Researchers found that healthy people who reported exercising regularly had a 30 to 40 percent lower risk of dementia.

The study, published Tuesday 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, seemed 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 ages 65 and older who showed no signs of dementia at the outset. The participants' health was evaluated every two years for six years.

Out of the original pool, 1,185 people were later found to be free of dementia, 77 percent of whom reported exercising three or more times a week; 158 people showed signs of dementia, only 67 percent of whom said they exercised that much. The rest either died or withdrew from the study.

The study could not say if exercise helped prevented dementia altogether, because not all of the participants were followed up to their deaths.

The frequency of dementia was 13 per 1,000 person years for those who said they exercised three or more times a week, compared with 19.7 per 1,000 person years for those who reported exercising less.

Other researchers said randomized studies - in which participants would be randomly assigned to either exercise or maintain their usual habits - are needed to confirm the findings.

Bill Thies, vice president for medical and scientific affairs for the Alzheimer's Association, said a randomized trial with more people could help answer questions such as what types of exercise might help more than others.

"You would have to start with a group that had roughly common habits, and change those habits in one group and not in the other," Thies said.