It’s easy to see why exercising can be addictive: it lifts your mood, gives you confidence, increases your social circle, and, unlike some other vices out there – chocolate, gambling, crystal meth – it’s good for your mental and physical wellbeing. But why does it make us feel good and what happens when you get hooked on that feel good feeling? We find out what goes on inside the brain of a fitness addict.

Changes occur in the brain’s reward circuits during and following exercise. These reactions cause you to feel like you’ve had a pleasurable experience, even when you’ve just been through a tough slog around a muddy park.

These changes act as helpful motivators for most of us, ensuring that we want to repeat the experience and perform exercise again. But for about 0.2% of the population, these neurological reactions can take over their lives, to the point where they’re skiving off work to hit the gym.

Behavioural addictions, such as exercise addiction, have only recently been recognised for inclusion in the Diagnostic And Statistical Manual Of Mental Disorders – a huge handbook that lists and classifies all known psychiatric illnesses.

The criteria used in the handbook to define behavioural addictions are remarkably similar to those defining substance dependence. The key characteristics that the two addiction types have in common are A) the development of tolerance over time, and B) a loss of control.

Addicts have to perform more and more exercise to achieve the same ‘high’

For exercise addicts, this means that A) they have to perform more and more exercise to achieve the same ‘high’, and B) they have a tragic inability to simply stop exercising, even if they want to.

A few ex-exercise addicts have blogged about their behaviour at its worst. The examples below should ring alarm bells if you recognise them in yourself or those around you.

• Addicts will spend huge chunks of their day doing, planning, or recovering from their fitness routines. This goes way beyond taking trainers to work to squeeze in a lunchtime run.
  Katherine Schreiber would squeeze in a two-hour gym session before work, skip lunch for another session, and then spend her entire evening there, too. Exhausting.
• These are no fair-weather exercisers. Come rain, hail, or fractured limb, an exercise addict will still hit the streets for a run. “I’ll just walk it off,” they think, before grimacing through a painful seven-mile limp around the park that may or may not cause them permanent damage.
  This happened to Tara Fuller. Although now recovered from a mental health perspective, she was told, at the age of just 26, that her body would never fully recover.
• Not being able to exercise for whatever reason is, for most of us, a good excuse to sit on the couch and watch some telly, guilt-free. But for an exercise addict, being unable to exercise is devastating. Psychologists have suggested that a gruelling training regime can be used by addicts to suppress emotional distress – so restricting exercise will cause past traumas to re-emerge and destabilise their fragile mental wellbeing.
  Carrie Arnold talks of becoming suicidal after a cast on her foot stopped her from exercising. These self-destructive urges were accompanied by a worsening of an underlying eating disorder.

These behaviours are, clearly, a million miles away from merely enjoying the buzz of getting a bit sweaty. But the fact that serious exercise-addiction exists teaches us something about the less obsessive – but still fairly consuming – exercise behaviours that the rest of us engage in.

Addictions of all forms often come in pairs: cocaine addicts also have a tendency to gamble, while alcoholics usually like a fag or two with their pints. The co-occurrence of two disorders within one person suggests that they share overlapping brain circuitries – and similarly, your penchant for chocolate cake is all down to an activation of the same parts of your brain as your love of spin classes.

This suggests that exercise doesn’t just give you a nicer-looking bum, but can also be used to hijack, and therefore dampen, your brain’s tendency to crave things that are bad for you.

A recent study published in the journal Medicine And Science In Sports And Exercise provided some evidence for this idea. The study found that reward-related regions in the brains of men who’d exercised for 30 minutes prior to a brain scan – during which they were presented with a “non-natural rewarding stimulus” (in this case, money) – were significantly less active than the same regions in a group of men who had not undergone exercise. This implies that exercise diminishes sensitivity to non-natural rewards.

In other words, if you’re trying to give something up – be it fags, booze or Haribo – a sensible amount of exercise should help you on your way.