Q: I'm in my late 60s. I've read that regular exercise helps prevent memory loss. I find that hard to believe. How does it do that? And how much exercise do I need to reap this benefit?
A: It's easy to understand why regular exercise would be good for your bones, muscles, lungs and heart. Regularly challenging those organs would make them stronger.
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But the brain? Regular exercise often feels mindless. Indeed, that's one of its pleasures. You feel like you're turning off your turbulent brain and experiencing the rest of your body.
Nevertheless, many studies show that regular exercise -- in humans and in animals -- improves brainpower. We don't know how this works, but there are some reasonable theories.
For one, a stronger heart and lungs improves the supply of oxygen throughout the body, including the brain. For another, exercise helps reduce the risk for diabetes, high cholesterol, high blood pressure and stroke. These are all diseases that can damage the brain.
There's also some evidence that exercise increases the production of a brain chemical that multiplies the connections between brain cells. The communication between our brain cells is what makes us smart.
Finally, we've recently learned that regular exercise causes muscles to release substances in the blood that have positive effects on health.
For example, my Harvard colleague Bruce Spiegelman recently discovered a hormone made in muscle, called irisin. Regular exercise causes muscle to release irisin into the blood; it travels around the body and does lots of good things. It fights obesity by burning calories. Recent studies also indicate it may slow the aging of cells.
We don't know precisely how much exercise is needed for good brain health. But studies suggest that although the exercise needn't be extreme, it should be moderately vigorous and regular.
Below are exercise guidelines for you and other readers who are 65 years or older and are generally fit. They will deliver benefits for your body as well as your brain. If you have a health condition that limits your activity, do as much as you are able to do comfortably and safely. Always check with your doctor before starting an exercise program.
Aim to do the following:
• Two and a half hours (150 minutes) of moderate-intensity aerobic activity (e.g., brisk walking) every week; or
• One and a quarter hours (75 minutes) of vigorous-intensity aerobic activity (i.e., jogging or running) every week; and
• Muscle-strengthening activities on two or more days a week that work all major muscle groups (legs, hips, back, abdomen, chest, shoulders and arms).
You can also build physical activity into your daily routine. When possible, walk instead of driving; use the stairs instead of the elevator; plant a garden.
Above all, remember you're not exercising just to feel good and keep a healthy weight, although those are reasons enough. You're also doing it for your brain.
• Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.