Exercise Can Be Like Heroin, "Gym Rats" Show

Matt Kaplan
for National Geographic News
August 20, 2009
Hardcore runners who can't bear to skip a workout may be hooked in a way that's similar to heroin addiction, according to a new study of rats.

The well-known "runner's high" may be the culprit: Human runners need to increase the distances they run to feel that euphoria, experts say.

And if these runners are forced to stop, they can show signs of depression.

Such "withdrawal symptoms" have led researchers to theorize that addictive chemicals are naturally released by the body during exercise.

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Anorexia Athletica

To explore this idea, a team led by Robin Kanarek at Tufts University in Massachusetts divided lab rats into two types of cages, ones with running wheels and ones without.

Over seven days, both male and female rats with exposure to wheels naturally increased how much they ran on the wheels.

This was not surprising: Rats offered wheels are known to steadily increase their use of them over time, said Kanarek, whose study appeared in the August issue of the journal Behavioral Neuroscience.

On day nine, both the active and nonactive rats were divided into groups. After having had food available at all times, about half of the running rats began to be issued just a single portion of food a day, and only an hour to eat.

This brought on "anorexia athletica" in the food-restricted running rats: They dramatically increased their running and started losing weight.

In humans, anorexia athletica can be a fatal mental disorder that makes its sufferers compulsively exercise to lose weight.

(Related: "Modified Mice Stay Super-Fit -- Without Exercise.")


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.

Running Rehab?

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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