You may have first become aware of trans fats in the mid-1990s, when reports began to surface that margarine, often touted as healthy, contains these sinister fats and may not be preferable to butter after all. It turned out that trans fats were in most processed foods in the supermarket and in many fast foods. In 1994 Harvard researchers went so far as to conclude that trans fats were responsible for more than 30,000 of the country’s annual deaths from heart disease. Other scientists dismissed these fears as overblown, and since then the debate has continued. Here’s the latest.

What are trans fats and why are they added to foods?

Manufacturers partially hydrogenate—that is, add hydrogen to—corn, soybean, and other highly unsaturated oils to make them more solid and stable. The result: some of the polyunsaturated and monounsaturated fatty acids in the oils become more saturated. This process also transforms the chemical structure of some unsaturated fatty acids in other subtle ways, producing trans fatty acids (or simply trans fats). Hydrogenation gives margarines, shortening, and puddings a creamy consistency, and prolongs the shelf life of crackers, cakes, cookies, chips, popcorn, chocolate candy, and other foods that contain the semi-solid oils. Because they are less likely to turn rancid, hydrogenated oils are also often used for deep-frying in fast-food restaurants.

Why are trans fats bad?

While regular unsaturated fats lower blood cholesterol, trans fats act more like saturated fats—raising total and LDL ("bad") cholesterol. In addition, in a sort of double whammy, trans fats lower protective HDL ("good") cholesterol, which makes them, overall, even worse than saturated fats. They may also increase the risk of heart disease in other ways: for instance, they boost blood triglyceride levels and seem to impair the ability of blood vessels to dilate. They have also been linked to an increased risk of diabetes.

A 1993 study of nearly 90,000 women found that those who ate the most foods high in trans fats (especially margarine) had a more than 50% higher risk of heart disease than women who rarely ate these fats. Two recent Dutch studies also found increased coronary risk among people consuming high levels of trans fats.

How much trans fats do we eat?

Estimates are that trans fats provide 2 to 4% of our total daily calories, but other estimates are higher. The fact is, no one really knows. Moreover, any such average means little, since some of us consume little or no trans fats, while others consume two or three times the average. A large order of fast-food fries, for instance, or a frosted donut by itself can put you over the average. In addition, food manufacturers often change the types of oils they use and the degree of hydrogenation of the oils, so it’s impossible to keep track. And food labels don’t help (see below).

Many researchers believe that since trans fats make up only a small portion of our fat intake, worries about them are exaggerated. It is true, however, that during the past 15 years we’ve been consuming more and more trans fats, since food makers have been using more hydrogenated oils.

Is there any way to tell from the label how much trans fat is in a food?

No. If you see partially hydrogenated oil on the ingredients list, you know the food contains some trans fats. But nutrition labels don’t specify how much. This may actually encourage manufacturers to use hydrogenated oils, since unlike the saturated fats they often replace, the trans fats remain invisible on nutrition labels. And trans fats are not currently counted as saturated fat on labels.

Three years ago the FDA proposed adding the trans fats content to nutrition labels, but the process has been put on a back burner. As things stand, foods that contain trans fats are even allowed to make heart-healthy claims. For instance, Triscuit crackers are high in trans fats, yet the box boasts "no cholesterol" and "low saturated fat." Labeling of trans fats is long overdue.

Some things are clear

• You needn’t worry about occasionally eating small amounts of margarine (or, for that matter, butter, which is rich in saturated fat but contains no trans fats). However, if you eat lots of margarine, try to use less, or switch to a more healthful type. The more solid the vegetable oil, as in stick margarine, the more hydrogenated it is, and therefore the more artery-damaging trans fats it has. Tub margarines are soft and contain lots of unaltered polyunsaturated fats, which actually help lower blood cholesterol. "Diet" margarines are softest and have more water—and less than half the fat of other margarines. Liquid "squeeze" margarines are also good alternatives.

• Even if you don’t eat margarine, you may be consuming lots of trans fats. Processed and fast foods that contain partially hydrogenated oils supply more than two-thirds of the trans fats in the American diet. If you eat these foods often, cut back. Trans fats are only one of the nutritional strikes against them.

Promising developments: Food makers, especially in Europe, have been finding new ways to reduce trans fats in foods. In this country, margarine makers have been reformulating many products to cut down on trans fat (and total fat). And the USDA has developed a new process called low-trans hydrogenation, which produces fewer trans fats and may soon be used for margarines and other spreads.