Avoiding saturated fat and cholesterol is already mainstream practice for health-minded shoppers. But there’s a new villain in town: trans fats. Ubiquitous in processed foods, trans fats increase heart-disease risk and may contribute to diabetes, stroke, and cancer.

“People are just starting to get that trans fats are bad, but they don’t know why,” says Kim Severson, food reporter and author of The Trans Fat Solution (Ten Speed Press, 2003). Food labels increasingly cluttered by health claims often confound the problem. Here, we break down what trans fats are and offer tips on how to avoid them in your diet.

How trans fats form
Trans fatty acids are created when hydrogen is added to a liquid fat, forcibly changing its melting properties so it remains solid at room temperature—a desirable quality for products such as margarine and shortening. The prefix “trans” refers to carbon bonds that, during hydrogenation, are folded against their natural direction, forming a fat that’s artificially saturated. Hydrogenation improves shelf life, flavor stability, and “mouthfeel,” but it’s the birthplace of trans fats.

Like saturated fat and dietary cholesterol, trans fats raise LDL (bad) cholesterol, which accumulates on artery walls. Even worse, trans fats may lower HDL (good) cholesterol, which transports cholesterol back to the liver for disposal. In fact, compared with saturated fats, trans fats consumption correlates to a considerably higher risk for heart problems caused by narrowed arteries (Annals of Nutrition and Metabolism, 2004, vol. 48, no. 2).

Although trans fats exist in small amounts in animal products, of greater concern are those hiding in processed foods. They’re abundant in cookies, crackers, chips, cakes, and margarine, foods that contribute 75 percent of the trans fats in a typical U.S. diet.