Neuroscientist Fred H. Gage and his colleagues at the Salk Institute examined brain samples from mice. All of the mice showed vivid proof of what’s known as “neurogenesis,” or the creation of new neurons. But the brains of more athletic mice in particular showed many more. These mice, the ones that scampered on running wheels, were producing two to three times as many new neurons as the mice that didn’t exercise.

Since Gage’s discovery, scientists have been finding more evidence that the human brain is not only capable of renewing itself but that exercise speeds the process.

“We’ve always known that our brains control our behavior,” Gage says, “but not that our behavior could control and change the structure of our brains.”

Functional M.R.I.’s have shown that a portion of each person’s hippocampus receives almost twice the blood volume after exercising as it did before. Scientists suspect that the blood pumping into that part of the brain was helping to produce fresh neurons. The hippocampus plays a large role in how mammals create and process memories; it also plays a role in cognition. Many neurologists believe that the loss of neurons in the hippocampus may be a primary cause of the cognitive decay associated with aging.

In one MRI study of how exercise affects the hippocampus, researchers at Columbia University noted significant neurogenesis after exercise. “It’s reasonable to infer, though we’re not yet certain, that neurogenesis was happening in the people’s hippocampi,” says Scott A. Small, an associate professor of neurology at Columbia and the senior author of the study, “and that working out was driving the neurogenesis.”

Other recent studies support this theory. At the University of Illinois at Urbana- Champaign, a group of elderly sedentary people were assigned to either an aerobic exercise program or a regimen of stretching. (The aerobic group walked for at least one hour three times a week.) After six months, their brains were scanned using an M.R.I. Those who had been doing aerobic exercise showed significant growth in several areas of the brain.

And the benefits aren’t limited to adults. Other University of Illinois scientists have studied school-age children and found that those who have a higher level of aerobic fitness processed information more efficiently; they were quicker on a battery of computerized flashcard tests. The researchers also found that higher levels of aerobic fitness corresponded to better standardized test scores among a set of Illinois public school students.

What substances might help neurogenesis?

The human brain undergoes neurogenesis — the creation of new cells — throughout a person’s life, although the amount depends on a variety of factors, not just exercise.

MARIJUANA: A 2005 study on rats found that stimulation of the brain’s receptors for marijuana increased neurogenesis.

ALCOHOL: A 2005 study found that mice that swallowed a moderate amount of ethanol showed more neurogenesis than teetotalers. Other studies on mice have suggested that heavier drinking can be damaging to the brain.

SOCIABILITY: One study suggests that rats that live alone and have access to a run ning wheel experience less neurogenesis than those that have access to a running wheel and live in group housing.

DIET: A diet high in saturated fat and sugar sharply diminishes the brain’s production of the proteins and nerve-growth factors necessary for neurogenesis. Exercise may mitigate that effect somewhat.

STRESS: Mice that are subjected to uncontrollable stress (like electric shock) suffer substantial deterioration in their ability to produce new neurons.

CHOCOLATE: In a study published this year, an ingredient in cocoa, epicatechin, was shown to improve spatial memory in mice, especially among those that exercised. Epicatechin can also be found in grapes, blueberries and black tea.