See the rest of Slate's Fitness Issue.
Soon these obsessive joggers were being described as victims of an exercise addiction, though in deference to diagnostic conservatives their affliction was also termed obligatory exercise, overtraining syndrome, or exercise dependence (the "other" ED). Some researchers felt the condition was merely an outgrowth or subset of anorexia nervosa and declared the combination of undereating and overtraining responsible for the Female Athlete Triad, or FAT. (Its three components are disordered nutrition, amenorrhea, and osteoporosis.) And since men were diagnosed as anorexics less often than women, it was further proposed that overtraining might be the masculine version of an eating disorder.
(In the following ngram, I've plotted the concurrent rise of the phrases brain reward system, recreational runners, and exercise addiction in Google Books.)
What's more, a very similar and startling phenomenon had been demonstrated in rodents. In 1967, a pair of psychologists at Northwestern developed a standard procedure for inducing a kind of overtraining syndrome in the lab: Under the right conditions, a captive rat would become exorexic. The scientists noticed that if they limited a rat's access to food to one hour per day, the animal would start to lose weight before adapting to the new schedule and consuming more food when it had the chance. But if they gave that same rat the opportunity to exercise in a running wheel, it never adjusted. Instead, the rat would get slightly deranged: running more and more, and eating less and less, until it became too scrawny and weak to move. Without intervention, the animal would starve to death within two weeks.
Further studies revealed that rats derived some kind of pleasure from running on the wheel, or at least they could become dependent on the behavior. The Lewis strain of lab rats, for example, is especially prone to drug addiction. Lewis rats are also inclined to habitual exercise: When given regular access to a wheel, they'll engage in longer and longer bouts of running, until they're doing more than 6 miles per day. (That's a serious haul for a critter with tiny legs.) Rats of the Fischer strain, which aren't as likely to press a lever for drugs, also aren't as vigorous on the wheel—they only run for about a mile. Rats can also be trained to do things in exchange for access to the running wheel, too. Instead of a food pellet reward, they get the chance to exercise.
The fact that rats find physical activity rewarding doesn't mean they're always killing themselves by doing too much of it. Aside from its many well-established physical effects, exercise provides mental benefits for rats as well as people. It staves off melancholy and anxiety, for one thing, and seems to improve the well-being of a particular strain of rat—the Flinders sensitive line—that's been bred as a model of human depression. (The animals are usually sluggish, with sleeping problems and small appetites.)
So is exercise a kind of "positive addiction," as William Glasser put it in 1976—a habit-forming behavior that can displace a habit-forming substance or mitigate its negative effects? What should we make of the many similarities between the biochemical effects of exercise and those of the drugs of abuse? Both stimulate the release of dopamine and neuropeptides in the striatum; both lead to changes in brain circuitry and the formation of new neural connections; both apparently induce tolerance, and—in some cases—withdrawal symptoms and physical injury. Yet as Vaughan Bell has pointed out in Slate, there's no straightforward relationship between brain chemicals and addiction, and knowing a bit of neurochemistry doesn't tell you whether an activity is harmful or beneficial. To take just one of many examples, falling in love might lead to dopamine release, and an unexpected breakup could produce very real feelings of withdrawal. Do we wring our hands and wonder whether love is an addiction?
It would be silly—and very unhealthy—to avoid exercise on account of its habit-forming properties. But we shouldn't ignore the facts. A regular exercise program may improve your mood and prolong your life. It may help you quit smoking and keep your wits about you as you age. That doesn't mean it's unreasonable to weigh the bad against the good, however. I try to go to the gym three or four times per week, and I get a little annoyed when I can't make it. That's OK; the downside is worth it. But a few years ago, I was addicted to stretching, too: Every morning, I'd spend 15 minutes limbering up before leaving the house. It felt great when I did it, but terrible if I missed a day. I'd twist my legs under my desk for hours, trying to make my calves less stiff and my joints less creaky. That would have been a small price to pay, I suppose, if stretching offered some other, more important benefit. But when I discovered that it doesn't actually prevent sports injuries, the whole thing started to seem like a bad habit. So one day I quit. Cold turkey.