Hydrogenation changes liquid oil to solid in creating stick margarines and spreads. That's why stick margarines have lots of trans fats. Rule of thumb: Softer is better. The American Heart Association recommends that consumers use canola oil or olive oil when possible; look for processed foods that contain no hydrogenated oils; use margarine instead of butter, but choose a margarine that lists liquid vegetable oil as the first ingredient and that contains no more than 2 grams of saturated fat per serving.

Trans fats turn up in foods made with hydrogenated margarine and vegetable shortening, such as pastry and fried foods. Meat and dairy products have small amounts, too.

A July 2002 report from the National Academy of Sciences found that trans fats offer no benefits. The best daily intake is no intake, the report says. Because trans fats show up in a lot of foods, that may not be feasible—so the report suggests an intake "as low as possible."

Although saturated fats take heat for causing heart attacks, trans fats are worse, because they also lower the level of HDL ("good") cholesterol, according to the American Dietetic Association. The ADA recommends using trans-fat-free margarines, as well as diet margarines that contain fewer trans fats.

Since 2006, food labels have had to list the amount of trans fat per serving, as well as the amount of saturated fat and dietary cholesterol.

Trans fats abound in processed foods.