http://win.niddk.nih.gov/publications/physical.htm
Weight-Control Information Network, National Institute of Diabetes and Digestive and Kidney Diseases
Physical activity helps you control your weight by using excess calories that would otherwise be stored as fat. Most foods you eat contain calories, and everything you do uses calories, including sleeping, breathing, and digesting food. Balancing the calories you eat with the calories you use through physical activity will help you reach and maintain a healthy weight.