A study in the United States has found that one of the warning signs of an eating disorder is excessive exercise.
Researchers at the University of North Carolina say the problem may be particularly common among anorexic women who vomit or use laxatives to lose weight.
According to the study authors such women may be putting themselves at particular risk of dangerously low weight and potentially fatal consequences, and treating the eating disorder might be best tackled by targeting the anxiety and obsessive tendencies.
Doctors have known for some time that excessive exercise is a common feature of eating disorders like anorexia and bulimia, but it has been unclear which women are most likely to have the problem.
For the study, Dr. Cynthia M. Bulik and a team of researchers from Chapel Hill looked at data from three international studies of women with anorexia, bulimia or both.
The women were asked standard questions on eating disorder symptoms, personality traits and exercise habits.
For the study excessive exercise was defined as exercising more than 3 hours a day or being so obsessed with daily physical activity that it interfered with other aspects of life, such as 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, personality traits which are common among anorexics who purge to control their weight.
Bulik says it makes sense that such women would be particularly likely to "use all available methods in their drive for thinness and control," and she believes the findings could help with treatment.
Bulik says patients are sent home and the strong drive to exercise can have a negative impact on their ability to maintain the weight gained in hospital.
She says excessive exercise is a symptom that "requires more vigilance and understanding," so that patients can be taught how to include healthy exercise levels in their lives, without losing any gains made in controlling the eating disorder.
Experts in Australia have also expressed concern over the number of children being hospitalised with eating disorders which has trippled in the past two years, with sufferers as young as 10 being admitted with anorexia and bulimia.
Doctors say society's obsession with rising obesity has led to four and five-year-olds worrying about getting fat, and an alarming rise of admissions to hospitals.
Of particular concern is the increase in admissions of those under 14, which rose from five in 2003 to 25 last year.
The study is published in the International Journal of Eating Disorders, September 2006.