The Medical Minute: What is trans fat?

March 08, 2006

By John Messmer
Penn State Family and Community Medicine
Penn State Milton S. Hershey Medical Center
Penn State College of Medicine

It's easy to be confused with all the conflicting information about fat in the news these days. Just when we get used to avoiding saturated fats and looking for polyunsaturated fats, they are changing the food labels to include "trans" fats. What exactly is a trans fat and why are trans fats bad? To understand this issue, it's nice to understand a little of the chemistry involved.

Fats are composed mostly of carbon and hydrogen with a little bit of oxygen, too. A long chain of carbon atoms with oxygen at the end in a particular arrangement is a fatty acid. There is a different name for each fatty acid based first on how many carbons are in the chain and how they are joined together. Fats have three fatty acids attached to a molecule called glycerol. It is called a triglyceride for that reason.

Each carbon atom can have one or two hydrogen atoms attached. If the fat has all the hydrogen it can hold, it is called "saturated;" if not, it is "unsaturated." Unsaturated fats tend to be liquid at room temperature and we usually call them "oils." As fats become more saturated, they tend to be more solid at room temperature. Animal fats, such as butter, are high in saturated fat. Vegetable oils in their natural state are typically unsaturated and are liquid. In an unsaturated fat, two adjacent carbons with only one hydrogen atom apiece can have the hydrogen on the same side, called a "cis" configuration or on opposite sides, called a "trans" configuration from the Latin roots for same and opposite.

So, trans fats are unsaturated. For years we have been told that unsaturated fats are better for us, so why the change? It happens that most naturally occurring unsaturated fats are in the cis configuration. About a century ago, the process of hydrogenation of fats was developed and partially hydrogenated vegetable oil was first marketed as Crisco. Hydrogenated oils are more saturated with hydrogen, making them more solid and less subject to deterioration. Foods made with hydrogenated oils retain freshness longer, giving them much longer shelf lives -- a benefit for manufacturers who worry about the quality and taste of a product. Unfortunately, artificially hydrogenated fats are trans fats.

It was assumed that margarine, for example, because it had more unsaturated fat than butter would be healthier. Recent studies have found to the contrary that trans unsaturated fats raise levels of LDL, the "bad" cholesterol and lower levels of HDL or the "good" cholesterol. It's not completely clear why this is. Some speculate that the trans configuration, because it bends the molecule in a shape different from the cis fats, interferes with proper use of the fat by the body's cells. Fats are the building-blocks of certain chemicals used in inflammation. The chemicals may not be properly produced by a misshapen fat.

Food labels have listed "partially hydrogenated vegetable oils" for years. Since it is listed as an unsaturated fat, it is difficult to determine how much is present. In order to help consumers identify what types of fats are present in foods, the FDA now requires manufacturers to list the amount of trans fat separately, although if the food has less than 0.5 grams of trans fat, it can be said to be "trans fat free."

Look for the new labeling to appear on commercially prepared baked goods, such as donuts, cookies, crackers, cakes and in microwave popcorn, cereals and fast foods and many other items. People should consume most of these products only in limited quantities anyway, but with the new labeling requirements, at least it's possible to make better choices.

For more information on trans fats and the FDA rule, go to online.

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Last Updated March 19, 2009