Scientific research has yet to show conclusively that exercise leads to permanent weight loss.
If lean people are more physically active than fat people—one fact in the often-murky science of weight control that’s been established beyond reasonable doubt— does that mean that working out will make a fat person lean?
The American Heart Association and the American College of Sports Medicine publish joint guidelines for physical activity and health. They suggested that 30 minutes of moderate physical activity five days a week is necessary to “promote and maintain health.”
But they didn’t say that more physical activity will lead to loss of weight.
The best they could say about the relationship between fat and exercise was: “It is reasonable to assume that persons with relatively high daily energy expenditures would be less likely to gain weight over time, compared with those who have low energy expenditures. So far, data to support this hypothesis are not particularly compelling.”