In a recently published article in the New England Journal of Medicine, a team of medical researchers at Harvard Medical School  reported on the effects of different types of dietary fat on the risk of coronary heart disease. Trans fatty acids have been added to the list of saturated, mono-unsaturated and polyunsaturated fats that consumers should know about. As with many diet and health related issues the information consumers are receiving is a bite confusing.  The thing to remember is that having trans fatty acids in your diet turns out to be bad news.

It wasn't too long ago that the message went out  that we should be switching from butter to margarine. Butter is high in saturated fats, which have been linked in particular to cardiovascular disease. Margarines are high in polyunsaturated fats which have been shown to be beneficial.  The base for most margarines is vegetable oil (soybean, corn, canola). Vegetable oils are liquid and so to make a solid margarine, the oils are hardened or hydrogentated. In the hydrogenation process, hydrogen is pumped through the vegetable oil and this causes some of the polyunsaturated fatty acids to pick up hydrogens. Fatty acids that are unsaturated contain places in their molecule where the carbons joined together are connected by double rather that single bonds. Normally at these sites of double bonds the hydrogens attached to the carbons are on the same side and are said to be in a "cis" configuration. However, during the hydrogentation process, the hydrogens can take up positions on either side of the double bond and are said to be in a "trans" configuration. The geometry, the chemistry and the metabolism of trans-fatty acids are very different from the normal cis-fatty acids.

So margarines are made from vegetable oils and therefore are rich in polyunsaturated fatty acids which is good. But margarines that have been produced by hydrogenation contain trans fatty acids and as the study reported by  Dr. Frank Hu and his co-workers showed, eating trans fatty acids leads to a greater risk of  coronary heart disease. Fortunately it is possible to buy margarines that are made by blending rather than hydrogenation. But read the food labels carefully. Not only  your margarine but a wide variety of other processed foods may use hydrogentated vegetable oils either as an ingredient or in the processing.

Details of the quantity and the type of fat in a food may be even more important as we learn about the role saturated, polyunsaturated and trans fatty acids play in health and disease.

Reference

N Engl J Med 1997; 337:1491-1499