Getting older - and a little rounder?

A person's body fat typically doubles between the ages of 20 and 50. While this middle-aged spread is usually attributed to eating too much and exercising too little, there may be more to it than that. The body's ability to break down and use large quantities of fat drops with age, a new study finds. As a result, more of the fat eaten at large meals will become body fat instead of being burned as energy, explains Susan B. Roberts of the Department of Agriculture's Human Nutrition Research Center on Aging at Tufts University in Boston.

On four different days, her team analyzed how 16 women--half of them in their twenties, the rest over 60--metabolized various portions of peanut-butter-and-jelly sandwiches and milk. The four test meals ranged from a 250-calorie snack to a 1,000-calorie feast that represents two-thirds of the average 60-year-old woman's daily energy intake.

The two groups processed the fat in the smaller meals with equal efficiency. However, the older diners burned only 70 percent as much fat from the big meal as those in their twenties, the researchers report in the October American Journal of Clinical Nutrition. One explanation may be the fact that older women possess more glucagon, a hormone that instructs the body to put more sugar into the blood. "That's not good," Roberts says, because the more readily used sugar discourages the body from burning fat. In their thirties, people begin to lose skeletal muscle (SN: 8/10/96, p. 90), the site of most fat metabolism.

To cope with these age-related changes, Roberts suggests, older diners should not only eat less at a sitting but also exercise more--to strengthen and retain fat-burning muscle.