Researchers found that healthy people who reported exercising regularly had a 30 to 40 percent lower risk of dementia.
The study, published today in the Annals of Internal Medicine, reached no conclusions about whether certain types of exercise helped more than others, but researchers said even light activity, such as walking, appeared to help.
"It seems like we are delaying onset," said Dr. Wayne McCormick, a University of Washington geriatrician who was one of the study's authors. "The surprising finding for us was that it actually didn't take much to have this effect."
Some researchers have theorized that exercise might reduce brain levels of amyloid, a sticky protein that clogs the brain in Alzheimer's patients.
The study, from 1994 to 2003, followed 1,740 people 65 and older who showed no signs of dementia at the outset. Their health was evaluated every two years for six years.
