Many runners contend that jogging alone offers the opportunity to enjoy nature or appreciate an urban landscape, while giving the brain and the body a beneficial workout.

But new research with rats suggests that running solo might not offer the perceived benefits and, in fact, may actually be bad for runners when combined with social isolation.

The scientists behind the study report that mice forced to live and run by themselves have less brain cell growth than those that get to run with other rats.

The implications for humans aren't clear.

But the study does show "that the social environment plays an important role in determining how a basic function like physical activity affects the brain and body," said Bruce S. McEwen, a professor of neuroendocrinology at The Rockefeller University in New York City, who's familiar with the study findings.

The study researchers launched their project because they wanted to understand how a stressful activity -- running -- turns out to be beneficial from a health perspective.

"Stress hormones go up in runners and yet, under certain circumstances, these hormones don't affect the brain in a negative way," said study co-author Elizabeth Gould, a professor of psychology at Princeton University.

Gould and her colleagues studied the effects of stress by housing rats either in solitary confinement or in groups. They also made the rats run on running wheels, either alone or with other rats, and studied how the exercise affected "neurogenesis" -- the creation of new neurons in the brain.

The rats who ran in groups did a better job of generating new neurons than those that ran in isolation. The researchers also found that the isolated rats had higher levels of corticosterone, a hormone related to stress.

The findings appear in the March 12 online edition of the journal Nature Neuroscience.

Why would social isolation matter?

"For most species, social interaction is very natural," Gould said. "This makes a lot of sense for species which thrive with cooperative activities, like sharing food and cooperative breeding. The stressful component of isolation may be adaptive because it seems to be aversive -- if given a chance, most rats would seek out social situations, especially positive ones -- and so animals are unlikely to stay in an isolated setting if given a chance."

In other words, social isolation may be stressful because it's a sign to rats that they should find company.

Does this mean anything for humans who live alone? It's hard to say it does, Gould said.

For one thing, "rats are highly motivated to run. If you give them access to a running wheel, they will run without fail. This is a universally motivating behavior. This is not true for humans -- many humans are not motivated to exercise."

And second, the rats in solitary confinement weren't exposed to any other rats at all. "Humans living alone typically have lots of other social contact -- at work, shopping, on the phone, social outings," Gould said.

Still, the findings do provide more evidence that social interaction can be healthy and "buffer the negative effects of stress," she said.

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