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.
 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. 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.
 Health claims
Partially hydrogenated oils were in use for decades as a cheap substitute for lard or tallow. Most products marketed as "vegetable shortening", including Crisco®, consist 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.
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" transfats provide health benefits, and other studies which found no difference in cholesterol impact between natural and artificial trans fats.
Current evidence suggests that trans fats — or at least, the trans fats produced by partial hydrogenation, which are the ones 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.
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 U.S..
 "Good" cholesterol and "bad" cholesterol
Consuming saturated fat tends to raise both ones level of LDL ("bad") cholesterol and ones level of HDL ("good") cholesterol. By contrast, current studies seem to indicate that consuming trans fat tends to raise ones level of LDL cholesterol but lower ones 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 ones 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.
 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.
- ↑ 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
- ↑ 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 milk contains a quarter of a gram of naturally-occurring trans fat, and could thus be labelled as "trans-fat free".
- ↑ 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. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2830458.
- ↑ 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.
- ↑ http://www.cspinet.org/transfat/about.html
- ↑ http://www.acsh.org/trans-fats-anatomy-of-a-scare-from-american-com/
- ↑ http://www.acsh.org/just-trans-fats-maam/