lack hepatic glucokinase and are poor at down-regulating amino acid catabolism and gluconeogenesis when fed low protein diets, indicating obligatory gluconeogenesis. For example, feeding cats a 17.5% protein diet resulted in little change in the activity of hepatic enzymes phosphoenolpyruvate carboxykinase (PEPCK; EC, alanine aminotransferase (ALT; EC, aspartate aminotransferase (AST: EC, and fructose-bisphosphatase (FBP: EC compared with cats fed a 70% protein diet (Rogers et al. 1977). Conversely, in the omnivorous rat (Eisenstein and Strack 1971, Kettelhut and Migliorini 1980, Peret et al. 1981) and Japanese quail (Featherston and Freedland 1973), PEPCK and amino acid transaminases are very adaptable to changes in dietary protein or glucose content.”

    This sounds similar to a person with Type 2 Diabetes!

    But it’s a discussion of actual carnivorous animals.

    But what if the person isn’t actually defective? What if the core assumption that all people are adapted to eating the same sort of omnivorous diet is incorrect?

    Actually, this sounds EXACTLY like my own personal experiences with extended water fasting:
    “Cats… are poor at down-regulating amino acid catabolism and gluconeogenesis when fed low protein diets, indicating obligatory gluconeogenesis.”

    I can water fast for several days, exercising every day, and if I ever don’t exercise, my blood sugar rises even though I’ve consumed absolutely no dietary energy in several days, and long since burned through all of the glycogen I started with.

    The only explanation I can think of is that my liver is continuing to produce significant quantities of glucose from muscle protein, at a rate which is perhaps “abnormally” high.

    The solution for me thus far appears to be to eat a high fat, moderate protein diet, and watch out for any dietary carbohydrates. Nevertheless, I have absolutely no sign of any sort of “glucose deficiency” and actually my blood sugar is still a bit higher than “normal” people even though I am consuming almost no carbohydrates and exercising daily.

    If I ever eat more than 150 grams of proten in a day, particularly if I don’t exercise, sure enough my blood sugar skyrockets.

    Perhaps my rate of gluconeogenesis will decline over time. But perhaps it won’t.

    Nevertheless, I find it interesting that while this topic is well-explored in other species, it hasn’t been in humans. Any deviations from what is assumed to be proper are simply written off as being defects, rather than individuation worthy of further research.

  • Steve:

    You're correct about the biology of carnivores.  Apparently gluconeogenesis isn't something that can be turned on and off quickly…it seems to be slowly upregulated and downregulated over time in both carnivores and omnivores.

    Based on my research so far, I think upregulated gluconeogenesis is more likely to be a consequence of impaired metabolic flexibility, rather than a cause.  However, yours is an intriguing hypothesis, and I'm open to evidence that it's the other way around.


  • Steve

    After reading “Perfect Health Diet” by Drs. Jaminet, today I experimented by eating ~100 grams of carbohydrates from white rice.

    My exercise performance cratered: both maximum and sustained power output on an elliptical machine declined by ~30%! No matter how hard I pushed myself, I simply could not exceed about 70% of my normal workout intensity on a HFVLC diet. To top it off, pushing myself was much more of a mental challenge and my form suffered as I tried to push harder, but couldn’t.

    This is quite different from what you talked about regarding metabolic inflexibility.

    My blood sugar has been elevated all day by about 20mg/dL and there are no more ketones in my urine, despite consuming my usual ~8tbsp of coconut oil.

    While I’m not certain of all the hormonal and biochemical changes which happened in my body today, my body is clearly telling me: “Don’t do that!”

  • Steve:

    If you're keto-adapted, that means you've likely been VLC for quite a while — and your tissues become insulin-resistant in order to save glucose for the tissues which absolutely require it (mostly your red blood cells and brain).  Petro at Hyperlipid named this state “physiological insulin resistance” to differentiate it from the pathological insulin resistance of T2D and the metabolic syndrome.

    Result: a single glucose bolus is likely to result in bad times, since you're insulin-resistant…and the results you saw aren't surprising.  To really test your met flex, you'd need to eat a ~15% carb diet for several days in a row in order to overcome the physiological insulin resistance.

    Whether you want to do that, of course, is your choice, because then you'll have to keto-adapt again going the other way (though it'll be much faster than the first time).

    I see this going the other direction, too: people try VLC for three days and go “OMG terrible brain fog and no energy, VLC sucks”.  It takes time to adapt either way, and the adaptation period is usually not very pleasant.


  • Steve

    Well, considering that blood sugar control issues got me started down this path in the first place, I think I’ll stick with a ketogenic diet for a while longer before I try experimenting with reintroducing carbs on a regular basis. To this point, being in a ketogenic state has done me nothing but good. However, life is change…

    Yeah, I know about the whole physiological insulin resistance thing from Hyperlipid.

    I was just surprised that my athletic performance went to crap so quickly after just one meal.

  • Steve:

    You have to train in the energy system you want to improve.  Just like squats exercise different muscles than pullups, aerobic performance while keto exercises a different energy system than HIIT sprinting. 

    Most people, of course, have the opposite problem than you: they're so used to training on a sugar high that they “bonk” the instant their glycogen runs low.


  • Steve

    FWIW, my exercise routine consists of several strength training exercises to failure, followed by HIIT on an elliptical machine. I do this as frequently as my body feels up to the task, about 3x per week.

    In addition to that, I use a Desk Cycle at low intensity for several hours per week while doing things like reading and commenting on blogs 🙂

    I’m not sure to what extent any of these things really stresses the “keto” energy system… that would seem to be endurance exercise at significant “aerobic” exertion levels.

    Come Spring, I will likely resume bicycling about 20 miles per day several days per week – that being perhaps more of a keto exercise than my current regimen.

  • Steve

    …and it was HIIT which took the hit from carb-refeeding… I’ve tried it a few times now with the same result.

    I can HIIT with higher power output while I’m in ketosis than after carb refeed.

    I speculate that’s probably due to insulin blocking fat metabolism while my skeletal muscles are simultaneously insulin resistant, leaving my muscles starved for energy.

    Additionally, my HIIT performance on a ketogenic diet is also higher than it was before I adopted a low-carb lifestyle.

    I believe this is probably due to my familial pre/type-2 diabetes. So, more than just a temporary thing which I could adapt out of in 3 days of carb-refeeding.

    I believe this is a longer-term situation which will probably only ever change when/if the underlying type-2 diabetic disorder is fully controlled. My fasting blood sugar, while now “normal”, is still higher than I would like.

  • Steve:

    Low-rep strength training takes place in the phosphocreatine energy system, so it won't be strongly affected by glycogen depletion.  The glycolytic (sugar-burning) pathway starts being stressed as the reps go up, anywhere from 5 to 10.

    HIIT is a combination of PCr (when you start out) and glycolytic (as you continue).  Usually HIIT/sprint/intense performance lasting longer than 8-10 seconds is what takes a hit when you're keto.  However, if you've got both a family and personal history of type II, it may be the case that your glucose metabolism has some intrinsic defects, probably mitochondrial.

    As you supposed, low-intensity long-term aerobic activity is definitely the aerobic fat-burning zone, and is what improves on keto.  That's why it's finding favor with some ultra-endurance athletes.


  • Rick

    I’m as confused as ever. Every time I add some carbs back into my diet like oatmeal, rice or a sweet potato, I get fat, and I am already at a 50 gms of carbs per day.

  • Rick:

    People are different, metabolically and hormonally. 

    For instance, an obese 55-year-old post-menopausal woman with a history of dieting and weight regain, and a family history of type 2 diabetes, is very different, metabolically, and hormonally, than a 20-year-old male competitive college swimmer sporting 6% bodyfat.  As such, their responses to the exact same diet will be very different!

    Yes, “eat more carbs” is the current fashionable solution to every problem…but some people have metabolic defects at the mitochondrial level that don't respond well to that no matter what they do, resulting in lack of met flex.

    In order to raise your basal rate of fat oxidation, try a combination of moderate-intensity aerobic exercise at least 3x/week (bonus: do this fasted) and heavy weight/strength work at least 1x/week.  Since exercise causes mitochondrial proliferation, this often helps…unless your base population is too trashed to even get that going: see, for instance, Hernandez-Alvarez 2010 and Ritov 2010.  However, it's worth a try, especially if you've hit a long-term plateau.


  • […] running my life. By far the biggest side effect during the Whole30 Challenge was the dreaded Carb Flu which only made me realize how bad my sugar and carb addiction was. After learning to recognize my […]

  • […] was running my life. By far the biggest side effect during the Whole30 Challenge was the dreaded Carb Flu which only made me realize how bad my sugar and carb addiction was. After learning to recognize my […]

  • tam

    Maybe this post should have a ‘metabolic flexibility’ tag.

  • tam:

    Added.  Also added links to my 2013 AHS presentation, which covers this subject in more detail.


  • […] Life Challenge, or I would definitely be struggling. There’s no way I would be able to handle “carb flu” symptoms along with the broken ankle. Things have a way of just working […]

  • […] tend to have when they first switch from a normal diet to a healthier low carb/low sugar one. This article tells us that low carb flu is generally temporary but that its effects vary from person to person. How badly you will […]

  • bea

    I know it was a while ago you asked for input from other women regarding meal timing but thought I’d comment anyway. I only eat 2 meals and tried to push it till later in the day but could not. My BG is always low normal in the mornings so I think I run on cortisol during the day if I don’t eat breakfast. I like to eat 60-70 % energy need for breakfast. With most of my carbs then since I’ve got the wiggle room in the BG#. Top myself off around 2:00pm and upper gut clear of food by bed. I don’t think women can handle the full belly load at bedtime. I just can’t comprehend 3 meals a day anymore. Too much food and too much time and work.

  • bea

    Steves ‘ post is very intriguing to me also about gluconeogenisis . Steve and I are both drawn to LC for weight and BG issues. All resolved for me. But I manifest completely the opposite problem. If I don’t eat a minimal amount of carbohydrates my BG just goes lower and lower and lower. Woke up one day in the 50s. I think my issue helps me have better BG control in the modern world but Steve could survive in the wild hunting and his liver could supply the glucose. I would die unless I could find some carbs. Thoughts? Could you get glucose needs from someones blood? Sorry Steve but in a matter of life and death:-)

  • Bea:

    Thank you for the input!  

    It seems like women, in general, can still eat two meals a day — but many don't seem to do well with the traditional breakfast-skipping 16/8 schedule, and prefer to eat breakfast (even if it's late) and dinner instead.

    And no, I don't know why you have trouble maintaining BG levels!  Given that you've got a problem especially in the AM, I'm wondering about hypocortisolism.  

    Alas, there's not a lot of glucose even in fresh blood: perhaps a couple teaspoons.  There will be some lactate, particularly if you've just run something down, but I don't believe it's significant.  Fresh liver has some glycogen in it, but fresh meat has very little: perhaps 1.1% by calories.

    Since liver has been promoted as a magic carb source, let's do some math.  A human liver weighs about 3.5 pounds (1500g) and contains perhaps 100-150g of glycogen.  So assuming Grok the Hunter can choke down 500g (over a pound) of raw liver after making his kill, he's getting perhaps 30-50g of glycogen from that.

    And supermarket beef liver has about 20g of glycogen per 500g (that's 11% carb), so Grok is getting maybe 10-30g extra carbs from “animal starch” if he's lucky enough to have made the kill that day or week.

    So yes, the dogma being promoted that “fresh meat has lots of carbs in it compared to supermarket meat” is simply wrong.


  • Neil Barstow

    Hi J.
    What a great resource. I like your style. Very through, yet easy to read. AND you include the all important references. I am grateful that you take the time.
    I’m an almost 60 year old recreational mountainbiker. I discovered low carb via Dr.Mercola’s resource and have been eating that way for about 3 weeks. I was very weak on the bike after a few days of my new regime. Good organic/grass fed meat, eggs and veggies, no grains or legumes.
    Few beers and glasses if wine (hmm, could that be my downfall).
    Anyhoo, thanks to your informative article above, in an attempt to gain metabolic flexbility, I started doing an hour of steady mountainbiking each day and I hope I am starting to feel a bit less weak. At first my legs felt like lead pretty quickly on even an easy hill.
    Yesterday was day 5 and I didnt have such painful weak legs, so I am hoping I am reaching the adaptation hump and soon might be over it. I’m eating plenty of varied veg, so I hope my minerals are ok too. I do take a multivit and liposomal vit C.
    My question?
    Do you have any thoughts about supplementing on exercise days with a carb drink? I like Torq Energy, it does apparently have a balance of glucose and fructose, but the makers assure me it’s not the HFCS fructose and should be used up during exercise ratger than stored sine Insulin production is halted during exercise (they say).
    Also I guess I’d need to load up a bit the night before, perhaps, maybe also in the morning. Maybe a proten and carb shake (MyProtein Hurricane Evo Velvet Vanilla) might fulfil that need.
    What I’d like to do is maintain (actually regain) some sprint capability and not risk damaging my metabolic adaptation by using these carbs to fuel during and before exercise.
    I do plan to read Phinneys book for low carb athletes as recolmmended, but I would certainly appreciate knowing anything you night have learned about this please?
    Thanks again for maintaining these pages.

  • Neil:

    I’m glad you find my articles useful!

    What you’ll soon find is that there’s often a difference between training and eating for maximum performance and actually achieving maximum performance on race day — particularly in regards to metabolic flexibility.

    In order to increase your metabolic flexibility and your ability to burn fat for energy — which will increase your baseline “go all day” pace, your maximum pace, and your ability to function while fasted and without a continual supply of food on the trail — fasted training is far superior, because that’s the energy system it stresses. In contrast, you’ll achieve maximum single-race performance when you’re carbed up and thereby replete with muscle glycogen (though it’s best not to eat right before the race…I believe 3-4 hours before should be your last meal).

    Similarly, when you’re trying to adapt yourself, it’s often best to eat low-carb with periodic refeeds so that you’re training the fat-burning energy system most of the time. Refeeds are best done right after exercise IMO because your muscles are primed to suck up the glucose and turn it straight into glycogen. Again, there’s a difference between training to improve fat-burning and met flex (low-carb) and training for maximum race day performance (however many carbs you need to keep glycogen topped up).

    Important: STOP TAKING VITAMIN C. It crushes your body’s natural adaptive response to exercise…this has been proven in multiple controlled studies. You won’t get stronger, faster, or better as long as you’re pounding Vitamin C…the difference is something like 25% vs. 170% improvement with training. Supplemental C has its place when dealing with sickness, but in general you should concentrate on getting it from food.

    I’m not a fan of “workout energy”except in dire circumstances, i.e. you’re bonking and totally crushed. Even then, little energy is necessary to get you going again (I just take some Halloween candy with me and munch some in the rare event, usually after 4+ hours). As your met flex improves you’ll find yourself needing less and less sugar during rides: I’ve hiked Mt. Whitney fasted!

    Let us know how things go for you!


  • Neil Barstow

    Hi J,
    Thanks for taking the time.
    Read your Whitney story, impressive sounds like a great day out. You mentioned that you’d have have eaten if you’d wanted to go faster? Carbs?
    I stopped the C, thanks, not read that before but, sure enough, there’s compelling evidence out there.
    Been biking regularly fasted and doing pretty good at a fairly easy pace. Coffee (with grassfed butter) helps as a start up booster for sure.
    Some seem to think one should avoid carb refeeds to stimulate growth hormone, but I do feel keen to down the (MyProtein Hurricane EVO, but shame it has oats in it) recovery drink with whey and some carb. I’ll try find something better when I’ve used it up.
    Did one quite hard ride the morning after a risotto dinner and with a pre ride coffee, felt as good as I ever have and really enjoyed it. Maybe I’m “back” ;-).
    Its a great resource, thanks
    I am enjoying reading The Gnoll Gredo too. Nice work.

  • Neil:

    If I wanted to go faster I’d just have brought some actual food!

    You bring up an interesting point about GH: ironically, the best way to get a big GH spike is to train and then not eat anything at all for several hours! Obviously this doesn’t stimulate muscle growth, but it dramatically stimulates autophagy — the process by which old, less-functional cellular machinery is broken down and recycled. This is basically a way to get most of the benefits of a longer fast without having to actually fast a long time…as such, it’s not something you do every session, but I believe it’s good to do periodically (e.g. once every week or two).

    Caffeine boosts the release of fat from fat cells, which is why it helps with fasted exercise…thus the classic coffee+butter (or, even better, coffee+MCT oil) pick-up. And it keeps you in the fasted training fat-burning mode, though you might try omitting the butter/MCT as things improve for you.

    Anyway, it sounds like you were somewhat glycogen-depleted from low-carbing. The same thing happened to me: I felt great and had great endurance, but felt “flat” and low on reserves for the extra push, even after several months! The combination of fasted training and being otherwise glycogen-replete should get you going…and when you do decide to go for a PR fully carbed up, you’ll probably find yourself doing better than before.

    Thanks for following up, and I wish you the best from one mountain biker to another.


  • Steve

    Update 6 months later – great progress after trying a few different things.

    The secret for me was an extended protein-sparing modified fast, and overall focus on very high protein intake. My fasting blood glucose is now 85mg/dL and I’ve lost a lot of body fat.

    For me, and I suspect other people in the stubborn Type 2 diabetes spectrum, the answer may well be both low-carb AND low-fat simultaneously. In other words, get the vast majority of calories from protein.

    For the past few months, I’ve been almost-exclusively eating skinless chicken breast and spinach with a bit of salsa and sea salt for flavor and egg shell for calcium.

    Depending on the reference, this is about:
    65% protein
    15% carb
    20% fat

    I also eat a slice of calf liver once or twice per week, and add protein powder or nonfat plain greek yogurt as “snacks” between meals. These would skew the actual protein % even higher.

    Copious amounts of coffee, but hey, that’s practically zero calories… :-)

    I’ve been mostly focusing on weight lifting, and that’s helped me get stronger and leaner. “Aerobic” exercises didn’t actually seem to have much of an effect for me besides increasing appetite. HIIT primarily seemed to adversely impact my strength and recovery from weight lifting, so I stopped doing that.

    I doubt every one would get optimal results from a lifestyle like this, but it could turn out to be just the thing for people with stubborn type-2 (and “pre”) diabetes who have not had success with other approaches.

    I’m still working on weight loss and am curious to discover whether I get more typical results once my body fat is in the 10% range vs. my current 20% or so, or if my body indeed just insatiably craves protein.

  • Steve

    To Bea: LOL!!! :-)

    FWIW, I also tend towards high hemoglobin and donate blood regularly to keep it in check… So…

    But, yeah, my primary source of carbs is gluconeogenesis and my liver doesn’t appear to have much trouble keeping up. I did get some “low-carb flu” at first, but I’m pretty well adapted now even though I’m not apparently in ketosis either.

    Although I can fast for days without ill effects, I do try to eat protein about 5-6 times per day to make sure that I’m not breaking down unnecessary amounts of lean tissue to create glucose.

    It’s nice to see that as I lose body fat I actually have a surprisingly good amount of muscle underneath and have recently been getting compliments from women.

    If only I’d figured this lifestyle out 20 years ago, lol! :-)

  • (I’ve been busy with my AHS 2014 presentation…now that AHS is over, I’m going back to many of the comments I didn’t previously have time for.)


    Congratulations on your success!

    HPLFLC is, by necessity, low-calorie — and if you’re in the process of losing a substantial amount of weight you will almost by definition be insulin-sensitive. Remember that protein requirements don’t decrease just because you’re in energy deficit, so as “calories” decrease, protein should make up a larger percentage of total “calories” even though the absolute amount of protein isn’t changing.

    Result: energy-restriction diets need to be “high-protein”.

    The other thing is that energy restriction tends to be stressful, so combining it with high-intensity cardio isn’t necessarily the best idea — as you’ve found out. Note that you’re doing exactly what bodybuilders do when they’re cutting fat for contest prep: high-protein, low everything else, keep moving the weight.

    So yes, if your goal is to lose weight and maintain muscle mass, you’re doing it right! Not everyone copes well with that degree of energy restriction, but if you do, it’s quick and effective.


  • Traceyb

    Was just reviewing this thread as I’m trying to find the balance for my carb intake now that I’m a lot more active than when I first went paleo (fixed my food and now I have the energy to do more than lay on the couch, go figure). A year ago I was doing 50-70 grams carb a day, but if I do that now ( I bike, weight train, amd I’m learning to play ice hockey) I’m DYING. So it’s very activity-specific for me.

    Like some of the other females in this thread, I tried IF by not eating breakfast for a while and it made me go off on deranged doughnut binges after I worked out (mmmm…Krispy Kreme….) but if I do big breakfast and dinner I frequently “forget” to eat lunch… Just eat dinner a tad earlier. No mad cravings post-workout, either. So there’s one more for your data set.

  • Traceyb:

    Yes, 50-70 carbs/day plus a lot of physical activity puts you right in the middle of “almost-ketosis”, which is a bad place to live. You get the pain of low muscle glycogen without the benefits of actually becoming keto-adapted.

    Fortunately, you’ve probably also discovered that having gone paleo and (at least relatively) LCHF for some time, and having begun exercising, you’ve also regained some metabolic flexibility — meaning that you can tolerate more carbs now than you could before! At this point you’ll have to experiment and find your own happy medium: increase carbs if performance suffers, decrease carbs if you start gaining fat or feel the blood sugar rollercoaster cranking up.

    Thank you for sharing your experiences!


  • […] The Science Behind The “Low Carb Flu”, and How To Regain Your Metabolic Flexibility Reply With Quote […]

  • […] body to deal with, and it voices its displeasure by making you feel like death. Read more about it here if you’re interested in the science behind the keto […]

  • […] who go straight from a Standard American Diet (SAD) to the AIP experience something called “The Low Carb Flu.” The SAD diet includes a lot of sugar and foods that convert to sugar (grains), and your body is […]

  • […] who go straight from a Standard American Diet (SAD) to the AIP experience something called “The Low Carb Flu.” The SAD diet includes a lot of sugar and foods that convert to sugar (grains), and your […]

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