It is so simple the way the doctors explain it: take in more calories than you burn, and you gain weight; burn more than you take in and you lose weight. That, they tell us, is why the miracle diet plans never work. There's no magic to it. Weight loss is simply a matter of taking in less and/or burning more.

The doctors are wrong. I've known it for a long time. Still it is reassuring to see someone else say so in print (as William Bennett and Joel Gurin have done in American Health magazine). For a rough model of how the doctors say weight gain and loss works, think of a balloon with a small leak. If you blow more air into the balloon than escapes through the leak, you will have a fat balloon. Substitute calories for air and exercise for the leak, and you understand why your body looks like a balloon. But if you can stop feeling guilty long enough to think, you will see why the analogy, like the theory behind it, is wrong. Keep blowing in more air than leaks out and the balloon gets bigger and bigger until it bursts. Your fat friends, on the other hand, tend to overeat by about the same amount every day and yet they don't keep getting bigger. Mostly they remain about the same size. According to the balloon theory, this shouldn't happen.

What seems to be the case, according to Bennett and Gurin, is that each of us has a "setpoint" for body fat, a level of fatness that our bodies try to maintain, much as a heating system strives to maintain the temperature level of the thermostat. "For practical purposes," they write, "you can think of your own setpoint as the weight you tend to stay with when you aren't trying to control calories. You might secretly believe that your weight would ballon up to an infinite number of pounds if you let yourself go. In fact, at some point--your setpoint-- you would level off."

The key, apparently, is that the body is able to adjust the efficiency with which it burns fat, getting by on very little when intake is reduced and turning into a veritable fat guzzler when intake is up. This is what makes dieting so frustrating and so futile. The theory squares with what the overweight among us have always known but were afraid to admit because it has the ring of rationalization: fat people don't have to eat very much to remain fat, and skinny people can eat like pigs and still look like gazelles. Only those blessed with low setpoints see overweight in moral terms.

I wouldn't be telling you all this if there were nothing to be done, if you were stuck for all time with your cursed setpoint. You can change it, the authors insist, with exercise. But again, it isn't like the doctors tell us. It has nothing to do with mathematical formulae--run nine times around the block if you want to eat a potato. "Although we burn only a few calories during a run around the block," say Bennett and Gurin, "the effects of jogging, rapid walking, or any regular endurance exercise are long-lasting. An active body is automatically 'set' by exercise to be leaner than an inactive one."

But again, no miracles. You might lower your setpoint a few notches by regular exercise, but you are not going to turn your naturally plump body into a svelte one. And you just might ruin your health in the attempt. Rotundity may be an unfortunate condition, considering the American notion that slimness is attractive. But it isn't evidence of a moral flaw.