Trans fats are man-made fats; only very small amounts are found naturally in foods like meat, butter and milk.
Most start out as liquid vegetable oils, and through a process called hydrogenation, hydrogen is added. This turns the liquid oil into a partially solid, or hydrogenated, product. The hydrogenation process makes the artificial fat capable of clogging arteries, much like saturated fat. Trans fats can raise levels of "bad" (LDL) cholesterol, much like saturated fats. They also lower levels of "good" (HDL) cholesterol.
Trans fats are also high in calories -- like all fats -- and, when eaten in excess, can contribute to being overweight.
The U.S. Department of Agriculture's 2005 Dietary Guidelines warn consumers to "limit intake of fats and oils high in saturated and/or trans fatty acids and choose products low in such fats and oils."
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