One of the most widely accepted assumptions (假设) in our culture is that if you exercise, you will lose weight．I exercise all the time, but I still have got fat．Why does this happen?

    More than 45 million Americans now belong to a health club, up from 23 million in 1993．We spend some $19 billion a year on sports．Of course, some people join the health club but never go．Still, as one main study—the Minnesota Heart Survey—found, more of us at least say we exercise regularly．

    However, the number of fat people has risen greatly: a third of Americans are fat, and another third count as overweight．Yes, it's highly possible that those of us who regularly go to the sports center would weigh even more if we exercised less．But like many other people, I get hungry after I exercise, so I often eat more on the days I exercise than on the days I don't．Could exercise actually be keeping me from losing weight?

    People believe that "exercise is important for weight control" is actually rather new．Doctors used to advise not to exercise too much, particularly for older adults who might hurt themselves．Today doctors encourage even their oldest patients to exercise because people who regularly exercise are at a lower risk for most illnesses, especially heart problems．They less often develop cancer and many other illnesses, but the role of exercise in weight loss has been completely overestimated (评估过高)．

    "In general, for weight loss, exercise is pretty useless," says an exercise researcher．