New research shows that in addition to causing the release of chemicals called endorphins—well-documented as the source of the "runner's high"—exercise may contribute to the formation of new connections among nerve cells in the brain and even to the growth of new cells. Other research has underscored the strong correlation between exercise and higher mental function.

It's possible that running, cycling and other sports may trigger a primal "flight or fight" response in us, and in doing so, may spur brain cell growth, much as it did in ancient hunters as they chased their prey or were chased by predators. Or it could just be that getting more blood to the brain is good for us.

Experts caution that no one knows what these new cells actually do in the human brain, or whether they really become part of functional circuits that affect behavior and thought. But what is no longer in doubt is that new cell growth has been found in the places in the brain where we think, learn and remember.

Not long ago, neuroscientists believed the brain stopped producing any new nerve cells, called neurons, at birth. Researchers knew that synapses, the connections between neurons needed for transmitting memories and thoughts, change and grow as a person ages, declining later in life. But their consensus was that the number of neurons had only one way to go: down.

During the past few years, researchers at institutions in the United States and abroad have turned that thinking on its head as they pioneered the study of neurogenesis, or new brain cell development. They have found in a number of test subjects, including humans, that neurons are continuously being formed, even by the elderly. And behavior has a significant impact on how many new cells are grown. The results are astonishing (see Mind-Body Exercise Connection Research).

"The surprise is that the rate at which new cells are being born in the brain are regulated by, among other things, your interaction with your environment," neurobiologist Fred H. Gage says. And exercise seems to be a key to that interaction.