It is a ritual performed in private in front of a mirror.

A young woman takes off her clothes and stands intently comparing herself to the arresting photo of a lean, taut-bodied female in a health and fitness magazine. Thighs too fat, she assesses her reflection, arms too flabby, waist too thick, body too imperfect, life too hopeless.

Unless, of course, she loses more weight. Too thin? That could never be a problem.

The young woman in the mirror has anorexia nervosa, and her ritualized comparison to an idealized, impossible body image is not unusual, said Steven R. Thomsen, an associate professor of communications at Brigham Young University in Provo, Utah, who interviewed a sampling of young women, mostly college students, diagnosed with eating disorders.

``They literally study these fitness magazines,'' Thomsen says. ``They tear out images and actually keep file folders. `These are the legs I want. These are the hips I want. This is the butt I want.' They organize their bodies according to images in these magazines.''

Inevitably, they don't measure up to the perfect body images. What follows is lower self-esteem, depression and more of the unhealthy behaviors that mark anorexia - severely restricted eating and use of diet pills, laxatives and diuretics.

``Anybody who doesn't think this is a serious disease needs to study this,'' Thomsen says.

Eating disorders, such as anorexia and bulimia nervosa - marked by binge eating and self-induced vomiting or use of laxatives - are not failures of character or will. They are psychiatric illnesses. Estimates of prevalence vary, but psychologist Michael Levine of Kenyon College in Gambier, Ohio, says 10 percent to 15 percent of girls and women from ages 12 to 30 are suffering from some combination of distorted body image and unhealthy eating patterns.

Bonnie L. Blankmeyer, clinical professor of psychology and head of the eating disorders program at the University of Texas Health Science Center medical school's department of psychiatry, says the problems start early. She cites studies showing 53 percent of American 13-year-olds are unhappy with their bodies. Seven-year-old girls agonize over chubby cheeks.

Indeed, Thomsen's interviews with college women spurred his research team to look at a younger sample. They surveyed 498 sophomores, juniors and seniors at two Salt Lake City high schools, asking girls how often they read health and fitness magazines, such as Shape, Sports Illustrated for Women and Self. They also asked whether, over the past year, they had used laxatives or diet pills, made themselves throw up or limited themselves to fewer than 1,200 calories per day, considered an extremely low caloric intake. The research was published in the American Journal of Health Education in June.

Thomsen's findings show that:

- 46 percent of the girls read the health and fitness magazines frequently, at least once a month.

- 73 percent of girls who used appetite suppressants or weight-control pills were frequent readers.

- 60 percent of those who used laxatives frequently read them.

- 80 percent of girls who made themselves vomit were frequent readers.

- Among frequent readers, twice as many girls restricted calories to fewer than 1,200 a day.

``We can't say that one causes the other,'' Thomsen says of the link between fitness-magazine reading and eating disorders. ``But the two appear to be associated. When one increases, the other increases. There's a correlation.''

It's a normal part of adolescence to compare yourself to those around you, the researcher says. But in this case, the girls may have used these magazine images to support their anorexic thoughts and feelings.

Beauty and fashion magazines, with their images of improbable, reed-thin, size 2 models, have been associated with unhealthy eating patterns.

``This is the first study I know of that has looked at health and fitness (publications) and a teen readership. It breaks ground,'' says Sammye Johnson, a professor of communications at Trinity University in San Antonio who studies magazine images of women. ``By reading Shape and Fitness, the girls are saying, `I can be thin and still be healthy.' There is a validation here. But is it realistic for a 15-year-old to look like a woman in Shape?''

Levine explains that while these muscle manuals tout healthful diets and exercise, in some ways they send out messages that are quite similar to the newsstand beauty books.

``The first message is that image is more important than substance,'' Levine says. ``The image becomes the substance. Second, adhering to a certain standard of beauty is very, very important. There's not a lot of margin for individual standards. Third, the gold standard of beauty and fitness is slenderness. If you're slender, you must be fit.''

Yet another message, ``Slenderness says a lot about you. It is heavily laden with meaning,'' the psychologist says. ``It says you are good-looking, feminine. You have self-control. You are a good person.''

Thomsen finds that in addition to taking away distorted messages from the visual images in fitness magazines, readers with eating disorders distort the messages in well-intentioned articles.

Thomsen says the subjects in his studies understand eating disorders are deadly serious, but many are in denial.

For them, ``the message in these fitness magazines has become very malleable. One young woman told us, `My friends are telling me I'm too thin. I need to eat more. Yet I can open (them) up and see women who are just as thin, and they look happy. How can what I'm doing be so bad?'''

From Healthy.net