These benefits included enhancing “executive” cognition, which includes higher-order functions such as planning and reasoning; improving some types of learning; and attenuating neural damage from stress. There is also emerging evidence that physical activity may be protective against neurological disorders, including Alzheimer’s and dementia, Parkinson’s disease, stroke, and spinal cord injury.

Animal studies have begun to tease apart the potential molecular mechanisms that may underlie the brain benefits of exercise. Certainly, neurogenesis is one possible explanation. Synaptogenesis, the development of new synaptic connections, is another.

Exercise also seems to benefit glial cells, the nonneuronal brain cells that support synaptic transmission and ensheathe neural fibers with myelin, a fatty substance that speeds nerve signaling. Each of these effects may be driven in part by increased blood flow to the brain, a well-documented benefit of physical activity.

“Exercise mobilizes the molecular machinery to improve brain health and cognition,” says Carl Cotman, a neurobiologist at the University of California, Irvine. “It increases metabolism in the brain and generally makes brain cells healthier; it even helps clear out Alzheimer’s pathology in mouse models.”

The question of how exercise does all this is still incompletely answered. “It really is sort of startling to think about,” Small says. “Physical exercise induces a series of biochemical processes within neurons that induces the birth of new cells. How that happens is a big black box. There are a lot of missing pieces at the molecular level.”

Cotman, for one, believes the key is a group of chemicals called nerve growth factors, particularly brain-derived neurotrophic factor, or BDNF. His studies and others have found increased BDNF levels in exercising animals, and these increases have been correlated both with neurogenesis and with improved learning.

“I think that BDNF is the central regulatory molecule for the cascade of cellular changes that get activated by exercise,” Cotman says. “It’s kind of the gatekeeper for all of the other effects of exercise.”