An enduring myth ties exercise to weight loss. What is often ignored is that physical activity generally stimulates appetite. People who exercise strenuously are also less active afterward than they would have been otherwise. Therefore, the tendency is to eat more and move less after exercising.
According to a 2000 study, willpower is a muscle. It paradoxically weakens with use. So the self-discipline that propels people onto a treadmill can be extinguished with the effort, making that post-workout beer much more inviting.
Another dubious claim is that greater muscle mass burns more calories at rest. A pound of muscle burns about six calories a day, compared with two calories for a pound of fat. Packing on 10 pounds of muscle, which is extremely difficult, would burn an extra 40 calories a day. That is the equivalent of six baby carrots.
What exercise can do is play a supporting role in weight control. The National Weight Control Registry chronicles the lives of those who have shed pounds and kept them off. About 90 percent of the people on the registry exercise in addition to eating less food.
Obesity is highly genetic. British researchers examined 20,000 people to determine the effects of 12 genes associated with a higher risk of obesity. They determined that an hour of daily exercise cut the genetic obesity risk by 40 percent.