How are they formed? And what's so bad about them? Get the stats on the new fat on the block.

By Jennifer Rapaport
Updated September 08, 2006
Pat of butter
Credit: Marcus Nilsson

The History of Hydrogenation

Solid shortening, the thick white paste that made your grandmother's piecrust so flaky, was created nearly a century ago by adding hydrogen to liquid oils to make them turn solid at room temperature (the process known as hydrogenation). Originally intended as a cheap substitute for butter and lard, partially hydrogenated oils―what we now call trans fats―became known for their ability to increase the shelf life and improve the texture of baked goods and other food products. Soon food manufacturers were adding them to everything from cookies to nondairy creamers to frozen foods, and restaurants were using them for deep-frying and more.

Reasons for a Trans Fat Backlash

In 1994 a study at Harvard University reported that people who consumed the highest amounts of trans fats had twice the heart-attack risk of those who consumed little. "The more we look at trans fat," says Walter Willett, who worked on the study, "the more we see it is a metabolic poison." Trans fats are particularly harmful because they lower levels of good cholesterol and raise levels of bad cholesterol.

New Rules for Food Companies

The U.S. Food and Drug Administration now requires food manufacturers to list trans fats (often called partially hydrogenated oils) on nutrition labels, which prompted many food companies, including Kraft, Campbell's, and Wendy's, to reduce or remove trans fats from their products.

Trans Fats vs. Saturated Fats

Unfortunately, many companies replace trans fats with saturated fats. That's dangerous because saturated fats already make up a larger percentage of most Americans' diets―around 11 percent, as opposed to just 1.5 to 2.5 percent for trans fats, notes Alice Lichtenstein, D.Sc., director of the Cardiovascular Nutrition Laboratory at the Jean Mayer Center on Aging at Tufts University, in Boston. It's important not to consume more saturated fat because you're looking only at the trans fat amounts. "We should try to cut down on trans fats as well as saturated fats," says Lichtenstein.