Vanessa Alford didn't get hooked on any of these. What Alford got hooked on should have been good for her. It's something that most of us ought to do more of. It was exercise.

Alford is 35 years old and lives in Melbourne with her husband and two young daughters. She has long, dark brown hair and olive skin, and is small and slightly built. Yet she is strong in the manner of a marathon runner, which is exactly what she is, or was. "I still run," she says. "But I don't push myself like I used to."

As a young girl growing up in Melbourne, Alford loved her sport: she began gymnastics at age six, netball at seven, tennis at 10. As she writes in her upcoming book, Fit Not Healthy, she was "born to compete": both her gymnastics and netball teams won the state championships in her age group. Yet running, per se, was never her thing: she ran to train and to keep fit, but it was an obligation, not a pleasure. At Melbourne University, where she studied physiotherapy, partying took precedence. "I would use any excuse not to run."

After graduating, however, Alford moved to Townsville, where, much to her surprise, she began to run regularly: she would set the alarm for 6am, jog eight kilometres along the beach and be home by 7am, which gave her time to eat breakfast before cycling 15 kilometres to work. Soon, her eight-kilometre runs became 10-kilometre runs, and 12 kilometres on weekends.

Before she knew it, both the runs and the ride became mandatory morning rituals, "just like a shot of coffee or booze", that left her euphoric, floating for the rest of the day on a magic carpet of dopamine and adrenaline. "This feeling of elation would sweep over me," she says. "I just couldn't get enough of it."

Within months, she had dropped six kilograms and a dress size. "I liked that, too," she told me. "Everyone likes feeling good about themselves, and I got a lot of compliments."

Alford started running marathons, first in Townsville, then Bangkok (where she came second), and on the Gold Coast. Nike and PowerBar sponsored her. In October 2005, she came third in the Melbourne marathon in what she describes as "by far the best moment of my life".

By now, Alford was running for Team Nike as part of an elite group, which received free clothing and support and mentoring from long-distance runner Steve Moneghetti. Her then boyfriend (and current husband), Brent, was a tennis coach and frequently worked overseas, leaving Alford free to focus almost exclusively on her runs, which became longer and more gruelling, and were soon accompanied by a tyrannical dietary regime in which she counted every kilojoule, monitoring like a jail warden every morsel that entered her mouth.

By 2006, she was running up to 160 kilometres a week while surviving on a diet from which she had banished all fats and carbohydrates; lunch and "snacks" for a typical day consisted of a banana, an apple, a plum, a handful of strawberries and small bunch of grapes. Soon her body, just 43 kilograms, began consuming itself, shutting down non-essential systems. She was exercising herself to death, running her tiny sparrow's frame off a physiological cliff. People warned her, they told her to stop: "You have lost your mind," Brent yelled at her one day. "Running has taken over your life." But she couldn't stop.

Then finally, one day in August 2006, her body stopped for her.

For those of us who can barely get off the couch to find the remote, exercise addiction can be hard to fathom. The idea of compulsively and pathologically exercising - doing something that is good for you beyond the point at which it is no longer good for you - seems counter-intuitive, not to mention faintly ridiculous, right up there with other addictions du jour such as those to sex, shopping and the internet.

There is no formal diagnosis for the condition, and it is not recognised in the latest edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), a text used by psychiatrists and mental health workers. Yet it has been observed since at least 1976, when US psychiatrist William Glasser first coined the terms "exercise addiction" and "runner's high".

Glasser had been studying long-distance runners, whom he noticed experienced withdrawal symptoms when they were unable to run. Researchers using diagnostic tools such as the Exercise Addiction Inventory have since suggested "exercise dependence" exists in up to 3 per cent of the general population, 8.5 per cent of adolescent boys and 52 per cent of triathletes. (A study of 300 gym clients in France concluded that 42 per cent of them were addicted.)

Glasser defined exercise as a "positive addiction", noting that running was particularly habit-forming "because it is our most ancient and still most effective survival mechanism". What he didn't anticipate was how this survival mechanism could become so self-destructive when coupled with an eating pathology. It's thought that up to 48 per cent of people with anorexia also suffer from exercise addiction. For these people, exercise is another form of weight control. ("Every time I put food in my mouth, I literally felt it travel through my body and place itself on my hips," writes Alford, whose default reaction was to work it off during a 35-kilometre run.)

There's a satisfying logic to this pattern, which is sometimes called anorexia athletica. But it may also have a biological imperative, as the famous Routtenberg experiment suggests. In 1967, two neuroscientists, Aryeh Routtenberg and Abby Kuznesof, put rats in a feeding cage with a spinning wheel, and then progressively starved them. They expected that as the rats' caloric intake dwindled, they would exercise less. But quite the opposite occurred: the less they ate, the more they exercised. Indeed, most of the rats died within a week, some of them dropping dead on the spinning wheel.

"From an evolutionary perspective, this makes sense," says Sydney psychologist Stephen Touyz. "If there was a famine and all you did was lay on the same spot, you would die." Touyz has observed this in his own practice, which specialises in eating disorders. "Patients who lose lots of weight become fidgety, and anorexics become incredibly restless."

This compulsion to exercise can be virtually irresistible. "With me, it got to the point where I could not not exercise," says 30-year-old Emma from Sydney. Emma developed anorexia at school and managed it, off and on, throughout university, where, ironically, she studied to be a dietician. After graduating, she began working in hospitals where many of the meals were taken together in staff kitchens. "I'd eat them because I was trying to hide my condition," she says.

But to compensate for that eating, she would run. And run and run and run. She ran before work, after work, and during her lunch break. (She also cycled.) Her activity peaked in 2011, while she was living in central NSW, when for three weeks straight she ran every night - for eight hours. "I'd eat dinner and leave the house at about 9pm," she says. "It was in winter and I would run along the highway and all around town."

Soon her body, quite literally, fell apart. First she broke her hip while running. Then, eight weeks later, she suffered a stress fracture in her heel and was forced off work for a week. After both injuries, she kept on cycling - in the case of the hip, she was on her bike the very next day. "It hurt a lot," she says. "But the anxiety from not doing the exercise was worse."

In the end she weighed 38 kilograms, and had developed osteoporosis. Only then did she seek help.

"These days I actually have some body fat," she says. (She can't tell me her weight, because she is too frightened to weigh herself.) As for exercise, that's a no-go. "I only walk now," Emma says. "And I mean, really I'm just dawdling."

Some doctors remain sceptical about exercise addiction. "With alcohol and drugs, yes, they are addictions," says Alex Wodak, the director of the Alcohol and Drug Service at Sydney's St Vincent's Hospital. "But I am appalled by the pathologising of everyday behaviours, which is where I think shopping and exercise fit in."

Yet, neurologically-speaking, strenuous exercise operates in much the same way as substance abuse. When you thrash out a hard hour of squash, for instance, you activate the sympathetic nervous system, which causes a rise in the concentration of noradrenaline and serotonin.

At the same time, the pituitary gland secretes beta-endorphins, which block pain signals reaching the brain. The result is a pronounced euphoria. (Endorphins are often referred to as endogenous opioids, so similar are their effects to those of opiates such as heroin.) Over time, though, the body's tolerance to these endorphins increases, necessitating an increase in exercise.

"You end up chasing the high," says Sydney man Ben Carter. "It's exactly the same as drinking: if you drink three beers a day for three months, you build up resistance, so then you have to drink four beers a day, and then five. It's the same with exercise."

Carter, who is 26, has long suffered from depression, and has been diagnosed with both Generalised Anxiety Disorder and Obsessive Compulsive Disorder.

"At first, I dealt with it by drinking," he says. "From 18 to 22, I was an alcoholic. When I was at university, I would drink a bottle of spirits every day, or a box of wine." Then, four years ago, at the suggestion of his therapist, he began exercising, and this too quickly spiralled out of control.

"It turned into a seven days a week, five-hours a day compulsion," he says. "It was terrible. I was getting up at 4am, so that I could run before work. In the end I was running 100 kilometres a week, and working out in the gym for hours every day."

Carter had managed to get off the booze, but the exercise was worse. "Alcohol was easy," he says. "But exercise is really hard work. I dreaded it: it made me cry to think of it and I would be in pain while I was doing it. No one at that stage actually enjoys it: you loathe it, but you exercise through the pain, injury, sickness, it doesn't matter."

The compulsion to exercise is also reinforced by guilt: "If you don't do it, you feel like you are being lazy, like you are wasteful," says Carter. This means that exercise addicts feel worse about themselves if they try to cut down, which is like an alcoholic feeling guilty for being sober.

Carter's experience was a function of his addictive nature; given half a chance, he could probably get addicted to vegetable gardening, an activity which, as it turns out, he has recently taken up. But it also reflects the mania that has infected the contemporary fitness culture as a whole. Once upon a time, getting fit was regarded as fun. Now, fitness has become extreme: instead of fun we have SAS-inspired workouts, with sledgehammer drills, rope climbs, kettle bells and instructors who bark at you like drill sergeants.

Fitness is now synonymous with self-abnegation, suffering and pain, with boot camps and the Tough Mudder and Spartan Race events, in which participants scale walls, swim dams and jump fires over a range of different distances. (As the Spartan Race website states: "For the ultimate challenge, try the 42km Ultra-Beast. You'll know at the finish line!")

"It's the culture of excess," says Jeff Bond, who spent 22 years as the head of sport psychology at the Australian Institute of Sport. "There's no balance anymore: at one end of the spectrum we have the obsessive people who run themselves into the ground, and at the other we have those who overeat and don't exercise at all." Bond puts it down to self-esteem. "When I was at the AIS, I found that many of the really top athletes did what they did because, deep down, they didn't feel too good about themselves. Many had been told they were dumb at school. And so extreme achievement was a compensating mechanism. That low self-esteem explains so many things: ambition, excessiveness, anorexia. If you get that core self-esteem right, then people are okay with not being the best, or not being famous, and just being themselves."

Vanessa Alford burnt out in August 2006, collapsing in the middle of a race after losing sensation in her legs. (On the way to hospital, the nurse told her she "was an inspiration", something Alford found bewildering, given that she was lying, near death, in the back of an ambulance.)

Tests revealed she was suffering from severe adrenal dysfunction and that her levels of the stress hormone cortisol were 20 times higher than normal. "My body was under immense stress, 24 hours a day," she says.

For months afterwards she felt dizzy and exhausted; her legs were heavy and often numb, and her reproductive system had shut down. It took six months just to restore her energy levels. Even then, the symptoms didn't disappear entirely until 2011 when she decided to get pregnant. "The doctor told me I'd have to put on weight and stop exercising so much," she says. "I had to change my priorities." Her treatment consisted of sitting on the couch for two months and eating. "This is most people's idea of a dream - but not exercising was so, so hard."

Alford became pregnant in March 2011. Within a couple of months of giving birth, her symptoms had gone. As she points out, it was her daughter who ultimately saved her - ending, so to speak, life on the run.

"Looking back, it's pretty bizarre," she says. "I was so obsessed, so self-absorbed. It's really hard to recognise the person I was back then. But I was locked in, and I couldn't get off. I was addicted."