This article is from the WebMD Feature Archive

What exactly are trans fats? How are they made? How bad are they, really? And just how solid is the science that the FDA consulted when they voted to list trans fats on nutrition labels? To get to the bottom of these and other questions about trans fats, WebMD spoke to leading nutritionists.

What Exactly Are Trans Fats?

Trans fatty acids or trans fats are formed when manufacturers turn liquid oils into solid fats. Think shortening and hard margarine. Manufacturers create trans fats via a process called hydrogenation. Hydro-what? In a nutshell, hydrogenation is a process by which vegetable oils are converted to solid fats simply by adding hydrogen atoms.

Why hydrogenate? Hydrogenation increases the shelf life and flavor stability of foods. Indeed, trans fats can be found in a laundry list of foods including vegetable shortening, margarine, crackers, cereals, candies, baked goods, cookies, granola bars, chips, snack foods, salad dressings, fats, fried foods, and many other processed foods.

Trans fatty acids are found naturally in small quantities in some foods including beef, pork, lamb, butter, and milk, but most trans fatty acids in the diet come from hydrogenated foods. So there is good news: When the new nutrition labels go into effect Jan. 1, 2006, it will be easier to screen these fats out of your diet. Until then, look at the package's list of ingredients. Products that contain partially hydrogenated oils or vegetable shortening may contain trans fats.

Where Did Trans Fats Come From?

Trans fats were developed during the backlash against saturated fat -- the artery-clogging animal fats found in butter, cream, and meats. Then food manufacturers realized that trans fats lasted longer than butter without going rancid. The result: Today trans fats are found in 40% of the products on your supermarket shelves.

"We used to use animal fats, and people said, 'saturated fats are bad,' so we switched to trans fats," says Ruth Kava, PhD, RD, director of nutrition at the New York City-based American Council on Science and Health. "This kind of gives us an unfortunate focus on ingredients rather than the whole diet when the problem isn't this fat or that fat, it's too many calories."

"Anything was good if it decreased saturated fat consumption in the 1950s through the 1980s," agrees Alice H. Lichtenstein, Dsc, professor of nutrition at Tufts University in Boston. "But then studies began to question trans fats," too. Finally, in the 1990s, the evidence became clear: When vegetable oil is turned into a solid, like butter, it acts like butter inside the body.

Next, learn about the risks.

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