Excessive exercise can be addicting, new ...( WASHINGTON Although exercise is goo...),Health

WASHINGTON Although exercise is good for your health, extreme exercise may be physically addicting. Rats given a drug that produces withdrawal in heroin addicts went into withdrawal after running excessively in exercise wheels, according to new research. Rats that ran the hardest had the most severe withdrawal symptoms.

The scientists who conducted the study reason that if excessive exercise is addicting, then maybe, to feel good, addicts could take moderate exercise instead of drugs. The findings also shed light on the potentially fatal eating disorder called anorexia athletica, in which exercise undertaken to shed pounds becomes as compulsive as taking drugs, resulting in even greater weight loss.

"Excessive running shares similarities with drug-taking behavior," the researchers wrote in the August issue of Behavioral Neuroscience, published by the American Psychological Association.

For those looking for an excuse to hit the couch, however, this study looked at excessive, not moderate, exercise. "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," said lead author Robin Kanarek, PhD, of Tufts University.

For several weeks, 44 male and 40 female rats were allowed to either run in exercise wheels or remain inactive. To simulate anorexia athletica, the researchers divided the active and inactive rats into groups whose members were either given food for one hour a day or around the clock. Rats in all four groups were then given naloxone, a medicine for heroin overdose that produces immediate withdrawal symptoms.

Active and inactive rats responded very differently to naloxone, which was given in proportion to their weight. The active rats showed withdrawal symptoms like those seen in narcotics addicts: trembling, writhing, teeth chattering, and
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