Low-Fat Diets Don't Necessarily Reduce Calorie Intake

The popular belief that dietary fat increases people's calorie intake lost credibility in a study of identical twins who ate both high-fat and low-fat diets. But the study also suggests that genes exert some control over a person's preference for a high- or low- fat diet.

Researchers at the USDA Human Nutrition Research Center on Aging at Tufts in Boston designed the two diets for equivalent palatability, fiber content and calories per ounce--factors that may affect calorie intake. USDA's Agricultural Research Service and the National Institutes of Health funded the study.

On average, the seven sets of male twins chose about the same number of calories when served a diet containing 20 percent fat calories and a diet containing 40 percent fat calories. Four sets of twins ate more calories from the high-fat menu. The other three pairs preferred the low-fat diet. This suggests fat is not the dominant factor influencing people's calorie intake, and the amount of calories per ounce of food may be more important, supporting some earlier, short-term studies. It also suggests that genes influence the tendency to overeat certain diets.

The men metabolized both diets with equal efficiency. Together, these findings may help explain why the U.S. population has added weight, although fat intake has dropped and low-fat and fat-free products have flooded the market. Many such products are calorically dense. Edward Saltzman led the study in collaboration with Gerard Dallal and Susan Roberts.

Roberts, who heads energy metabolism research at the Boston center, is conducting a larger study involving 90 sets of identical twins. It is designed to determine to what extent people's genes contribute to body fat and overweight.

Roberts is scheduled to appear on tomorrow night's broadcast of "The Pulse," a new ABC television show on health and nutrition scheduled to air at 10 p.m. E.S.T.

Scientific contacts: Edward Saltzman and Susan Roberts, Energy Metabolism Laboratory, Human Nutrition Research Center on Aging at Tufts, Boston, Mass. 02111, Saltzman's phone (617) 556-3245, Roberts' phone (617) 556-3237, fax (617) 556-3344, saltzman_em@hnrc.tufts.edu, roberts_em@hnrc.tufts.edu.