Exercise can be habit-forming, as many of us know. But a new study suggests it can be really addictive—in the same way as heroin or other opiates. Regular exercisers need not fear, as this research applies only to very intense bouts of exercise. But the researchers hope their work might have implications for both drug addiction and exercise-related eating disorders.
The study, led by researchers at Tufts University, was particularly interested in triggering some of the effects of anorexia athletica, an eating disorder whose sufferers exercise compulsively in the pursuit of constant weight loss. So, the researchers divided rats into high-activity and inactive groups, and then divided both groups by how much food they received: a mere hour per day for one group versus all-you-can-eat for the other. To add complexity, the rats were all given naloxone, a drug that is prescribed for heroin overdose but that presents immediate symptoms of withdrawal in opiate users. If exercise is an addiction, the researchers reason, it will impact the opiate receptors in the brain, much like heroin. Add naloxone to block those receptors, and, if the exercise was in fact addictive, the subject should go into withdrawal. Withdrawal has unmistakable physical symptoms, and is therefore measurable. An addicted rat in withdrawal behaves just like a person in the same situation, with trembling, writhing, teeth chattering, and drooping eyelids.
And that, in fact, is exactly what happened. The active rats that were food-restricted chose to run the most on an exercise wheel, and had the most severe symptoms of withdrawal. They lost substantial amounts of weight, just like a person with anorexia athletica. On the other hand, the inactive rats, food intake notwithstanding, had little reaction to the naloxone. The conclusion, according to researcher Dr. Robin Kanarek of Tufts University: "Exercise, like drugs of abuse, leads to the release of neurotransmitters such as endorphins and dopamine, which are involved with a sense of reward." A really intense workout gets you genuinely high.
The average person has little to fear from this research, which pertains to very extreme exercise. As Dr. Kanarek summarizes, "As with food intake and other parts of life, moderation seems to be the key. Exercise, as long as it doesn't interfere with other aspects of one's life, is a good thing with respect to both physical and mental health." Still, the researchers hope that research of this sort, that investigates how behavior might trigger the release of pleasure-inducing brain chemicals, will have potential for treating both anorexia athletica and drug addiction. Though these were until recently believed to be completely distinct behaviors, increasingly research is linking the diverse ways our brains seek pleasure from our environment.