Older adults burn less fat during exercise.

Couch potatoes often loll in front of the television instead of taking their extra pounds for a jog around the block. Research shows that inactivity is not the entire problem. When an aging couch potato waddles into the gym to exercise, those extra pounds are harder to shed, partly because muscles lose the ability to burn fat as individuals get older. Researchers at Washington University School of Medicine in St. Louis and the University of Texas Medical Branch in Galveston reached this dismal conclusion after a study that found older people burn less fat than younger ones when the two groups do similar exercise.

In the young adults (average age, 26), about one-half of the fuel metabolized during exercise came from fat, with the other half from carbohydrate. In the elderly subjects (average age, 73), about two-thirds came from carbohydrate and one-third from fat. According to Samuel Klein, director of Washington University's Center for Human Nutrition, burning carbohydrate rather than fat is not unhealthy. it simply substitutes one fuel source for another. However, increased use of carbohydrate makes it harder for people to continue their workout. Carbohydrate oxidation leads to quicker fatigue and depletes blood sugar...