The truth about trans: hydrogenated oils aren't guilty as charged.
They're in Ritz Crackers, Hostess ho Hos, Oreos, Kellogg's Pop Tarts, and hundreds of other processed foods. They're the principal ingredient in margarines and shortenings. They're hydrogenated fats.
Many journalists--and a few researchers--view hydrogenated fats with great suspicion. And more than a few consumers won't touch margarine. Some aren't sure why they distrust the stuff. Others know exactly why. Margarine contains "trans" fats.
Manufacturers hydrogenate (add hydrogen to) liquid oils to make them semi-solid. In the process, some hydrogens in the fats get rearranged. Scientists refer to fats with the ordinary arrangement as "cis." The new ones are called "trans" (see diagram).
Despite the rumors, there is little good evidence that trans fats cause any more harm than other fats. Though new questions can always be raised, some of the standard accusations can be laid to rest.
Does Trans Equla Trouble? In the Goldbecks' Guide to Good Food, Nikki and David Goldbeck state that trans fats "are suspected of interfering with fath metabolism, disrupting normal heart functions, enhancing fatty deposits in the arteries, inhibitng the production and utilization of substances in the body which influence the immune system, and reducing the body's ability to rid itself of carcinogens, drugs, and other toxins." Other authors have raised similar concerns.
Much of the anxiety over trans fats stems from their reputation as "unnatural." Yet ruminants, such as cows and sheep, hydrogenate oils in one of their several stomachs. So although most of the trans fat people consume today is man-made, about 5 to 20 percent comes from beef, lamb, and dairy products.
In rat studies, trans fats appear safe. Animals absorb them just as well as they absorb other fats and oils.  And rats fed high levels of trans fats for 46 generations lived as long as other rats, reproduced as well, and appeared normal. 
Hydrogenated Hearts. But these studies have not stilled all fears. Some claim, for example, that trans fats raise blood cholesterol levels. That's a serious charge against trans-fat-containing margarines, which are marketed as heart-healthy.
In some animals, trans fats do raise blood cholesterol; in others, they don't.  But it makes sense to look most closely at the evidence from human studies.
Although some human studies suggest that trans fats do raise blood cholesterol, most of these had serious flaws. Several, for example, used an unusual fat with two trans groups. This fat is not present to a significant extent in commercial margarines or oils. [4,5]
Only a few studies were well designed, and these showed that hydrogenated (trans-containing) and non-hydrogenated fats produced similar cholesterol levels. [6,7] However, even in the weaker studies, trans fats did not raise cholesterol as much as saturated fats.  "In general," says Fred Mattson, a noted researcher at the University of California in San Diego, "studies show trans fats to have the same effect on human blood lipid levels as cis fats."
Cancer Worries. In 1978, Mary Enig and others at the University of Maryland examined the rise in our vegetable fat intake since 1910. This rise, they asserted, was linked to n increase in the number of people who developed or died of cancer, especially breast and colon cancer, during that period. The trans fats in vegetable fats, suggested the scientists, could best account for the "significant positive correlation" with cancer rates. 
At most, associations of this type can only provide clues about the cause of a disease. In no way do they prove cause and effect. But this particular analysis had serious flaws.
For example, experts pointe out that most cancer rates, including deaths from colon cancer, had remained essentially stable during much of the period in question. What's more, the authors failed to consider other changes in America's eating habits over more than half a century. Commenting on the paper, John C. Bailar III, editor-in-chief of the Journal of the National Cancer Institute, concluded, "Evidence for a [cancer--causing] role of the trans fatty acid component is no better--and no worse--than for countless other dietary components." 
In most animal studies, trans fats don't increase the incidence of tumors. But non-hydrogenated corn, safflower, and sunflower oils, which contain high levels of polyunsaturated fats, are relatively strong tumor promoters, at least in animals. 
Unfortunately, few people are familiar with these studies. Instead, many have heard of the cancer "correlation," but not of its flaws. So the rumor that trans fats cause cancer persists.
Cell Membranes. Animal studies show that trans fats, like other fats, may beome incorporated into the membranes that surround all cells in the body. Some fear that the incorporated trans fats might alter the normal passage of substances into and out of cells, perhaps permitting carcinogens to enter cells with greater ease.
But Edward Emken, a biochemist with the Department of Agriculture's Agricultural Research Service in Peoria, Illinois, notes that the body selectively excludes trans fats from key membrande positions.  "Trans fats are only incorporated into these positions when an animal is fed so much trans fat that it has no other choice," he explains. "But those dietary levels are unrealistically high."
According to Emken, charges that trans fats impair the immune system are based on inadequate data. "There's no hard evidence to suggest that, at current levels of intake, trans fats pose any more danger than other kinds of fat," he maintains.
Although trans fats don't present a clear-cut threat, it makes sense to avoid excesses. "Humans have been consuming trans fats for eons," explains John Kinsella, a food chemist at Cornell University, "but only in very limited amounts. As long as you are not flooding the system wiht them, it's probably all right."
Kinsella believes an excess of trans fats in the diet may increase the body's need for linoleic acid, a fatty acid called "essential" because it must come from the diet. (That is, the body can't make its own.) However, because linoleic acid is found in soybean, safflower, corn, and other polyunsaturated vegetable oils, we get a more-than-adequate supply of this nutrient. In fact, some studies suggest that our generous linoleic acid intake actually promotes cancer.
All told, the charges against trans fat just don't stand up. And by extension, hydrogenated oils seem relatively innocent.
Although hydrogenation can be used to make fats completely saturated, it is rarely used that way. In ordinary "partial hydrogenation," most of the change is from polyunsaturated to cis and trans monounsaturated fats, with relatively little saturated fat produced. Soybean oil, for example, starts out 15 percent saturated, but after hydrogenation to make margarine, it ends up about 17 to 20 percent saturated fat. [12,13]
That means margarine is much less saturated than butter, which is 66 percent saturated. In addition, the kind of saturated fat (stearic acid) produced by hydrogenation does not seem to raise blood cholesterol as much as other saturated fats. 
In countries where heart disease is rampant, that makes margarine preferable to butter. True, if you cut back on saturated fats in meat, cheese, and pastries, and don't have high blood cholesterol, you can probably afford an occasional pat of butter on your toast. But that's a matter of taste, not health.
As for processed foods, you're better off choosing products made with hydrogenated soybean, corn, or cottonseed oil than those containing butter, lard, beef fat, or palm, palm kernel, or coconut oil. But you're even better off cutting back on total fat. That may reduce your risk of cancer and heart disease and should help keep your waistline within reasonable limits. And it will make concern over trans fats the small issue it should be.
 Ann. Rev. Nutr. 4: 339, 1984.
 J. Nutr. 63: 241, 1957.
 Fed. Am. Soc. Exp. Biol. Health Aspects of Dietary Trans Fatty Acids, 1985, pp. 65-67.
 J. Nutr. 75: 388, 1961.
 J. Am. Oil Chem. Soc. 58: 260, 1981.
 Am. J. Clin. Nutr. 28: 726, 1975.
 Bibl. Nutr. Diet 7: 137, 1965.
 Vergroesen, A.J., ed. The Role fo Fats in Human Nutrition (Academic Press, New York) 1975, pp. 1-41.
 Fed. Proc. 37: 2215, 1978.
 Fed. Proc. 38: 2435, 1979.
 Cancer Res. 44: 1321, 1984.
 Handbook No. 8-4, Composition of Foods: Fats and Oils, 1979.
 J. Am. Oil Chem. Soc. 60: 1788, 1983.
 Am. J. Clin. Nutr. 23: 1184, 1970.