Anorexia nervosa is regarded as an eating disorder. But Sarah Leibowitz looks on it as an appetite disorder. Where does body image come into it? After the physiology. She believes it's a case where physiology sets the stage for the psychology, and the neurochemical switching on of fat appetite is the trigger event.

"If physiology is telling you you're going to be a little bit heavy, anyone who has a problem with body weight, body shape is going to have the stage set for a problem." Leibowitz insists that anorexia is a no-holds-barred attempt to avoid fatness in those predisposed to plumpness either because of family history or metabolism. "They are," she says, "trying to avoid the inevitable."

Explaining her neurochemical model of anorexia, Leibowitz points out that anorexia typically starts at puberty - just when the brain's eating center suddenly switches on the taste for fat in foods. "You see yourself getting fat, and the culture is telling you it's wrong. Your boyfriend is telling you not to get fat. Your mother is telling you not to get fat. Then you come to this bottleneck in development. You're asking, 'Who am I?' 'Am I going to be as fat as my mother?' Psychology plays into it, but it is also a matter of just fighting a natural tendency."

Her view contradicts conventional wisdom, and regards the failure of sexual maturity that typifies anorexia as a consequence of the disorder, not its raison d'etre. Before puberty, appetite in girls is dominated by a physiologically based preference for carbohydrate. Production of Neuropeptide Y in the supraventricular nucleus of the hypothalamus presides over this taste preference.

At puberty, rising levels of estrogen turn on production of the neurochemical galanin, which drives an everyday appetite for fat. It's part of nature's attempt to ensure a future for the species; reproduction in women requires a substantial amount of body fat. With their energy needs so intertwined, the brain center for eating behavior and the brain center for reproduction, located just next to it in the hypothalamus, coordinate their activities through regular neurochemical cross talk.

When the switched-on taste for fat alerts young women to the possibility of getting fat, some women swing into high gear - notably those with obsessive-compulsive personality traits. Performing a behaviorally heroic feat, they shun fat altogether. They cling instead to the prepubertal pattern of food intake - very small, frequent meals based on carbohydrate. "They're trying to keep going all day on what is normally that early-morning feeling. It takes very little food." They are literally starving.

Here's the catch. Starvation pumps up abnormally high levels of Neuropeptide Y in the brain. Neuropeptide Y confines their dietary interest to foods with carbohydrate. But the high levels of Neuropeptide Y have an effect on the sex center next door as well; they turn off production of gonadal hormones, which diminishes sexual function.

"It's important for anorexics and bulimics to know that there's this chemistry of the brain that they're fighting. Then they don't feel that they're just crazy. They're fighting nature. Of course, the approach is to alter that tendency before they become anorexic."

Not surprisingly, Leibowitz's unconventional view of the disorder leads her into a new approach to treatment. She wants to temporarily shut off production of galanin at puberty, the neurochemical that turns on the appetite for fat. "Eating disorders set in very close to the onset of menstruation. We find that estrogen increases the production of galanin, and it makes us want to eat. It makes us want to deposit fat, and it makes us want to eat fat."

She has administered to animals a substance that blocks production of the appetite stimulant for fat. It is an antagonist to galanin. "It is a newly developed experimental drug, M40. The animals just stop eating fat. It doesn't affect carbohydrate or protein intake. Now, if we can work with an individual who is getting all stressed out about having to eat fat, we can help her over the hump with the drug. Then we have a fighting chance to bring on board behavioral modification, nutrition, and education, which work more gradually to control appetite."