This is a bonus article: I usually update on Tuesdays. I’ve got something astoundingly excellent lined up for this next Tuesday, so subscribe to my RSS feed to remind yourself!
Tireless media coverage, and government-mandated nutritional labeling, has convinced everyone that trans fats are bad for us.
Fortunately, unlike most dietary scares of the past 50 years, the government and the ADA appear to have got this one mostly right: trans fats are indeed toxic. According to this data (yes, it’s a prospective study and therefore contaminated by associational confounders), consuming just 2% of your calories from trans fat doubles your risk of heart disease! They’re also associated with obesity, Alzheimer’s, and infertility in women, and they may interfere with liver function.
What’s a Trans Fat? (You can skip this if you’re not interested in chemistry)
“Hydrogenation” means that a hydrogen atom is forced into the space where a double bond once was, making it into a single bond. For instance, hydrogenating a monounsaturated fat makes a saturated fat.
Dietary fats are either saturated, monounsaturated, or polyunsaturated. “Saturated” means that there are no double bonds in the molecular structure, and no hydrogens can be added. “Monounsaturated” means one double bond, and “polyunsaturated” means…well, more than one.
The interesting part is that double bonds can be cis- or trans-…basically the chemical equivalent of right- or left-handed. It turns out that fats created (or hydrogenated) by enzymes, in mammal bodies, are all cis-handed. But chemical hydrogenation creates a mixture of cis- and trans- fats that actually favors the unnatural trans- configuration 2:1. And the resulting trans- molecules have a dramatically different shape!
This is why trans fats wreak havoc in your body: they’re the wrong shape, and your body simply doesn’t know what to do with them. It’s like putting brake fluid in your engine oil, or antifreeze in your gasoline.
(The alert observer will note that it is impossible to create a saturated trans fat.)
Hidden Trans Fats: They’re Everywhere
So you think “I’ll only buy products with 0g of trans fat on the nutrition label. Then I’ll be safe.” Right?
Trans fat hides in plain sight.
Trans Fat: Hiding on the Nutrition Label
Here’s the first place it hides: on the nutrition label. If a ‘serving’ of food has 0.5 grams or less of trans fat, the label can say “0 grams”. But how many ‘servings’ are you eating? If a ‘serving’ is 50 calories, you can easily eat eight servings at a sitting—or four grams of trans fat!
How can you tell? Look on the ingredient list. If you see the words “hydrogenated” or “vegetable shortening”, you can guarantee the presence of trans fats—no matter what the nutrition label says.
Trans Fat: Hiding in ‘Heart-Healthy’ Seed Oils
Well, aside from the fact that seed oils contain mostly pro-inflammatory n-6 (“omega-6″) polyunsaturated fats, both n-6 and n-3 polyunsaturated fats are less chemically stable than saturated fats. It turns out that the process of extracting and deodorizing them (which requires both hexane, a poisonous industrial solvent, and high heat) turns some quantity of them into…trans fats!
SEAN. O’KEEFE, SARA. GASKINS-WRIGHT, VIRGINIA. WILEY, I-CHEN. CHEN. LEVELS OF TRANS GEOMETRICAL ISOMERS OF ESSENTIAL FATTY ACIDS IN SOME UNHYDROGENATED U. S. VEGETABLE OILS. J Food Lipids Vol 1 #3 pp.165-176 Sept. 1994
Concentrations of trans isomers of 18:2w6 and 18:3w3 were measured in soybean and canola oils purchased in the U. S. […] The degree of isomerizations of 18:2w6 and 18:3w3 ranged from 0.3% to 3.3% and 6.6% to 37.1%, respectively. The trans contents were between 0.56% and 4.2% of the total fatty acids.
Yes, that’s the ‘heart-healthy’ canola oil that they put in everything nowadays because it has ALA in it (the least useful omega-3). Yet the average canola oil contains over 2% trans fat! (Remember: 2% of calories = doubling of heart disease risk.) And if extraction under carefully-controlled conditions creates that much trans fat, how much more does the uncontrolled heat of cooking and frying create?
(We don’t know—but we do know that n-3 fats are less chemically stable than n-6 fats, and generally get hydrogenated first. So all those “Omega-3 Enriched!” oils become “Trans-Fat Enriched!” when you cook with them. For evidence of this, we move to the next section…)
Trans Fats: Hiding In The Deep Fryer
The third place trans fats hide is in the deep-fryer. Canola oil is the most common frying oil, because everyone knows canola is ‘heart-healthy’…right?
Unfortunately, since polyunsaturated oils are unstable under the continuous heat of the deep fryer, canola oil is hydrogenated so it’ll last longer. (This is why we fried everything in saturated fats, like beef tallow, before the now-discredited “Lipid Hypothesis” took over American nutrition theory: saturated fats can’t hydrogenate by definition, and they don’t degrade nearly as much or as quickly under heat.)
Industrial canola oil for deep-fat frying contains 27% trans fat. (Source.)
Still want that order of French fries?
Live in freedom, live in beauty.
(Did you enjoy this article? Share it with your friends! For more articles about fat, try “Why Humans Crave Fat” or “Fat and Glycemic Index: The Myth of ‘Complex Carbohydrates’“.)
Postscript: What Should I Use Instead?
Use saturated fats, which don’t hydrogenate—and monounsaturated fats, which hydrogenate to saturated fats, not trans fats. Butter, beef tallow, and coconut oil each contain only a tiny fraction of polyunsaturated fats…and if you buy grass-fed beef or butter, that fraction contains more healthy n-3 fats and less unhealthy n-6 fats.
And since a surprising number of people still believe that saturated fat is bad for you: no, it isn’t.
Patty W Siri-Tarino, Qi Sun, Frank B Hu, and Ronald M Krauss. Meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease. Am J Clin Nutr Jan 2010
“A meta-analysis of prospective epidemiologic studies showed that there is no significant evidence for concluding that dietary saturated fat is associated with an increased risk of CHD or CVD.”
And here’s the layman’s version, from Scientific American: “Carbs Against Cardio: More Evidence that Refined Carbohydrates, not Fats, Threaten the Heart“, Scientific American, May 2010