Weighing Benefits Of Exercise, Diets: Both Work For Weight Loss, But Dieters Also Lose Muscle

Both those who dieted and those who exercised lost a significant amount of weight, according to findings from an NIH-funded study on whether a calorie-restriction diet can extend lifespan. However, while exercisers maintained their strength and muscle mass and increased aerobic capacity, those who dieted lost muscle mass, strength and aerobic capacity.

"Exercise-induced weight loss provides the additional benefit of improving physical performance capacity," says Edward Weiss, Ph.D., lead author and assistant professor of nutrition and dietetics at Saint Louis University's Doisy College of Health Sciences.

"If push comes to shove and somebody wants to know if they should diet or exercise to lose weight, I would suggest exercise, provided they are willing to put in the extra time and effort and not offset the gains they make by eating more."

Weiss is a part of a Washington University team of scientists who studied healthy 50- to 60-year olds whose body mass index was between 23 and 30, placing them at the high end of normal weight or overweight.

Of the 34 study participants, 18 dieted and 16 exercised to lose weight.

The goal of dieters was to reduce their calorie consumption by 16 percent the first three months, and by 20 percent the next nine. Likewise, exercisers strove to burn 16 percent more calories the first three months, increasing to 20 percent the next nine months.

Both groups lost between 9 and 10 percent of their total body weight. Those who exercised engaged in 60 minutes of cardiovascular activity six times a week, such as a brisk three- to four-mile walk. Those exercisers who worked out for 90 minutes a day took off more pounds -- 15 to 20 percent of their body weight.

Those who dieted lost muscle mass while those who exercised did not. This is because exercisers routinely challenged their muscles, which prevented muscle tissue from degrading. Dieters didn't work their muscles as vigorously as those who exercised.

"In addition, once a person loses weight, his or her muscles don't have to work as hard at everyday movements, such as rising from a chair, walking up steps or getting out of a car," Weiss says.

"Because they're carrying a lighter load, less demand is placed on them."

The muscles dieters use to carry their bodies "detrain," in much the same way that a weight lifter using less weight in the gym doesn't develop as large of muscles as one who uses heavier weights.

Weiss was careful not to rule out the benefits of dieting, though.

"It's important that dieting not be seen as a bad thing because it provides enormous benefits with respect to reducing the risk of disease and is effective for weight loss," Weiss says. "Furthermore, based on studies in rodents, there is a real possibility that calorie restriction provides benefits that cannot be achieved through exercise-induced weight loss."

In addition, those who exercise may be under the mistaken impression that they can eat more and still lose weight.

"A critical requirement for exercise-induced weight loss is that food intake does not increase," he says.