We all know a fitness buff who can’t seem to get enough exercise, but keeping fit may actually be addicting, finds a study published in the December issue of Behavioral Neuroscience. When a mouse’s running wheel is taken away, its brain shows a jump in neurological activity much like the symptoms seen during withdrawal from drug addiction.

Justin Rhodes, a postdoctoral fellow at Oregon Health and Science University, examined the neurological activity of two sets of mice. One was a group of ordinary typical lab mice; the other had been bred for 29 generations to have a predisposition for running.

In the study, conducted at the University of Wisconsin-Madison, each mouse was allowed to run as much as or as little as it wanted for six days. As expected, the runner mice tended to go longer distances, covering six miles a day compared to just two for the normal mice. On the seventh day, researchers took the running wheels away from half of both mice groups.

Five hours later, at the time of the day when mice usually hit their wheel-running peak, researchers measured the brain activity of every mouse by examining levels of Fos, a gene expressed in response to neurological excitement. The exercise-denied mice had higher brain activity in 16 out of 25 brain regions. The more a mouse had run during the previous days, the more brain activity it had.

“These were the same brain regions that become activated when you prevent rats from getting their daily fix of cocaine, morphine alcohol or nicotine,” explains Rhodes. In withdrawal, stronger neurological activity creates the desire to relapse, or return to an addictive habit.

The researchers note that human research will also be needed before they can conclude that people also become addicted to exercise.