Women such as these may be at particular risk of dangerously low weight and potentially fatal consequences, according to the study authors. Targeting the anxiety and obsessive tendencies so common in these women might aid in treating the eating disorder, they report in the International Journal of Eating Disorders.

Doctors have known that excessive exercise is a common feature of eating disorders like anorexia and bulimia, but it hasn't been clear which women are most likely to have the problem.

For the current study, researchers led by Dr. Cynthia M. Bulik of the University of North Carolina-Chapel Hill used data from three international studies of women with anorexia, bulimia or both.

The women completed standard questionnaires on eating disorder symptoms, personality traits and exercise habits. Excessive exercise was defined as exercising more than 3 hours a day or being otherwise obsessed with daily physical activity - letting it interfere with other aspects of life, for example, or exercising even when injured or ill.

Although excessive exercise was common regardless of the type of eating disorder, the study found, it was most common among anorexic women who purged. Of these 336 women, more than half exercised excessively.

Women with high levels of anxiety, obsessiveness and perfectionism were also particularly likely to exercise to an extreme degree, the study found. Such personality traits are common among anorexics who purge to control their weight.

It makes sense that these women would be particularly likely to "use all available methods in their drive for thinness and control," according to Bulik.

The findings could aid in treatment, she told Reuters Health, by helping doctors know which eating disorder patients need to be more extensively screened for extreme exercise habits.

"Clinically," she explained, "we know that when we send people back home and they have a strong drive to exercise that it can negatively impact on their ability to maintain the weight that they worked so hard to gain in hospital."

Excessive exercise is a symptom that "requires more vigilance and understanding," Bulik said, so that patients can be taught how to include healthy exercise levels in their lives, without losing any gains they've made in controlling the eating disorder.