For years physicians have warned diners to sharply reduce their consumption of saturated fats, the leading source of fat calories in the U.S. diet. Saturated fats-which, like butter, remain solid at room temperature-can increase levels of serum cholesterol and the low-density lipoproteins (LDLs) that deposit cholesterol in heart arteries.
But what about unsaturated fats-such as corn and soybean oil-that have been “hardened” through hydrogenation for use in margarines and shortening? A new study shows that this process subtly reconfigures ordinary unsaturated fatty acids into “trans” fatty acids, which elicit serum cholesterol changes comparable to-or even worse than-those caused by saturated fats.
Trans Fatty Acids and Cholesterol
For three consecutive 3-week periods, 25 men and 34 women, mostly students, ate diets that varied nutritionally only in their major fats. One of the randomly assigned cycles featured liquid oleic acid, the main monounsaturated fatty acid in olive and canola oils; another offered meals high in the solid “trans” oleic acid. Saturated fats replaced oleic acid in yet another cycle.
The trans fatty acids raised serum cholesterol 5.8 percent above the levels seen with the liquid oleic-acid diet-or about half as much as the saturated fat diet, reported Ronald P. Mensink and Martijn B. Katan of the Agricultural University in Wageningen, the Netherlands. However, the trans fat’s effect on lipoproteins “was more unfavorable than is suggested by the small increase in the serum total cholesterol concentration,” they note in the New England Journal of Medicine.
Compared with saturated fat, trans fat sparked a greater increase in LDL-carried cholesterol and a greater reduction in high-density lipoproteins (HDLs), the so-called “good” lipoproteins. It thus produced the highest-that is, unhealthiest-ratio of total cholesterol to HDL. That ratio, considered a powerful gauge of the risk of coronary artery disease, rose an average of 22.6 percent in volunteers on the trans-fat diet and only 13 percent during the saturated fat cycle.
“It would seem prudent for patients at increased risk of atherosclerosis to avoid a high intake of trans fatty acids,” the researchers conclude. In the typical U.S. diet, 2 percent to 4 percent of the calories come from trans fatty acids.