for National Geographic News
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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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.
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