Addiction: Why We Crave Exercise

Exercise Addiction Activates Reward Center in Brain

From the WebMD Archives

Dec. 3, 2003 -- Like some humans, there are also mice that have an exercise addiction, a new study shows.

It's been debated: What happens in the brain during exercise? Does exercise addiction really exist, like other addictions such as drugs, alcohol, or nicotine?

In this study, researchers bred exercise-loving mice -- that is, mice with a genetic predisposition to run longer distances on the wheel, explains researcher Justin S. Rhodes, PhD, a zoologist with the University of Wisconsin-Madison.

His report appears in the Dec. 1 issue of Behavioral Neuroscience.

"All mice run on wheels, and, therefore, have a motivation to run," says Rhodes in a news release. However, the specially bred mice are much like people who have an intense desire or compulsion to run -- exercise addiction, he explains.

His goal was to analyze the brain of each mouse -- to look at brain activity changes in addicted mice and in "regular" mice.

And They're Off ...

For six days, both the specially bred, exercise-compelled mice and the control mice ran in their cages -- on their wheels -- as much as they wanted. Researchers documented the distances that each covered.

The compulsive mice covered more ground than the control mice, although each group ran for the same time period, he reports. On the sixth day, the compulsive mice averaged about 6 miles, while control mice logged about 2 miles.

On the seventh day, half the mice in each group were blocked from using their wheels. The other half in each group got free-running time as usual. As the mice reached their running peak, about five hours later, researchers looked at their brain activity. They also looked at brain activity of mice blocked from running.

The specially bred mice had extremely high levels of brain activity in certain regions -- the same regions that become activated when rats can't get their daily fix of cocaine, morphine, alcohol, or nicotine, Rhodes explains.

For these specially bred mice, running activates the brain's "reward" circuitry, which drives them to keep running, he says. "These mice have run for six days. They want to run, and they're ready to run, but they can't," he explains. "Changes in brain activity is an indication of their motivation to run."

Whether this truly applies to humans is another matter, Rhodes says. He likens himself to the control mice in his study; he likes to bike but feels no compulsion to do it regularly. No exercise addiction in his blood, Rhodes admits.

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SOURCES: Rhodes, J. Behavioral Neuroscience, Dec. 1, 2003: vol 117. News release, University of Wisconsin-Madison.
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