If you lose 4.5 kg of body weight, you may be lighter, but if 2.3 kg of that loss are muscle, you sure won’t be stronger, and your performance can suffer. Appearance-wise, you can still look flabby when muscle tissue is lost. Exercise is one of the best ways to make sure you are shedding weight from fat stores rather than from muscle stores. Researchers have put this principle to the test. In a study of 10 overweight women, half of the women were placed in a diet-plus exercise group and half of the women in an exercise-only group. The women in the first group followed a diet that reduced their calories by 50 percent of what it took to maintain their weight, and they worked out aerobically six times a week.

The women in the exercise-only group followed the same aerobic exercise program but followed a diet designed to stabilize their weight. After 14 weeks, it was time to check the results. Here is what happened: Both groups lost weight. But the composition of that loss was vastly different between the groups. In the group that dieted and exercised, the weight lost was 67 percent fat and 33 percent lean mass. In the group that only exercised, the women lost much more fat—86 percent fat and only 14 percent lean mass! Not only that but RMR declined by 9 percent among the dieters, whereas it was maintained in those who only exercised.

What does all this tell us? Sure, you can lose weight by low-calorie dieting. But you risk losing muscle. Also, your metabolic rate can plummet, sabotaging your attempts at successful weight control. With exercise and a nonrestrictive diet, you preserve calorie-burning muscle and keep your metabolism in gear.