It’s not news that we should exercise. We know it, but we don’t do it. It’s hard to get started and harder still to keep at it. It’s so easy to come up with excuses.

But Harvard psychiatrist Dr. John Ratey thinks he has the nudge you need to get moving. It’s not six-pack abs or thinner thighs. It’s far more important and fundamental: "Exercise keeps your brain from eroding," he says.

"Exercise is the one thing we’ve proven again and again that prevents the ravaging of aging on our brain," and that’s "one thing people are still afraid of."

Ratey is trying to spread the word through lectures, books ("Spark: The Revolutionary New Science of Exercise and the Brain") and website (sparkinglife.org).

Other hazards of a sedentary life — obesity and Type 2 diabetes — have lost the fear factor. "People aren’t afraid of diabetes. People aren’t afraid of obesity." They think "we’ll get pills," he says, "But they are (still) afraid of losing their minds."

Ratey points out that a recent Mayo Clinic review of more than 2,000 scientific papers concluded that exercise is "the one thing you can do to prevent the onset of cognitive decline and Alzheimer’s disease."

And for those who really detest the idea of exercise, Ratey says, "This might be the clincher. … You get the most bang for your buck if you haven’t been exercising. The biggest changes are seen there."

By changes he means "improved intellectual capacity along with what we call emotional regulation — if you’re sour, you get a little more happy; if you’re anxious, you get a little less stressed and anxiety-driven." And this is true of kids too, who learn better and get higher test scores if they exercise regularly. The more intensity and time spent, the bigger the payoff.

"If you exercise three to six months on a regular basis, your brain actually grows," Ratey says.