Stronger, Faster, Smarter

The first step toward that understanding is to figure out exactly where the new brain cells are growing—and whether that is a part of the brain that needs to be rejuvenated. In Small and Gage's experiment, the new neurons created by exercise cropped up in only one place: the dentate gyrus of the hippocampus, an area that controls learning and memory. This region, tucked under the temporal lobes, helps the brain match names to faces—one of the first skills to erode as we age. Fortunately, the hippocampus is especially responsive to BDNF's effects, and exercise seems to restore it to a healthier, "younger" state. "It's not just a matter of slowing down the aging process," says Arthur Kramer, a psychologist at the University of Illinois. "It's a matter of reversing it." Kramer's work also has implications for the frontal lobes, the seat of "executive functioning"—a type of higher thought that entails decision-making, multitasking and planning ahead. With scanning technology, he has found that exercise causes the frontal lobes to increase in size. And in dozens of previous studies of men and women in their 60s and 70s, brisk walking and other aero-bic workouts have yielded improvements in executive functioning. Subjects have fared better on psychological tests, answering questions more accurately and quickly. With the new brain studies, researchers can now begin to explain why.

As far as scientists know, new neurons can't grow throughout the rest of the brain. But other regions benefit from exercise in many secondary ways. Blood volume, like brain volume, increases with exercise, says Small: "Wherever you have the birth of new brain cells, you have the birth of new capillaries." Active adults have less inflammation in the brain. They also have fewer "little bitty strokes that can impair cognition without the person even knowing," says University of California, San Francisco, neuroscientist Kristine Yaffe. Still other researchers have found that athletes have more astrocytes, or cells that support neurons and mop up neurotransmitters after they're used to send messages from cell to cell. And even the levels of those neurotransmitters are higher in people who exercise frequently. "Dopamine, serotonin, norepinephrine—all of these are elevated after a bout of exercise," says Ratey. "So having a workout will help with focus, calming down, impulsivity—it's like taking a little bit of Prozac and a little bit of Ritalin."

Unlike neurogenesis, which can take weeks to occur, most of these additional effects appear almost immediately. Get off the treadmill after a half-hour workout, says Hillman, and "within 48 minutes" your brain will be in better shape. But alas, these benefits are somewhat transient. Like weight, mental fitness has to be maintained. New neurons, and the connections between them, will stick around for years, but within a month of inactivity, "the astrocytes shrink down again, and then the neurons don't function as well anymore," says William Greenough, a psychologist at the University of Illinois. Let your body go, then, and your brain will follow.

To keep the effects, you've got to keep working out. "If you're thinking that by exercising at age 20 you're going to have some effect on what you're like at age 70," Greenough adds, you'd better be willing to commit to 50 years of hitting the gym.

Unless, that is, you're a kid. Most studies of exercise and cognition have focused on older people—the folks who are just starting to worry that their minds aren't what they used to be—but the effects of physical exertion on the brain aren't limited to that group at all. In fact, in young children, they're even more potent. Exercise probably has "a more long-lasting effect on brains that are still developing," says Phil Tomporowski, a professor of exercise science at the University of Georgia. In kids, as in adults, the hippocampus reaps many benefits from exercise. This won't surprise parents of kids with ADHD, many of whom already use physical activity as a substitute or supplement for drugs. In children with the disorder, the hippocampus is enlarged, and it may be wired to the rest of the brain in abnormal ways that affect its function.

But a good workout, or for that matter a boisterous session of kickball, can also have much more widespread effects on children's brains. Until about 20, kids don't have fully developed frontal lobes, so they "recruit" other parts of the brain to perform necessary functions, including those involved in learning. In Hillman's look at third- and fifth-grade PE students, exercise sped up not just executive functioning, but a broad variety of skills ranging from math to logic to reading, all of which rope in many regions of the brain. "In kids you have a tremendous amount of growing brain tissue, particularly in the frontal lobe," says Tomporowski. "So we can't just break it down to hippocampal function in them. Exactly what else is going on in there, I don't think anybody knows."