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Trans fat is a form of unsaturated fat that has undergone the chemical process of hydrogenation to make it more saturated. While hydrogenating unsaturated fats offers certain benefits, it also raises health risks.

During hydrogenation, hydrogen is added to vegetable oil. In most cases, hydrogenation is partial, leaving the fatty acid between 5 and 60 percent saturated. The end result is known as a trans fatty acid. Partially hydrogenated vegetable oils provide about three-fourths of the total fatty acid in the U.S. diet, according to the American Heart Association.

Hydrogenation achieves two goals. First, it takes a liquid oil and makes it more stable. The more hydrogenated an oil is, the harder it is at room temperature. For instance, olive oil (which does not contain trans fat) is liquid at room temperature, while stick margarine (which does contain trans fat) is hard at room temperature. Secondly, hydrogenation extends a product’s shelf life by making it less susceptible to spoilage. 

However, hydrogenating a fat also makes it less healthy. Consuming trans fats causes an increase in both total cholesterol levels and low-density lipoprotein (LDL) – or so-called “bad” – cholesterol. Trans fats also may cause a drop in the levels of high-density lipoprotein (HDL) – or so-called “good” – cholesterol.

Hydrogenated oil is the source of most trans fatty acids. For example, margarines are created from vegetable oil. Other foods that frequently include hydrogenated fats are commercially prepared baked goods, fast foods and vegetable shortening.

Most experts do not recommend that people try to eliminate trans fats from their diets. This would be largely impractical, as these fats are found in too many foods. However, people are urged to limit their consumption of trans fats as much as possible. This can be accomplished by reading the “Nutrition Facts” section of food labels and looking for phrases such as “hydrogenated” or “partially hydrogenated.”

As of January 2006, the U.S. Food and Drug Administration (FDA) requires manufacturers to list the trans fat content of foods on packaging labels. This information also can be found on the “Nutrition Facts” label.