Sad, but unfortunately true - exercise alone is not enough to ensure weight loss. In a recent small study, most of the 81 overweight and sedentary women who volunteered for a 12-week supervised exercise program ended up putting on weight, in the form of fat. Some gained as much as 10 pounds despite walking on treadmills three times a week for 30 minutes at a pace representing 70 percent of their maximum endurance. The upside here is that when the program ended, all of the women were aerobically more fit than they had been at the start. In addition, several of the participants had more positive results: some of the women were losing weight four weeks into the program and continued to do so after the exercise program ended. The probable explanation for the varied results of the participants is caloric intake. When they joined the study, the women were asked not to make any changes in their diets, but study leader Glenn Gaesser suggested that the 70 percent of the women who gained weight were likely to have eaten more and moved less than they usually did when they weren't on the treadmills. Dr. Gaesser noted that if you really want to lose weight when undertaking an exercise program, keep checking your bathroom scale and if you're not losing weight, cut back on your food intake.
My take? My own experience is that exercise alone is much less effective at promoting and maintaining weight loss than exercise combined with a positive change in eating habits. According to the American College of Sports Medicine (ACSM), to maintain weight you have to perform between 150 - 250 minutes of moderate intensity physical activity per week - that translates to between 2 1/2 and about four hours per week. But if your goal is to lose significant weight, the ACSM says you'll have to put in more than 250 minutes (four hours plus) per week. And to prevent regaining any weight you do lose, you'll need to continue to exercise more than four hours a week.