Research reveals that one out of three Americans is obese, making overfatness nothing short of a national epidemic. In spite of the fact that over $30 billion is spent annually on diet programs, the incidence of obesity has increased by 25 percent in the last 15 years. What's wrong with this picture?

First, exercise works, but too few people are doing it. Second, everyone is dieting, but diets don't work. Well, not everyone is dieting, but research reveals that one out of two American women and one out of four American men are presently on a calorie-restricted diet.

Why don't diets work? If you cut down the number of calories you consume, you should lose weight, right? Yes, and you definitely do lose weight on calorie-restricted diets. But your weight loss is almost always short-lived. Studies on calorie restriction and weight loss show that 95 percent of all dieters regain all of the weight they lost within the following year. Whatever happened to willpower?

Actually, willpower has very little to do with whether or not the lost weight is regained. It's more a matter of physiology than psychology. About 25 percent of the weight loss is muscle tissue, and less muscle results in a lower metabolic rate. In other words, you have less vital tissue to service, so you burn fewer calories daily. Meanwhile, your body adjusts to the low-calorie diet by slowing down your metabolism even further.

You may not notice the detrimental effects of a slow metabolism while dieting, but as soon as your calorie intake is increased, which must happen for you to survive, you will be surprised at how quickly that weight you lost comes back.

So why do people work so hard on something that works so poorly? Primarily because diets do produce satisfying short-term results. With exercise, short-term changes in total bodyweight are relatively small because exercisers typically add lean weight and lose fat weight at the same time.

For example, in a classic study conducted at Tufts University, 12 senior subjects were placed on a basic program of strength exercise. They performed about 30 minutes of strength training three days per week for 12 weeks. As a result they added 3 pounds of lean (muscle) weight and lost 4 pounds of fat weight. Although their bodyweight changed by one 1 pound, they actually made a 7-pound improvement in body composition.

What's more, to keep the participants' bodyweight relatively constant, the researchers had to increase their caloric intake throughout the study period. During the last few weeks of the study, the exercisers were eating 15 percent more calories every day to maintain their bodyweight. Unlike dieting, the strength exercise resulted in more muscle and the need for more calories as the subjects lost fat. Of course, if the subjects had not increased their calorie consumption throughout the study, they would have lost even more fat weight.

In our own larger study, 383 men performed 25 minutes of strength exercise and 25 minutes of endurance exercise two or three days a week for eight weeks. This combination exercise program produced a 6.4-pound fat loss and a 3.7-pound muscle gain, for a 10-pound change in body composition.