Excessive running can be as addictive as taking drugs, find researchers, and can also lead to similar withdrawal symptoms.
The scientists believe that extreme exercise sparks a reaction in the brain that is similar to that caused by such drugs as heroin.
Sudden withdrawal of the activity leads to symptoms like those seen in addicts: trembling, writhing, teeth chattering, and drooping eyelids.
The researchers hope that the findings could one day lead to exercise therapies for drug addicts and also treatment for anorexia athletica – excessive exercising to lose weight.
“As with food intake and other parts of life, moderation seems to be the key,” lead author Professor Robin Kanarek, of Tufts University in Massachusetts.
“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.
“Excessive running shares similarities with drug-taking behaviour.”
It has long been known that exercise produces feel good chemicals known as endorphins and dopamine and these can lead a high after activity.
But Professor Kanarek, a psychologist, has also proved that they are likely to produce similar withdrawal symptoms.
Her team observed a group of rats, some of which exercised excessively on a wheel as if they were addicted and others which did no exercise.
Then they administered naloxone to the rodents, a drug used to block the effects of heroin, especially in overdose victims.
Active and inactive rats responded very differently to naloxone, which was given in proportion to their weight. The active rats showed serious withdrawal symptoms similar to rats that had been addicted to heroin.
The non-active rats were unaffected.
Because of the way the active rats responded to naloxone, they seemed to have undergone the same changes in the brain’s reward system as rats addicted to drugs, said Professor Kanarek.
Rats and humans share many nervous-system traits, said Professor Kanarek and the laboratory findings should be extrapolated to the real world.
The research is published in the journal Behavioural Neuroscience.