During the study, 71 of the 716 study participants developed Alzheimer's. Study authors say this is the first study to use an objective measurement of all physical activity in addition to self-reports. Participants wore an actigraph on their wrists to assess levels of activity. The mean score for participants was 3.3 hours per week. Intensity of exercise also mattered: People in the bottom 10% of intensity of physical activity were almost three times as likely to develop Alzheimer's.

Unfortunately, there's no concrete evidence that any protective step—be it brain games or dietary supplements—lowers the chances of Alzheimer's, according to a 2010 report by the National Institutes of Health. Although observational and animal studies show promise that the disease can be sidestepped, there's a lack of robust clinical studies involving humans, the gold standard in clinical research. "Can I say that exercise or diet will reduce your risk of Alzheimer's or delay onset so you get it at a later age? No, because we don't know for sure at this point," says Laurie Ryan, program director of Alzheimer's Disease Clinical Trials with the National Institute on Aging. "But the data we do have suggests that healthy living promotes healthy aging, which can only be good for you, even if it doesn't prevent or slow down onset."

"We've shown for cognition that an active lifestyle increases the quality of life in old age and lowers the levels of disability," Dr. Aron S. Buchman, associate professor of neurological sciences at Rush and one of the study's authors, said. "When you hear the results, they sound kind of intuitive: If older people are more active, you can see they're doing better. This kind of information is helpful. Even at age 82, you can improve independence and quality of life."