If you are a lonely long-distance runner, exercise may not be doing your brain much good, scientists suggest.
Running with someone else may be better for you
Many people find it hard to maintain an exercise schedule on their own - and a rat study may explain why.
Researchers from Princeton University in the US say exercising with others is better for the brain.
Writing in Nature Neuroscience, they suggest social contact negates the negative effects exercise can have on activity in the brain.
Running is known to increase levels of stress hormone corticosterone, which can reduce the creation of new brain cells - a process known as neurogenesis.
But the activity has also been found to increase spatial awareness and to boost communication between neurons.
The researchers in this study looked to see what factors might explain this apparent anomaly.
They looked at the effects of running on adult rats who were in groups or on their own.
Running was found to increase neuron generation only when rats were housed in groups.
In rats that ran in social isolation, neurogenesis was suppressed.
Running caused similar elevations of the stress hormone - which can impair neuron generation - in both groups.
But only animals that ran alone were vulnerable to its negative influence.
They also had higher levels of the hormone compared to group runners.
When isolated rats ran for a long time, they did see the same brain benefits as their short-term runner peers - but only when they had been running for a much longer period.
Writing in Nature Neuroscience, the researchers led by Dr Elizabeth Gould, say: "In the absence of social interaction, a normally beneficial experience can exert a potentially deleterious influence on the brain."
The research comes after previous studies recommended exercising as key to reducing the risk of developing dementia.