If you are sitting down reading this and are not otherwise active, your major source of fuel is stored fat. This is surprising to many, since if humans use stored fat as a major source of energy during rest, why are so many people obese? The answer is that the amount of fat released while sedentary is minuscule and doesn't make a dent in the overall systemic body fat stores. To do that you need to exercise and control daily caloric intake. The tried and true rule about this is that to lose body fat you need to ingest fewer calories than you oxidize or burn through physical activity. Most health authorities suggest that all diets, no matter what their composition is, work only if the calories are reduced enough to make inroads into stored body fat. Other self-styled "experts" often write or say that exercise is futile for attempting to lose superfluous body fat. They often point out that a pound of fat contains an average of 3,500 calories, yet the average workout only burns between 200 and 500 calories per session, depending on the intensity level and the time involved, as well as the size of the person. Several problems exist with this "exercise doesn't help you lose body fat" idea. For one, if you just reduce total caloric intake and don't do any exercise, a large percentage, up to 50%, of your weight loss will consist of muscle mass. This is bad because muscle mass is directly related to resting metabolic level. If you lose a lot of muscle while dieting, your resting metabolic rate will significantly decline. In practical terms, this means that you would have to consume fewer calories than before you dieted to maintain your weight loss.
An example of how extreme dieting can adversely affect the resting metabolic level and thus promote weight regain involves contestants who participated in the popular "Biggest Loser" television show. Many of these contestants lost as much as 100 pounds over a period of a few months through a drastic reduction in their caloric intake coupled with extreme exercise. While there is little doubt that these people lost a lot of body fat, their extreme dieting also cost them a great deal of muscle mass. For this reason, nearly all of the past contestants on the Biggest Loser regained their lost body fat. A study of some of them published two years ago showed that those who gained the weight back still showed significant drops in their resting metabolic rate six years after they participated in the show. On the other hand, the few who managed to keep the weight off after six years continued to stay on a reduced-calorie diet, but more importantly continued to exercise regularly, although far less intense than they did as contestants . . .