Jamie-Lynn DiScala's exercise routine once rivalled, and probably exceeded, that of many Olympic athletes. The 24-year-old actress, who plays Meadow in the TV show The Sopranos, used to wake at 3am to work out for several hours and then try to squeeze as much exercise as possible into the rest of her day. Such extraordinary self-discipline did not, however, leave her a picture of health, nor did she experience any of the mood-boosting highs associated with keeping fit. Instead, DiScala admitted in an interview with CNN that she "seriously contemplated suicide" after struggling with exercise bulimia, the relatively new, but increasingly common disorder in which the purging of calories through workouts becomes an obsession.

It started, she said, by "maybe doing 20 minutes on the treadmill in the morning , then deciding I wasn't going to have any dessert any more". She then began pre-breakfast workouts. Before long, exercise occupied her every thought. "I would set things so far apart on the [breakfast] counter where I would have to walk up and down the kitchen to make it just so I could constantly be burning calories."

Exercise bulimia is a disorder that experts believe has risen sharply in the past decade as a result of our preoccupation with gym culture and media pressures not just to be thin, but toned. It bears similarities to the eating disorders anorexia and bulimia nervosa in that most sufferers have underlying issues with low self-esteem and body image. But rather than employing a socially unacceptable method of dealing with calories (like food restriction and self- induced vomiting), exercise bulimics choose to purge their bodies of nourishment solely through sweaty endeavour.

"Unlike so-called 'exercise addicts', they are not dependent on exercise for its health benefits," says Professor Ian Cockerill, an exercise psychologist and research fellow at the university of Birmingham. "In some ways it is a psychological bid for them to gain a feeling of discipline and control in their lives."

How many people suffer from the condition is uncertain. Eating-disorder experts estimate that between 1% and 4% of the population display exercise bulimic tendencies, but by its very nature it is difficult both to diagnose and to quantify. "It's hard for professionals to draw the line between what is reasonable and what is out of control when you see someone exercising a lot," says Stuart Biddle, a professor of exercise psychology at Loughborough University.

Warning signs include increased workouts, a preoccupation with exercising before or after meals, and visible weight loss. "It's not uncommon for them to purge before a meal, to justify eating and to make sure they are in calorie debt," says Cockerill. "If they have run two miles or flogged themselves up and down the pool, they have rid themselves of some guilt."

Still, DiScala's behaviour, says Steve Bloomfield, a spokesperson for the Eating Disorders Association, "is not at all unusual". Unlike anorexia, exercise bulimia is not a predominantly female disorder and is more prevalent among those in their late 20s and older than younger age groups.

The consequences of the condition can be severe. Within four months of embarking on her fitness frenzy, DiScala weighed just five and a half stone. "I was wearing children's clothes," she says. "Every week I would see the reflection of my back and see more bones coming out." Most sufferers develop overuse injuries such as shin splints, many experience anaemia and poor circulation and women often have exercise amenorrhoea, which causes their periods to stop. When this happens, it is a warning sign that levels of the hormone oestrogen have plummeted to those of a postmenopausal woman. Since oestrogen is vital for the normal development of bone, it can mean a woman may start losing bone mass instead of building it, putting her at risk of osteoporosis.

But most exercise bulimics never seek treatment, partly because they excuse their excessive exercise habits, both to themselves and others, as a healthy obsession. Julie Timmins, 32, a marketing executive from Kent, says she dismissed concerned comments from family and friends for 18 months before accepting that her exercise habits had spiralled out of control. "It got to the point where I was constantly agitated and would do anything to keep moving," she says. "If I ate a biscuit I would go upstairs and perform 100 press-ups or run on the spot just for 10 minutes to make sure the calories didn't go to my hips. At the time I felt a bit superior to friends who thought I'd lost the plot. I thought I was super-fit and super-healthy."

Eventually, because she became depressed about her routine, Timmins was referred to a consultant psychiatrist by her GP. "It took a couple of years for me to realise that I didn't really enjoy all the working out. It didn't leave me on any sort of high - it was just a stress in my life and something I did out of guilt," she says. Now, she does little more than a daily walk to keep fit and has "never felt more healthy or fulfilled".

For DiScala, the turning point came when she was late for an inline-skating session, meaning her strict exercise schedule was disrupted - a disaster for an exercise bulimic. "I was shaking and crying in the back of the car and my parents were crying because they didn't know what to do," she says. The following day, she admitted her problem, and saw a nutritionist and a therapist who prescribed Prozac.

Now a spokesperson for the National Association of Eating Disorders in America, DiScala raises awareness about the illness that plagued her early career. "I thought that that was my life. I was set. This was the way I was going to have to live," she says. "And knowing that I was able to overcome it and be healthy and happy again is amazing."

Cockerill says treatment for the condition varies according to its severity, but, as with eating disorders, counselling and antidepressants are options. "We should remember that exercise is a very positive thing when you organise it to fit in with your life," he says. "But once someone begins to fit their life around exercise, it becomes a negative factor and they usually need help."