In TV commercials and in supermarkets across the country, butter and margarine face off in an effort to win over consumers. But which is considered South Beach Diet-friendly?

The answer is trans-fat-free margarine. The reason lies in the different types of fats contained in butter and margarine. Butter, made from animal fats, is high in artery-clogging saturated fats. Margarine, on the other hand, is made with vegetable oil, so it contains mainly polyunsaturated fat. But don't be deceived! While margarine contains less saturated fat than butter, the hard varieties (like stick margarine) are high in dangerous trans fats.

Trans fats are manufactured in a process called hydrogenation, in which liquid vegetable oils are transformed into solid fats. Trans fats are used in foods like crackers, cookies, and pastries to improve their flavor and texture and to extend their shelf life. Like saturated fats, trans fats raise LDL ("bad") cholesterol, but trans fats have also been shown to lower levels of HDL ("good") cholesterol. Trans fats may therefore pose greater health risks than saturated fats.

Even so, when it comes to spreads, butter isn't best! The healthiest choice is a trans-fat-free liquid or tub margarine. And because in January 2006 the Food and Drug Administration required that the trans fat content of packaged foods be included on the Nutrition Facts panel, it's easy to make an informed decision about which products to buy.

If you've been diagnosed with high cholesterol, you may elect to use one of the cholesterol-lowering margarine spreads made with phytosterols, also known as plant stanol esters. These compounds, found in sources like soybean oils, can assist in reducing blood cholesterol levels. One last piece of advice: Use all fats in moderation.