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We posed your question to Dr. John Ratey, an associate clinical professor of psychiatry at Harvard Medical School, author of ,��and a Reebok ambassador for active children.

Exercise, he says, has three effects on the brain that make people smarter:

1. IT MAKES PEOPLE BETTER LEARNERS
“Exercise improves concentration, attention, motivation, and general overall mood, and decreases impulsivity,” Ratey says. “One of the ways that’s done is by raising the neurotransmitters dopamine, serotonin, and norepinephrine. A bout of exercise is like taking a little bit of Ritalin and a little bit of Prozac.” Neurotransmitters are chemical substances released in the brain, and these three in particular have been linked to mood and alertness.

2. EXERCISE PRIMES THE BRAIN TO GROW NEW BRAIN CELLS
“The only way we learn something is when our brain cells grow. They have to make better connections with one another to make it easier for us to retrieve information and hold information in.” Ratey says. Exercise stimulates the increase of BDNF, brain-derived neurotrophic factor, a protein that promotes the growth of brain cells, in a process Ratey likens to fertilizing the brain.

3. EXERCISE MAKES NEW BRAIN CELLS GROW
“We spawn new brain cells every day,” Ratey says. “A major area we do this in is the hippocampus, which is like learning Central Station in the brain. Nothing we know of—no drugs, no activity—nothing else competes with what physical exercise does to increase the number of new brain cells that we make every day.” That neurogenesis has been  flowing to the brain during exercise.

How much exercise do you need to do to get these brain benefits? Ratey says even four minutes of cardiovascular exercise—working out at 55 to 60 percent of your max heart rate or more—can help change the brain. Too much exercise, however, can have a negative effect on the brain. “When your body is stressed and you have high levels of ,” a hormone released in response to physical and psychological stress, Ratey says, “the brain cells will shrink, and eventually they’ll die. But usually, it’s just a matter of shrinking.”