Trans-fats have been in the news sporadically in recent years, thanks largely to the bans passed by the New York City Health Department and the Indiana State Fair. Even consumers who don’t read the news have undoubtedly become familiar with the term because of food manufacturers labeling their products “0 Trans Fats!” (often with a small-print “per serving” which usually means there are trans fats in the product, just less than .5 g per serving) or “Trans-fat free!” whether or not they ever contained trans-fats in the first place.

But there still seems to be considerable confusion about what trans-fats are and why they might be bad for your health, which has likely been complicated by the long, stupid demonization of fats qua fats and saturated fats as a supposed cause of high cholesterol and heart disease.

Trans-fats are trans isomers of fatty acids, and although they occur naturally in small amounts in meat and dairy products from ruminants like cows and goats (in the form of vaccenic acid), the primary source of trans-fats in most Americans’ diets is hydrogenated vegetable oils. Most vegetable oils are composed primarily of unsaturated fats, which tend to be liquid at room temperature. In the early 20th Century, when the U.S. started importing soybeans as a source of cheap protein, soybean oil became readily available as a byproduct and was far cheaper than butter or lard. However, liquid and unsaturated fats get rancid much more quickly than solid fats, have a lower smoke point, and were unsuited to many American culinary traditions—biscuits and pastry crusts or all “short breads”* absolutely depend on solid fats to create their flaky texture, as explained in the note at the end.

Hydrogenation, a process first developed by French and German chemists around the turn of the century,  provided the solution: heating the liquid, unsaturated fats in the presence of hydrogen turned them into solids at room-temperature. Apparently, cottonseed oil was also far cheaper than the beef tallow used in candles, so that one of the first uses of hydrogenated oils. It took a little marketing work to convince people it was also good eating--the major campaign for years was “Use Crisco, it’s digestible!” (okay, actually that probably just reflected the central nutritional concerns of the early 20th C: indigestion and dispepsia, see Hillel Schwartz’s Never Statisfied.

However, the process of hydrogenation also creates trans fats, and a different kind from the ones present in beef and dairy products. Unlike saturated fats, trans-fats produced through hydrogenation have been repeatedly correlated with coronary heart disease, including fatal heart attacks, in large, long-term epidemiological studies, including the Framingham Study. A review article on the available research on the relationship between dietary fat and coronary heart disease (CHD) published this past September concluded:

According to the classic ‘diet-heart’ hypothesis, high intake of SFAs [saturated fatty acids] and cholesterol and low intake of PUFAs [poly-unsaturated fatty acids] increase serum cholesterol levels and risk of CHD. However, few within-population studies have been able to demonstrate consistent associations with any specific dietary lipids, with the exception of trans fats and n–3 fatty acids.

In other words, everything you've heard in the last decade about trans fats (bad) and omega-3s (good) actually seems to be supported by the available research, unlike everything you've been told for the last five decades about saturated fats.

However, many of the claims about the threat posed by trans-fats allege that trans-fats raise LDL levels. And it's not at all clear to me that anyone should be concerned about the fact that trans fats might be associated with increased cholesterol, even "bad" cholesterol, for reasons I discuss in the second entry on saturated fat. The only thing makes me think trans fats might actually be bad for people's health is the consistent, strong association between trans fat consumption and increased risk of CVD and myocardial infarction. I know correlation =/= causation, and I haven’t found any good evidence about a proposed mechanism. On the basis of the current evidence, it seems like there’s a difference between the naturally-occurring trans-fats and the ones produced by hydrogenation:

The association was only seen for for trans fatty isomers from hydrogenated vegetable oils. The mainly different trans isomers from ruminant fats did not show such an association. A case-control study in 239 people suffering an acute myocardial infarction found that after adjustment for age, sex and energy intake, intake of trans fatty acids was directly related to risk of myocardial infarction [241]. Those with the highest intake of trans fatty acids had twice the risk of myocardial infarction as those with the lowest intakes after adjusting for other cardiovascular risk factors. As with the Nurses Health Study, the association was only seen for trans isomers from partially hydrogenated vegetable oils. (from a 1995 lit review in the British Food Journal)

Is that because the hydrogenated oils contain linoleic acid, and meat and milk with naturally-occurring trans-fats don’t? I don’t know. I think the most interesting finding from Harvard's Nurse's Health Study, which is one of the studies that did show a weak but significant correlation between saturated fat consumption and CVD, is that the consumption of trans fats was associated with a much higher incidence of CVD than saturated fats. That finding, which is nearly two decades old, should probably be pretty infuriating to anyone who’s eaten hydrogenated-vegetable-oil margarine or shortening in the last two decades because it was supposedly a “healthier” alternative to butter or lard. Similar findings in several other studies prompted the more recent review article to conclude:

The observational evidence that TFA are independently associated with increased risk of CHD events is convincing, though based on a more limited body of evidence.

However:

There is probably no direct relation between total fat intake and risk of CHD.

So the persistent recommendation from public health and nutrition authorities to reduce total fat consumption? Not supported by the available evidence. But the reason this isn’t a  "things that won't kill you" entry is because, well, trans-fats might.

What it means for how I eat

The short version: lard before margarine or shortening, except when the lard is shelf-stable or the margarine/shortening is produced using fractionation and palm oil. Read more