You ask Margaret Bright her age and she says she's 24. Then she give a nervous little laugh. She knows she looks a lot younger. Maybe 14, or less.

Her eyes are bright, determined, intelligent. She weighs 90 and a fraction, not much on her small, not quite 5-feet-1-inch frame, but that's almost twice what she weighed two years ago. And she's confident, really confident, that now she has this thing under control, this thing that she's spent half her life fighting. This thing called anorexia nervosa that binds its ivctims to a rigid ritual of controlled and methodical starvation from which they will not -- and then, later, cannot -- free themselves.

There is no one who will even estimate the number of cases of anorexia or its sister disorder, bulimarexia, there may be. Many anorexics, of course, end up in hospitals, but even there they often are misdiagnosed. And too often, help is only temporary.

Bulimarexia is even harder to spot. It involves repeated cycles of binge eating and self-induced purging -- by vomiting, by constant use of laxatives or diuretics, or all three.

Bulimics may not exhibit the outward signs of starvation that are the hallmark of the anorexic. They may be of normal weight, even slightly overweight. Some women have been bulimics for years. (It is a common and, within that cultural unit, a more or less accepted phenomenon of ballet schools and companies.) Even when hospitalized for a host of physiological ills their bizarrre behavior can produce, bulimics may escape detection, and continue to lead secret lives of desperation, built, fear, isolation and loneliness.

As a general rule, these are the very emotions which also mark the anorexic.

In the case of both eating disorders, the victims have an overriding terror of losing control. Their weight may be the one thing in their lives that they believe they can control, and control they do, with the iron discipline of a Spartan warrier, the singlemindednes of a suicidal lemming.

Margaret Bright has been through it all. Her father died when she was 10 and her mother, a retired schoolteacher, moved with Margaret and her blind grandmother from their Northwest Washington home to rural Maryland. Margaret says she felt isolated, lonely and altogether miserable as she started junior high in a neighborhood she felt rejected her.

Now she can say, "I think I might have been a little bit of a snob."

But when a classmate teased her about being chubby -- she weighed only five pounds more than she does now -- she says, "I thought if I went on a diet that I would be more acceptable to people." She has been as low as 49 pounds and during the height of the ailment rarely broke 60.

"Of course this disorder has cultural determinants," says Dr. Michael Ebert, chief of the section on experimental therapeutics at the National Institute of Mental Health. "You see it in societies that value the extremely thin person."

Dr. Ebert has been heading an anorexia project at NIH for the past three years in which 50 or so patients have been studied and treated. He is particularly interested in biological aspects and changes in brain chemistry which may control appetite. Early indications from this study, and from other research, point toward a conclusion that although the illness may be psychological or environmental in origin, it may be prolonged by starvation-induced physiological changes.

It may be, Dr. Ebert feels, that the point -- at which appetite is satisfied -- is somehow reset, as a thermostat is reset, so the body no longer perceives itself as hungry. Some recovering patients have spoken of a "high" they experience as they starve, even to death.

Pinpointing chemical changes will make it easier to diagnose the disorder in its early stages, says Dr. Ebert, but even more important, "It may led physicians to find ways to get that faulty appetite thermostat -- the appestat -- to resume normal functioning." Once that appestat malfunction kicks in, it is difficult for even the best motivated patient to reverse the process. i

Margaret Bright has classic anorexia nervosa. She is known as a "starver" as opposed to the bulimarexic "purger."

"I've gone for periods when I have not eaten," says Margaret Bright, "and I've lost a lot of weight, and of course, a lot of other things accompanying it. Your hair falls out of your head, but grows on other parts of your body; you feel cold most of the time. Our blood pressure is low and you feel physically weak.

"But it seems like when you reach a certain point when you're not eating that you feel very energetic. I could go, go, go on a kind of nervous energy that didn't seem to run out."

There is much disagreement among specialists about who is prone to which eating disorders -- chronic obesity, it is felt, for example, may be the other side of the same coin.

But most experts cite the studies of Hilde Bruch (professor of psychiatry at Baylor College of Medicine), who has described it as primarily an illness of adolescent girls from usually affluent families, and possibly most importantly, those eager to please. Her work of the past decade is considered a classic base line from which other research proceeds.

The disorder is not limited to girls or adolescents, although they seem to constitute the largest victim population. According to Vivian Meehan, a nurse in the Chicago area who heads the non-profit organization, Anorexia Nervosa and Associated Disorders (ANAD), it has been seen in children of both sexes as young as 8 and its victims extend well beyond adolescence.

Margaret Bright (who grew 3/4 of an inch last year, more than she'd grown in a decade) and Mary Hall, a Maryland social worker, who has dealt with recovering anorexics and bulimics, are part of a small support group which they would like to extend and expand. Elsie Bright, Margaret's mother, has found considerable help from a parents' support group in suburban Maryland.

The anorexia support group (which includes both "starvers" and "purgers") is, proponents believe, particularly useful -- in conunction with individual therapy -- because it helps dispell the sense of isolation and shame so often found among the mostly young, often gifted victims.

Pat Howe, a recovered anorexic, is head of the National Anorexic Aid Society, Inc. (NAAS) in Columbus, Ohio. She has helped form support groups in 24 cities and can barely keep up with a constant flood of inquiries. Both NAAS and ANAD help match sufferers with therapies in all parts of the country.

There is no widely accepted "cure" for the problem, but with sensitive psychotherapy -- both individual and family -- some behavior modification techniques, support groups and community understanding, many young people are being helped.

Ohio's Pat Howe, who vanquished the disorder a decade ago, is optimistic that cures are possible. Social worker Mary Hall notes that, after all, "It is just a caricature of a problem we all fight . . ." in a culture in which success is defined as much in thinness (and now, fitness) as in wealth.

For women, now pressured to achieve more than ever before, it is even more prevalent to think that "if only, magically, we had this perfect figure . . . If only I were just a little thinner, I'd hit that magic place where all will go right with my life . . ."