Exercise fanatics 'suffer withdrawal like drug addicts'

Weaning them off their health kick produces the same trembling of the body of drug addicts who go "cold turkey", experts say.

What starts as an attempt to stay active can become as compulsive as narcotics, particularly for those who exercise to shed the pounds, the findings show.

The report, by scientists at Tufts University in Boston suggests that as soft drugs build up to hard drugs, so mild exercise like jogging or bike riding can turn into triathlons and 100-mile rides.

Excessively exercising to shed weight can also lead to the potentially fatal condition anorexia athletica, they add.

But the researchers, writing in the journal Behavioural Neuroscience, warned their study was not an excuse to become a couch potato.

They reported: "Although exercise is good for your health, extreme exercise may be physically addictive. Excessive running shares similarities with drug-taking behaviour. 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."

The scientists studied rats who were split into two groups, one given exercise wheels and the other remaining inactive. A rat's nervous system is similar to a human's, they said.

All the rats were given the medicine naloxone which heroin addicts take to produce immediate withdrawal symptoms - so only addicted rats would show the symptoms. The results showed the active rats displayed withdrawal symptoms much more than those who had remained inactive during the experiment. The more the rat had been running, the worse the withdrawal symptoms after naloxone, said the findings.

These symptoms included "trembling, writhing, teeth chattering, and drooping eyelids", which are the same as heroin addicts coming off their fix.

The researchers said: "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. Exercise, like drugs of abuse, leads to the release of neurotransmitters such as endorphins and dopamine, which are involved with a sense of reward."