It’s well established that exercise can sharpen the mind: People and mice who work out do better on cognitive tests, and elderly people who are physically active reduce their risk of dementia. Now, in a surprising finding, researchers report that blood from a mouse that exercises regularly can perk up the brain of a “couch potato” mouse.

This effect, traced to a specific liver protein in the blood, could point the way to a drug that confers the brain benefits of exercise to an old or feeble person who rarely leaves a chair or bed. “Can your brain think that you exercised, from just something in your blood?” asks aging researcher Saul Villeda of the University of California, San Francisco (UCSF), who led the rodent research.

The study grew out of research in Villeda’s lab and others suggesting blood from a young mouse can rejuvenate the brain and muscles of an old mouse. Some teams have since claimed to find specific proteins that explain the benefits of this “young blood.” Graduate student Alana Horowitz and postdoc Xuelai Fan in Villeda’s group wondered whether  exercise—not just youth—could confer similar benefits via the blood.

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