.For now, trans fatty acids may be lurking under the "polyunsaturated" label on your margarine, but labeling rules issued by the Food and Drug Administration will change in 2006 to state specifically whether packaged foods contains trans fats. A label identifying "partially hydrogenated" oils also indicate that a food contains trans fats.
If these listings are among the first few ingredients, the food may have high amounts of trans fats.
Though a few trans fats occur naturally in small amounts, most are unsaturated liquid vegetable oils, like corn oil, that have been hydrogenated - that is, treated to add hydrogen to their molecules. This transforms the molecules into a shape that makes the fats form a mouth-friendly solid at room temperature, rather than a liquid.
Technically, the hydrogenated fats are still unsaturated; unfortunately, the chemical change also makes the fats act more like saturated oils. They tend not only to raise undesirable low-density lipoproteins in the bloodstream but also to reduce beneficial high-density lipoproteins.
Many food makers are phasing out trans fats, and labeling will soon make it clear which foods still have it.