Q**A recent article indicated there are some risks to the trans fatty acids found in margarines. Could you explain what is meant by trans fatty acids? Must we now avoid margarine?
A**Unsaturated fatty acids contain one or more chemical double bonds, or sites in the fatty acids where the backbone of carbon atoms lacks some hydrogen.
The carbon and hydrogen atoms at these sites come in one of two different shapes called "cis" or "trans."
Most naturally occurring fats and oils contain only cis fatty acids. The major exception is the fats of cows and other ruminant animals. Trans fatty acids account for 3 percent to 6 percent of the fat in beef, milk and butter.
About one-third of the trans fatty acids in our diets is derived from margarine and products that contain margarines. These trans fatty acids are formed when hydrogen is added to vegetable oils to make them firmer or harder.
A major issue about trans fatty acids in the diet -- their effects on blood cholesterol levels -- remains controversial. A recent article in the New England Journal of Medicine reported that replacing more than half of dietary monounsaturated fatty acids (in this case the oleic acid of olive oil) with trans fatty acids led to a modestly increased LDL (bad) cholesterol and decreased HDL (good) cholesterol.
It is not clear from this study whether these undesirable results were caused by the adverse effects of the added trans fatty acids or to a loss of the beneficial effects of the oleic acid.
Moreover, the amount of trans fatty acids included in the test diet (10 percent of total calories) was considerably greater than the average person eats in this country (2 percent to 4 percent of total calories). Although this study was carefully done, the findings conflict with those of previous trials on the effects of trans fatty acids on blood cholesterol.
The best way to avoid trans fatty acids is to follow the recommendations made by the American Heart Association and other groups -- reduce total dietary fat to less than 30 percent of calories.
Remember that beef and milk contain some trans fatty acids. While it is true that margarine is a major source of trans fatty acids, margarine is still a better choice than butter. Butter contains trans fatty acids as well as cholesterol (which is absent in all margarine) and a much higher percentage of saturated fat than margarine.
Tub margarines are preferable because they contain more polyunsaturated fatty acids and only about half the amount of trans fatty acids contained in stick margarine.Dr. Margolis is professor of medicine and biological chemistry at the Johns Hopkins School of Medicine and associate dean for academic affairs at the school.