What are trans fatty acids?
Unsaturated fats, found in such foods as avocados and olive and corn oils are heart healthy, but in the air they can go rancid by absorbing oxygen and then decompose, C&EN explains. Manufacturers can stop this process by bubbling hydrogen (hydrogenation) through the fat at a high temperature in the presence of a catalyst like nickel and in the absence of oxygen.

The process raises a fat's melting point, turning liquid vegetable oil into products ranging from soft margarine to solid shortening, according to the newsmagazine. When the healthful unsaturated fats are partially hydrogenated, the double bonds are rearranged, converting some to the trans configuration and shifting the double bonds along the chain. Unfortunately, this newly created trans fatty acid is an artery-clogger.

Amid the criticism of cookies, chips and other products containing trans fat, a number of companies have either developed foods without partially hydrogenated oils or have pledged to explore ways of replacing the fat. PepsiCo's Frito Lay, for example, has already eliminated trans fats from some of its products.

Which foods contain trans fatty acids?
Trans fats are produced commercially in large quantities to harden vegetable oils into shortening and margarine. Food manufacturers also use partial hydrogenation of vegetable oil to destroy some fatty acids, such as linolenic and linoleic acid, which tend to oxidize, causing fat to become rancid with time. The oils used to cook french fries and other fast food are usually this kind of partially hydrogenated oil, containing trans fats. Commercial baked goods frequently include trans fats to protect against spoilage. A small amount of trans fat is also produced in the gastrointestinal tract of cattle, so that low levels of these isomers are found in dairy and beef fat.

Commercial production of partially hydrogenated fats began in the early 20th century and increased steadily until about the 1960s as processed vegetable fats displaced animal fats in the diets of the U.S. and other Western countries. Lower cost was the initial motivation, but health benefits were later claimed for margarine as a replacement for butter.

Although the average level of trans fat in margarines has declined with the advent of softer versions, per capita consumption of trans fatty acids has not changed greatly since the 1960s because of the increased use in commercially-baked products and fast foods.

How are trans fatty acids harmful?
In clinical studies, trans fatty acids or hydrogenated fats tend to raise total blood cholesterol levels and LDL ("bad") cholesterol and lower HDL ("good") cholesterol when used instead of cis fatty acids or natural oils. These changes may increase the risk of heart disease. It's not clear if trans fats that occur naturally have the same effect on cholesterol and heart disease as those produced by hydrogenating vegetable oils.