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Trans fats are fats whose fatty acid molecules contain one or more trans double bonds between neighboring carbon atoms. They exist in small quantities naturally in some foodstuffs (notably meats and dairy products), but are created in greater quantities through the partial hydrogenation of food oils. Starting in the 1990s, trans fats gained an overly bad reputation for causing all manner of ill health effects, both real and imagined. Nevertheless, Harvard Medical School lists trans fat as a "bad" fat to avoid on their website (the "good" ones include monounsaturated—think olive oil — while saturated fats — beef fat, lard, and other tasty substances—are in the middle).
Partially hydrogenated oils
Normal food oils like soybean oil consist primarily of cis-unsaturated fatty acids. These fatty acids contain either a single cis double bond between neighboring carbon atoms (in which case they're called "monounsaturated" fatty acids) or multiple cis double bonds (in which case they're "polyunsaturated" fatty acids). Most of the carbon atoms in a fatty acid chain are single-bonded together, and have two hydrogen atoms bonded to each of them; but the carbon atoms on either side of a double-bond are missing a hydrogen atom and thus only have one each.
The hydrogenation process for food was invented in the 1920s. As its name implies, hydrogenation involves adding additional hydrogen atoms to the unsaturated fatty acid molecules. Basically, the oil is heated under pressure and hydrogen gas is forced through it. In full hydrogenation, all double-bonds are converted to single bonds, resulting in each carbon atom ending up with two hydrogen atoms bonded to it -- in other words, all the unsaturated fats are converted to saturated fats. In partial hydrogenation, some double-bonds are converted into single bonds and some aren't. As a side effect of the partial hydrogenation process, some of the cis double-bonds are converted into trans double-bonds, resulting in trans fatty acids.
The names cis and trans are strict organic chemistry terminology. Since each carbon atom on either side of a double bond has one hydrogen atom bonded to it, the two carbon atoms can be aligned either with the two hydrogen atoms next to one another (cis means "next to"), or with the two hydrogen atoms on opposite sides from one another (trans means "across from"). A cis double bond results in a bend or kink in the carbon chain; a trans double bond does not.
Under American food labeling laws, either fully hydrogenated oil or partially hydrogenated oil can be listed on a product's ingredients list as simply "hydrogenated oil." There's no way to tell from the ingredients label whether the "hydrogenated oil" is partially hydrogenated or fully hydrogenated, unless the manufacturer is nice enough to put a "fully" or "partially" in front of it.
If heating, pressurizing, and bubbling hydrogen through an oil creates trans fat, what about heating alone? Are you creating trans fats whenever you fry with plain old soybean or canola oil?
Yes, but only in very tiny quantities. If you heat an oil to 245°C (470°F), and keep it there for 16 hours, about 1% of the polyunsaturated fatty acids in it will be converted into trans fatty acids. Even used restaurant frying oil, which will have been heated near its smoke point off-and-on for several days before it's thrown out, will not have a significant amount of trans fat in it. (Assuming the restaurant starts with liquid oil and not partially hydrogenated oil, of course.)
Unfortunately, a rumor got started some time ago that heating canola oil creates oodles of trans fat, and like most rumors that are embraced by the woo crowd, it has yet to die.
Partially hydrogenated oils were in use for decades as a cheap substitute for lard (pig fat) or tallow (beef or mutton fat). Most products marketed as "vegetable shortening", including Crisco®, consisted of partially hydrogenated soybean and/or cottonseed oil. Until the 1990s, not much attention was paid to the existence of the trans fats in such oils, and the fact that their saturated fat content was actually lower than that of lard or tallow implied they were less detrimental to cardiovascular health.
In the 1990s, however, some results from the Nurses' Health Study brought trans fats into the public light. Subjects who'd consumed trans fats had a much higher incidence of coronary heart disease than subjects who'd consumed an equal amount of cis unsaturated fats. As a result of this, combined with a sense that trans fats were "unnatural" because partially hydrogenated oils were "processed", a sort of public health scare went up. Trans fats became the food villain du jour. One attention-seeking man even went so far as to attempt to sue Nabisco because he "didn't know that Oreos had trans fat."
As of 2006, the FDA requires the trans fat content of all foods to be listed on their Nutrition Facts label alongside their Total Fat and Saturated Fat counts; however, as much as 0.49 grams of trans fat per serving can still be rounded down to 0g trans fat on the label. As a result, many processed foods have been reformulated to eliminate ingredients that contain trans fats, or at least to reduce their trans fat content below 0.5 grams per serving. Modern Oreos, for example, are made with palm oil instead of partially hydrogenated soybean oil.
Current evidence suggests that trans fats — or at least, the elaidic acids produced by partial hydrogenation, which are the trans fatty acids that have been studied the most — significantly increase the risk of coronary heart disease, measured at about two-and-a-half times the heart disease risk of saturated fats.
Naturally-occurring trans fats
Trans fat is not just synthetic. It is a naturally occurring substance in ruminant animals, like cows, sheep, and goats, and in the milk produced by these ruminants. One tablespoon of butter, for example, contains 0.4 grams of trans fat. Since it is the synthetically-created trans fats in partially hydrogenated oils that have been the target of study for their health effects, little data exists on the health impact of the naturally-occurring trans fats in meat and dairy products. These naturally-occurring trans fats are chemically distinct from the trans fats produced by partial hydrogenation. There have been studies which seem to indicate that "natural" trans fats provide health benefits, and other studies which found no difference in cholesterol impact between natural and artificial trans fats.
Of particular note is the fact that conjugated linoleic acid, one of the essential fatty acids, is itself a naturally-occurring trans fatty acid. Conjugated fatty acids in food are, by law, not included in the "Trans Fat" value on the Nutrition Facts label in the US.
"Good" cholesterol and "bad" cholesterol
Consuming saturated fat tends to raise both one's level of LDL ("bad") cholesterol and one's level of HDL ("good") cholesterol. By contrast, current studies seem to indicate that consuming trans fat tends to raise one's level of LDL cholesterol but lower one's level of HDL cholesterol.
This number-free fact has been touted as damning proof that trans fat is worse for you than saturated fat. However, it must be taken into proper perspective. The amount by which one's HDL or LDL cholesterol levels are altered is just as important as the direction in which they were altered.
Let's say, for example -- using fictitious numbers -- that 1 gram of saturated fat increases your LDL cholesterol by 15 mg/dL and increases your HDL cholesterol by 1 mg/dL. And let's say that 1 gram of trans fat increases your LDL cholesterol by 1 mg/dL and decreases your HDL cholesterol by 1 mg/dL. In the saturated fat case, the spread between your LDL and HDL cholesterol has shot up by 14 mg/dL, while in the trans fat case, the spread between your LDL and HDL cholesterol has only increased by 2 mg/dL. (In the trans fat case, as well, your total cholesterol is 16 mg/dL lower than in the saturated fat case.) It is the difference between your LDL and HDL cholesterol levels which is significant, and if these numbers were true (which they're not, but if they were), trans fat would be less detrimental to your overall cholesterol picture than an equal amount of saturated fat.
Unfortunately, when real numbers are available, the situation darkens for trans fats considerably. The net increase in LDL-to-HDL ratio with trans fat is approximately double that due to saturated fat. This seems to be related to its effects on cholesterylester transfer protein (CETP).
Who's for it, who's against it?
There isn't a public issue woo meisters don't have an opinion on one way or the other. However, in the case of trans fat, there is disagreement over what the public policy should be even between different science-based organizations.
The American Council on Science and Health, which appears to have a libertarian bias, believes most of the public fears about trans fat are groundless, stating on its website: "The problem here is that today’s chronic diseases are highly associated with personal lifestyle factors — cigarette-smoking, obesity, poor diets, alcohol abuse, and failure to use life-saving technologies like seatbelts and bike helmets. Interventions by the Health Department run head-on into personal decisions and lifestyle choices."
The FDA chimes in
On 7-November-2013, the U.S. FDA issued a Federal Register notice with a preliminary determination that partially hydrogenated oils are no longer Generally Recognized As Safe (GRAS). ("Gras" is also French for "fat". Insert rimshot sound effect here.)
In response to this announcement, Stephen Colbert commented:
“”Yes, those who eat trans fats are making an informed choice. Just ask the average Joe filling his cart with frozen cinnamon buns. He'll tell you he had a hankerin' for the process of adding a hydrogen atom to a fatty acid, thus reducing the number of double bonds to create a malleable mixture of fats that stay solid at room temperature and cannot be metabolized, causing it to accrete in the tissues. At least, that's what I think he was saying; his mouth was full of unthawed bun.
|—The Colbert Report, 3-Dec-2013|
(It should be noted that the notion that trans fats aren't metabolized appears to be a nature woo scare story. There's no evidence for it, and there's evidence that at least some trans fats are metabolized in a manner similar to saturated fats.)
On 16-June-2015, the FDA's decision became final. Artificial trans fats are no longer be allowed in any food sold in the U.S., unless the food manufacturer gets a case-by-case waiver where they can prove that their use isn't harmful. They had until 18-June-2018 to comply.
At least one professor at the Harvard School of Public Health liked the idea. Professor Walter Willett, chair of the Department of Nutrition, says the Centers for Disease Control estimated trans fats are responsible for 7,000 premature deaths annually. (They actually said 3,000-7,000).
- What's the truth on trans fat? on The Straight Dope (from 2006)
- The truth about fats: the good, the bad, and the in-between, Harvard Health Publishing.
- Heat-Induced Cis/Trans Isomerization in Vegetable Oils and Oleic Acid, Incite, 30-Jan-2012
- Wolff, R. L. 1993. Heat-induced geometric isomerization of alpha-linolenic acid: effect of temperature and heating time on the appearance of individual isomers, Journal of the American Oil Chemists Society, vol. 70 no. 4, pp. 425-430.
- Przybylsk et al., Formation of trans fats during food preparation, Canadian Journal of Dietetic Practice and Research, 2012 Summer, vol. 73 no. 2, pp. 98-101
- See e.g. this paleo diet webpage: Canola Oil: The blob that ate butter, olive oil, coconut oil and peanut oil threatens American cuisine
- Crisco® has changed its formulation; it now consists of liquid soybean oil and fully hydrogenated palm oil, blended with a smaller amount of partially hydrogenated vegetable oils. This puts it under the magical 0.5-grams-of-Trans-fat-per-serving mark, thereby allowing them to claim "trans fat free!".
- Nurses' Health Study
- Willett et al., Intake of trans fatty acids and risk of coronary heart disease among women, The Lancet, 1993 Mar 6;341(8845), pp. 581-5
- Lawsuit dropped as Oreo looks to drop the fat
- Part of the impetus for that half-a-gram-minimum reporting requirement came from the meat and dairy industries -- an 8 U.S. fluid ounce glass of whole milk contains a quarter of a gram of naturally-occurring trans fat, and could thus be labelled as "trans-fat free".
- Trans Fatty Acids and Cardiovascular Disease, Dariush Mozaffarian, M.D., M.P.H., Martijn B. Katan, Ph.D., Alberto Ascherio, M.D., Dr.P.H., Meir J. Stampfer, M.D., Dr.P.H., and Walter C. Willett, M.D., Dr.P.H.; N Engl J Med 2006; 354:1601-1613, April 13, 2006
- Hu, FB; Stampfer, MJ, Manson, JE, Rimm, E, Colditz, GA, Rosner, BA, Hennekens, CH, Willett, WC (1997). "Dietary fat intake and the risk of coronary heart disease in women". New England Journal of Medicine 337 (21): 1491–1499. PMID 9366580.
- Gebauer et al., Effects of Ruminant trans Fatty Acids on Cardiovascular Disease and Cancer: A Comprehensive Review of Epidemiological, Clinical, and Mechanistic Studies, Advances in Nutrition, July 2011, vol. 2 pp. 332-354
- Bassett et al., Dietary Vaccenic Acid Has Antiatherogenic Effects in LDLr−/− Mice, The Journal of Nutrition, vol. 140 no. 1 (January 2010), pp. 18-24
- Brouwer, I. A.; Wanders, A. J.; Katan, M. B. (2010). Reitsma, Pieter H. ed. "Effect of Animal and Industrial Trans Fatty Acids on HDL and LDL Cholesterol Levels in Humans – A Quantitative Review". PLoS ONE 5 (3): e9434. PMC 2830458. PMID 20209147.
- Mensink RPM, Katan MB. Effect of dietary trans fatty acids on high-density and low-density lipoprotein cholesterol levels in healthy subjects.] N Engl J Med 1990; 323:439-45
- Mensink et al., Effects of dietary fatty acids and carbohydrates on the ratio of serum total to HDL cholesterol and on serum lipids and apolipoproteins: a meta-analysis of 60 controlled trials., American Journal of Clinical Nutrition, vol. 77 no. 5 (May 2003), pp. 1146-55
- Abbey M, Nestel PJ (1994). "Plasma cholesteryl ester transfer protein activity is increased when trans-elaidic acid is substituted for cis-oleic acid in the diet". Atherosclerosis 106 (1): 99–107. PMID 8018112.
- Kinsella et al., Metabolism of trans fatty acids with emphasis on the effects of trans, trans-octadecadienoate on lipid composition, essential fatty acid, and prostaglandins: an overview., American Journal of Clinical Nutrition, vol. 34 no. 10 (Oct 1981), pp. 2307-2318
- U.S. Bans Trans Fat - Bloomberg Business, 16-June-2016
- Phasing out trans fats is a long, costly process, Chicago Tribune, 26-June-2015
- An end to trans fats?, Harvard Gazette, 7-Nov-2013
- http://www.cdc.gov/nutrition/downloads/trans_fat_final.pdf CDC's source was JAMA: Dietz WH, Scanlon, KS. 2012. "Eliminating the Use of Partially Hydrogenated Oil in Food Production and Preparation." JAMA. 2012;308(2):143-144.