If you're not familiar with the number of calories burned during exercise, you may be discouraged when you first learn about it. For instance, if you weigh 150 pounds and go on a brisk, 1-mile walk for 20 minutes, you'll expend about 100 calories, considerably short of the 3,500 calories needed to drop a pound. But such efforts add up. If you expend an extra 300 calories a day through activity and reduce your dietary intake another 200 calories, then by the end of a week, you'll have a calorie deficit of 3,500, comparable to a one-pound weight loss. This is precisely the kind of gradual success that experts recommend for long-term weight management.
Exercise also has other body-slimming effects.
It builds muscle and displaces fat. A given volume of muscle weighs more than the same bulk of fat. So your bathroom scale may not record dramatic changes, but your clothes will be looser, and you'll have a trimmer body shape. Because exercise builds muscle, it may also help counter a problem caused by dieting. When you reduce calories, your body metabolism may slack off and burn calories more slowly.