Trans-fatty acids (TFAs) and partially hydrogenated oils are not the same, but they are related. The process of hydrogenation, which turns liquid fats into solid ones, creates TFAs, making them one of the components of partially hydrogenated fats. (Fully hydrogenated fats do not present this problem.) Chemically, TFAs are individual molecules of fatty acids that have unnatural configurations. They are worse for the heart and arteries than saturated fats - indeed, recent studies indicate saturated fats probably don't raise the risk of coronary disease, and a component in full-fat dairy products may actually be cardioprotective.
TFAs, on the other hand, increase total cholesterol, raise LDL ("bad") cholesterol and lower HDL ("good") cholesterol. Beyond that TFAs may have adverse effects on cell membranes and the immune system and may promote inflammation, cancer and aging.
Trace levels of TFAs are found naturally in milk fat (created by bacterial action in the stomachs of cows), but, even in butter, the amounts are so small that they are probably not a concern.
Partially hydrogenated fats include such products as margarine and vegetable shortening. In addition to the hardening effects, the process of hydrogenation makes fats more stable. This extends the shelf life of the foods that contain them - principally, commercial baked goods as well as other processed foods including French fries, chips, microwave popcorn and other snack foods, most frozen meals, breakfast cereals and low-fat ice cream.
Beginning in 2006 food manufacturers will be required to include on labels the amount of TFAs their products contain, but the fact that fewer processed foods may contain these unhealthy fats doesn't mean that partially hydrogenated ones are acceptable in a healthy diet.
Partially hydrogenated oils contain mixtures of saturated, monounsaturated and polyunsaturated fats. Even if some labels claim that foods include no TFAs, I would avoid those listing partially hydrogenated oils. They are markers of low-quality foods that are often unhealthy in other ways and have no place in the optimum diet.
Andrew Weil, M.D.