In spite of the insouciance with which dieting advice is dispensed, no one has properly investigated the question of whether slimming down to "ideal" weight produces longer life. The proper study would compare the longevity of people who are at their ideal weight without dieting to people who achieve their ideal weight by dieting. Without this study the common medical advice to diet down to your ideal weight is simply unfounded.

This is not a quibble; there is evidence that dieting damages your health and that this damage may shorten your life.

MYTHS OF OVERWEIGHT

Physical inactivity is a major cause of obesity. Probably not. Fat people are indeed less active than thin people, but the inactivity is probably caused more by the fatness than the other way around.

Overweight shows a lack of willpower. This is the granddaddy of all the myths. Fatness is seen as shameful because we hold people responsible for their weight. Being overweight equates with being a weak-willed slob. We believe this primarily because we have seen people decide to lose weight and do so in a matter of weeks.

But almost everyone returns to the old weight after shedding pounds. Your body has a natural weight that it defends vigorously against dieting. The more diets tried, the harder the body works to defeat the next diet. Weight is in large part genetic. All this gives the lie to the "weak-willed" interpretations of overweight. More accurately, dieting is the conscious will of the individual against a more vigilant opponent: the species' biological defense against starvation. The body can't tell the difference between self-imposed starvation and actual famine, so it defends its weight by refusing to release fat, by lowering its metabolism, and by demanding food. The harder the creature tries not to eat, the more vigorous the defenses become.

BULIMIA AND NATURAL WEIGHT

A concept that makes sense of your body's vigorous defense against weight loss is natural weight. When your body screams "I'm hungry," makes you lethargic, stores fat, craves sweets and renders them more delicious than ever, and makes you obsessed with food, what it is defending is your natural weight. It is signaling that you have dropped into a range it will not accept. Natural weight prevents you from gaining too much weight or losing too much. When you eat too much for too long, the opposite defenses are activated and make long-term weight gain difficult.

There is also a strong genetic contribution to your natural weight. Identical twins reared apart weigh almost the same throughout their lives. When identical twins are overfed, they gain weight and add fat in lockstep and in the same places. The fatness or thinness of adopted children resembles their biological parents--particularly their mother--very closely but does not at all resemble their adoptive parents. This suggests that you have a genetically given natural weight that your body wants to maintain.

The idea of natural weight may help cure the new disorder that is sweeping young America. Hundreds of thousands of young women have contracted it. It consists of bouts of binge eating and purging alternating with days of undereating. These young women are usually normal in weight or a bit on the thin side, but they are terrified of becoming fat. So they diet. They exercise. They take laxatives by the cup. They gorge. Then they vomit and take more laxatives. This malady is called bulimia nervosa (bulimia, for short).

Therapists are puzzled by bulimia, its causes, and treatment. Debate rages about whether it is an equivalent of depression, or an expression of a thwarted desire for control, or a symbolic rejection of the feminine role. Almost every psychotherapy has been tried. Antidepressants and other drugs have been administered with some effect but little success has been reported.

I don't think that bulimia is mysterious, and I think that it will be curable. I believe that bulimia is caused by dieting. The bulimic goes on a diet, and her body attempts to defend its natural weight. With repeated dieting, this defense becomes more vigorous. Her body is in massive revolt--insistently demanding food, storing fat, craving sweets, and lowering metabolism. Periodically, these biological defenses will overcome her extraordinary willpower (and extraordinary it must be to even approach an ideal weight, say, 20 pounds lighter than her natural weight). She will then binge. Horrified by what this will do to her figure, she vomits and takes laxatives to purge calories. Thus, bulimia is a natural consequence of self-starvation to lose weight in the midst of abundant food.

The therapist's task is to get the patient to stop dieting and become comfortable with her natural weight. He should first convince the patient that her binge eating is caused by her body's reaction to her diet. Then he must confront her with a question: Which is more important, staying thin or getting rid of bulimia? By stopping the diet, he will tell her, she can get rid of the uncontrollable binge-purge cycle. Her body will now settle at her natural weight, and she need not worry that she will balloon beyond that point. For some patients, therapy will end there because they would rather be bulimic than "loathsomely fat." For these patients, the central issue--ideal weight versus natural weight--can now at least become the focus of therapy. For others, defying the social and sexual pressure to be thin will be possible, dieting will be abandoned, weight will be gained, and bulimia should end quickly.

These are the central moves of the cognitive-behavioral treatment of bulimia. There are more than a dozen outcome studies of this approach, and the results are good. There is about 60 percent reduction in hinging and purging (about the same as with antidepressant drugs). But unlike drugs, there is little relapse after treatment. Attitudes toward weight and shape relax, and dieting withers.

Of course, the dieting theory cannot fully explain bulimia. Many people who diet don't become bulimic; some can avoid it because their natural weight is close to their ideal weight, and therefore the diet they adopt does not starve them. In addition, bulimics are often depressed, since binging-purging leads to self-loathing. Depression may worsen bulimia by making it easier to give in to temptation. Further, dieting may just be another symptom of bulimia, not a cause. Other factors aside, I can speculate that dieting below your natural weight is a necessary condition for bulimia, and that returning to your natural weight and accepting that weight will cure bulimia.