Kanarek wondered whether the anorexia condition is caused by activation of the same chemical pathways that create narcotic addiction.
To find out, she and her team injected all of their rats with the drug naloxone, a chemical compound that is often used to help drug abusers recover from addiction.
When injected into human addicts, the drug induces withdrawal symptoms that include writhing, chattering teeth, and swallowing movements.
Kanarek had observers unfamiliar with the experiment note down the rats' behaviors.
They found that the most hardcore rat runners showed the greatest degree of withdrawal symptoms, while rats that did not have access to wheels displayed fewer withdrawal symptoms.
Kanarek is not worried about mass exercise addiction in people.
"While we saw naloxone-withdrawal symptoms in active rats, these symptoms were not as severe as those typically seen during morphine withdrawal—suggesting that exercise is not as addictive," she said.
What's more, the addictive effects of exercise could be used in a positive way.
"We think a bright side to our findings is that exercise may be one way to actually help [drug] addicts recover," she added.
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