If you can’t remember the last time you exercised, consider this: Every time you jog, swim, bike or lift weights, you’re building more than just muscle. A new report finds that exercise also helps build a healthy supply of memory-enhancing neurons.

The link between exercise and memory is nothing new. Many studies show that active people can recall information more accurately than their sedentary peers. But scientists have finally discovered why regular exercise sharpens your mental edge, even into old age.

Most healthy adults begin losing their memory around age thirty, when a specific area of the hippocampus, called the dentate gyrus [dentate GY-russ], gradually slows production of nerve cells. Neurogenesis, as it’s called, is important for proper memory formation; when the process breaks down, forgetfulness sets in.

But how does exercise fit into the picture? To find out, scientists at Columbia University sent eleven out-of-shape volunteers to the campus fitness center for forty-minute aerobic workouts, several times a week, for three months. The volunteers took memory tests before and after completing the exercise regimen. Using a revolutionary M-R-I imaging technique, the scientists snapped the world’s first-ever photographs of live neurogenesis taking place within the dentate gyrus region of human brains.

The ground-breaking study reveals that physical exercise can help improve memory by stimulating generation of nerve cells and combating the neurological side effects of old age. These findings are especially important for the over-thirty crowd… exercise seems to be more important the older you get.