"One of the things that happens with exercise is an increase in the micro blood vessels in the brain," he says. Those blood vessels are associated with the birth of new cells. "So there is a physiological link between exercise and neurogenesis," he says.

When you exercise, muscles begin to use oxygen at a higher rate, and the heart pumps more oxygenated blood through the carotid artery to the brain. In fact, the brain uses about 25 percent of the oxygen that you take in. Because exercise creates endorphins, people who exercise regularly have more energy, feel alert and have an increased sense of well-being and better memory retention.

Prior to the recent studies, scientists assumed that increased cerebral blood flow was the factor linking exercise and better brain function. Now, we are beginning to understand more about the workout-brain connection (see A Brain Primer).

In The Laboratory
In 1998, Gage of the Salk Institute and Peter Eriksson at the Sahlgrenska University Hospital in Göteborg, Sweden, studied hippocampal tissue in deceased cancer victims ranging in age from 57 to 72 years old at death. Each patient while living had received an injection of bromodeoxyuridine, or BrdU, for diagnostic purposes.

BrdU is absorbed only by cells that are dividing, meaning they are creating new cells. In cancer patients it is used to see if cancer cells are multiplying. When BrdU-labeled neurons were found in the hippocampus, it was a the equivalent of a smoking gun for Gage and Eriksson. They could only conclude that neurogenesis was taking place.

"The way you detect (new neurons) is in part by determining whether or not they're undergoing cell division," Gage says. More important, these new cells weren't just floating around aimlessly.

"We found these new neurons were known and recognized by the other cells. And they looked as though they're wired up to the appropriate area" to potentially play a role in cognition.

Meanwhile, Gage and other Salk scientists were engaged in different research, this time on the effects of learning on the brains of mice. What they found could have big meaning for us humans. The mice were exposed to a rich environment of toys, treats and other incentives to think.

Almost as an afterthought, running wheels—mouse treadmills—were introduced as another variable. The mice ran at their own pace, as often and for as long as they liked. And the mice that did the running grew twice the new brain cells as mice in a control group.