Could you be a fitness junkie?

"Keep-it-up-keep-it-up. Up! Up! Up!" The manic, repetitive beat booms out of the fitness centre in Crouch End, north London, where evening aerobics is in full swing. Up front on a raised dais, Sally Plant ("Super Sal") drives her third class of the day to ever higher peaks of V02 max (work rate) and leads by example. She is superbly tuned, toned, sleek, focused and driven, and to her acolytes - the 25 leotards and tracksuits puffing away on plastic mats - a figure to admire, perhaps desire. "Stretch- two-three-four" ... her torso unfurls with obscene grace, her metronome- like chant reinforcing what we already know: that regular repeated exercise is "boring, boring, boring", but that if we persist it will make us feel "good, good, good". In tight, black midriff-top and Lycra thigh-squeezing shorts, Sally, like so many aerobics and gym instructors, appears the seductive image of total health and fitness.

Nothing could be further from the truth. Sally, a 28-year-old single mother who hails from "a middle-class cul-de-sac in the Midlands", is by her own admission a "compulsive exerciser" who last felt good after working out more than two years ago. Her body is so run down, so exhausted, that she struggles to get up in the morning. Sex? Forget it. Last year she had flu most of the time.

Four weeks ago, a vertebra rotated out of joint and her neck got stuck. Her muscle and joint alignment was so bad that the chiropractor was convinced she had been in a car accident and was suffering from whiplash. Doctors plead with her to give her body a break, but to no avail. "It would be sensible to stop for a while, but I can't because the physical pain I feel from training is not nearly as bad as the anxiety, self-loathing and flabbiness I feel when I don't work out," she says, munching an apple after class.

Sally started exercising as a teenager because she had a complex about her "chubby legs", but it was only when she split up with the father of her son a few years ago that she started becoming obsessive. "I felt demoralised and worthless when the relationship broke down, but I wanted to prove that I still had a life and that I was strong," she says. "Initially the exercise made me feel good about myself; I got attention from men and my self-esteem was boosted. I started teaching once a week, then once a day, then two to three times a day, doing more and more to get the same high. Then the high seemed to disappear and now I need constant calorie burn just to feel normal.

"My whole life has been affected. Men don't like coming second to sit- ups, so my relationships tend to be transitory. Holidays are a disaster - I can't go anywhere unless there's a gym to work out. I push myself to the max every day and anything less is unfulfilling. I'm a case, an addict in every sense of the word."

It is not known how many people are "addicted to fitness" in the UK, though a few classic case studies abound. One woman was so committed to her exercise that after having a baby, she plonked her infant in a pram in the middle of a park and ran round it. According to the Fitness Industry Association, only 10 per cent of the adult British population exercise regularly, twice as much as a decade ago, but for most of us the problem is still too little exercise rather than too much. The widespread discovery in 1975 of the existence of endorphins (endogenous morphines, the opiate- like substances produced by the body during sustained physical exercise) and the prevalence of the fitness craze since its crash- landing from the US in the Eighties has brought a growing clinical interest in exercise addiction. (Today, more than pounds 500m is spent annually on entrance fees for fitness facilities in the UK.)

Does addiction to exercise really exist? If so, is the addiction - defined as an inability to spend any length of time without exercise - physiological or psychological? Last week, in an attempt to raise the profile of this controversial subject, the British Psychological Society published a research booklet entitled Exercise Addiction.

Hannah Steinberg, former professor of psycho-pharmacology at University College, London, now visiting professor at Middlesex University and a contributor to the booklet, says that much of the research is in its infancy and is still incomplete, but exercise addiction seems to resemble drug addiction in five ways: the activity is initially pleasurable and leads to repetition; the activity becomes excessive and compulsive and begins to take over their life; pain thresholds are raised, withdrawal symptoms (anxiety and irritability) occur if addicts are suddenly deprived, but the feelings disappear as soon as the activity resumes; and finally, the body becomes "tolerant' to the activity so that more and more is required to produce the same effect. (Sally Plant, interestingly, appears to satisfy all of these criteria.)

But what exactly are exercise addicts "hooked" on? Does their addiction have a natural physiological or bio-chemical explanation? "We know that exercise causes endorphins to be released into the blood and that endorphins have a similar addictive potential to opiate drugs," says Steinberg. "Like opiates, endorphins are responsible for increased pain tolerance, favourable moods, feelings of calmness and euphoria (sometimes known as the 'runner's high'). If administered externally - by intravenous injection - endorphins can induce withdrawal and craving in much the same way as morphine.

"But in order to prove that this addictive potential can cause the long- term addiction associated with drugs, we need to prove that endorphins have the same effect in the living human brain as morphine and, as yet, that has not been measured. The endorphin hypothesis is attractive but it may be that other substances, like adrenalin, are as much part of the story. It seems probable, though, that exercise addiction is a more benign addiction than drugs."

Dr Ian Cockerill, sports psychologist at the University of Birmingham, distinguishes between two forms of addiction - positive and negative. "Positive addiction is when individuals make exercise an essential part of their lives and their well-being is enhanced; negative addiction is when individuals organise their lives around exercise and so are ruled and ruined by it."

People who exercise daily, the Dianas and Madonnas of this world, are not necessarily in danger. Lorenzo Blasi, interviewed below, is an example of positive addiction. But at what point, and why, does a person who is in control of their fitness suddenly cross the threshold? No one knows, though it is well documented that women with eating disorders, for example, are four times more susceptible to exercise addiction than the general populace.

Sally Plant's friend and colleague, Bonny Saunders ("Bonny Biceps"), aged 25, is a case in point. "Aerobics became a way of controlling my weight and taking the stress off my eating disorder. I have since sorted out my eating disorder, but I'm still addicted to exercise and I am constantly worn out with it," she says.

Dr Clive Long, psychologist at St Andrew's Hospital, Northampton, and an expert in eating disorders, says that it "is only since the Seventies, when jogging and marathons became the rage, and then in the Eighties, when aerobics and Jane Fonda tapes led to the professionalisation of fitness, that doctors began to notice how, for some anorexics, addiction to exercise predated their eating disorder. [Previously they had assumed that hyperactivity was merely an outcome of severe weight loss.] Recent research has shown that 80 per cent of anorexics show an addiction to exercise and that it seems to persist in patients even after they have 'recovered' from anorexia."

But exercise addiction is not confined to females. The conventional view was that women exercised for looks, men for athletic performance. Men jogged, ran or worked out, while women did aerobics. These distinctions are more blurred in the Nineties but, more importantly, the motives for men pursuing fitness have changed. The male body, traditionally a symbol of strength, has become a symbol of desire, with the result that the pressures on men are remarkably similar to those on women. Commercials for Levi Jeans and Pepsi Cola (in which a group of women gaze lasciviously at a bare-chested man) reflect the new fetishisation of the male body.

"Nowhere is this more evident than in the gay community," says Mark Simpson, author of Male Impersonators. "Gay men are more likely to work out than straight men, perhaps because other men tend to be more appreciative of big muscles than women. The gay scene is a market place and the bigger your pecs [pectorals] ... the greater your pulling power. But the pressures on men are by no means confined to the gay community. Action heroes, like Arnold Schwarzenegger and Sylvester Stallone, developed the male body to the point where muscle became body armour, something to hide behind. Men who feel inadequate and inferior in their manliness are more at risk of becoming addicted to pumping muscles because their identities start to depend on it."

There is also new evidence that men who develop eating disorders do so after first becoming addicted to exercise. But how different is fitness addiction to, say, work addiction? Endorphins are released by stress, as much as by exercise. "Keep-it-up! Up! Up!" is also the silent bidding of the workaholic. Do we all need to be dependent on something? If so, is there a gradation of addictions, where perhaps exercise is not as bad as, say, drug, alcohol or gambling addictions? The question is significant. Steinberg wonders whether morphine addicts can be weaned by getting them addicted to exercise (internal opiates) instead.

Dr Barry Cripps, a Devon-based occupational psychologist who has seen half a dozen cases of exercise addiction, insists it is treatable. "I use hypnotherapy to deal with the depth issues behind the addiction and to break the link. At the same time, I treat it behaviourally, encouraging the individual to introduce other interests - hobbies, culture - into a life that's become boringly uni-focused. It's not quick or easy but patients can make a complete recovery."

For Sally, who has being seeing a counsellor for 11 months, there is greater insight into her problem but no improvement. "I don't hate exercise, I just hate that I have to do it. My biggest grudge is against people who eat what they like, don't exercise and have brilliant figures. They should be drowned in a sea of cellulite."