Trans fatty acids have been linked to heart disease and diabetes, and are worse for you than saturated fats. It is estimated that between 30,000 and 100,000 premature cardiac deaths could be prevented by replacing partially hydrogenated fats with natural non-hydrogenated oils.
Trans fatty acids are produced commercially in large quantities to harden liquid oils into solids. Trans fatty acids are created when a naturally occurring unsaturated fatty acid, which would be a liquid at room temperature, is heated in the presence of metal catalysts and hydrogen. This process, called partial hydrogenation, causes carbon atoms to bond in a straight configuration and remain solid at room temperature. Naturally occurring unsaturated fatty acid molecules are curved, and this new straight molecule is what is called a "trans fatty acid".
Trans fatty acids are in almost all commercially processed food. Partially hydrogenated oils are used in foods to help maintain their shape and to give them a longer shelf life, as hydrogenated oils do not spoil as quickly as natural ones. The FDA now requires information about trans fatty acids to be listed on nutrition labels. The way to identify trans fatty acids in foods is to read the ingredient list. Look for the word HYDROGENATED. If anything has been hydrogenated, it has gone through the process of partial hydrogenation and should be avoided.