We’re normally warned about the dangers of exercising too little, but it seems that too much physical activity can also be a problem. A drug which causes withdrawal symptoms in heroin addicts can have the same effect in rats after excessive use of exercise wheels. Rats which exercised the most had the severest symptoms.
A study published in the journal Behavioral Neuroscience split rats in to an active and inactive group, and gave each group either one hour of food a day, or a round-the-clock feast. To examine their addiction to exercise, all rats were given naloxone, a medicine for heroin overdose that produces immediate withdrawal symptoms.
The active rats who ate for only one hour a day were the heaviest exercisers, and also the worst hit by withdrawal symptoms. Their behaviour mimicked a potentially fatal eating disorder called anorexia athletica, in which exercise undertaken to lose weight becomes as addictive as taking drugs. Inactive rats had little reaction to the drug, regardless of how much they ate.
It seems that exercising activates the same part of the brain as drugs. Working out releases endorphins and dopamine, giving a sense of reward. This research should not be used as an excuse for avoiding exercise though, warns lead author Robin Kanarek:
“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.”
Instead, the researchers hope their work may lead to new treatments for addiction that incorporate moderate forms of exercise. Addicts could be weaned off drugs by replacing their missing sense of reward with exercise.