obvious jokes notwithstanding

(…) The researchers were trying to replicate earlier work in which the brains of mice given free access to running wheels subsequently fizzed with new brain cells, a process known as neurogenesis, and the mice performed better on rodent intelligence tests than those without access to wheels. To the Princeton researchers’ surprise, when they performed the same study with rats, “which are a little closer, physiologically, to humans,” said Alexis Stranahan, the lead author of the Princeton study, running did not lead to neurogenesis. The rats’ brains remained resolutely unaffected by exercise.

Hoping to discover why, the researchers examined how the rats and mice had been housed and learned that while the mice in the earlier experiments had lived in groups, the rats were kept in single-occupancy cages. Rats, in the wild, are gregarious. They like to be together. The researchers wondered whether isolation could somehow be undermining the cerebral benefits of exercise at a cellular level.

Putting this idea to the test, they divided young male rats into groups housed either in threes or singly and, after a week, gave half of them access to running wheels. All of these rats ran, but only the rats with cagemates experienced rapid and robust neurogenesis. Not until after weeks of running, long after the other socially engaged rats’ brains had sprouted plentiful new neurons and neural connections, did the lone rats start to produce brain cells. Social isolation had dramatically suppressed and slowed the process.

(…)Does this happen in lonely human exercisers? No one knows, Dr. Stranahan said, since comparable experiments on people are impossible. (The animals were sacrificed.) But she added, “There is abundant epidemiological literature in people that loneliness has cognitive consequences, contributing to depression, strokes, Alzheimer’s and so on.”

(…)Taken together, these otherwise varying studies of rodents and humans suggest that while exercise may seems a simple physical activity engaged in by individuals, it is not. It is in fact a behavior plaited with social and emotional concerns that can influence how often you work out and with what physiological consequences. “It may take longer” for lonely people to improve the state of their brains with exercise, Dr. Stranahan said, just as it may take a divorce to get some men in shape. But thankfully, there are some aspects of exercise and interpersonal relationships that remain stubbornly unambiguous. In a 2010 study from the Neuroscience Institute at Princeton, male rats given access to “sexually receptive” females enthusiastically engaged in procreative activity, a moderate workout in its own right and, despite raising their stress hormones, vigorously pumped up the amount of neurogenesis in their brains. Sex improved their ability to think, obvious jokes notwithstanding.

gretchen reynolds for the new york times

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