By Amanda Gardner

WEDNESDAY, April 18, 2012 ( — In recent years, several studies have found that older people are less likely to develop Alzheimer's disease and other forms of dementia if they engage in vigorous exercise, such as jogging, swimming, or brisk walking.

People who are too frail or out of shape to hit the pool or treadmill shouldn't despair, however. According to a new study, even mundane, low-key tasks like gardening, cooking, and washing dishes can lower the risk of Alzheimer's if they're performed often enough.

The study, which was published this week in the journal Neurology, included 716 dementia-free men and women in their 70s and 80s. Compared to the most active people, those with the lowest levels of overall physical activity had more than double the risk of going on to develop Alzheimer's disease. Greater physical activity was also associated with a slower rate of aging-related memory and cognitive decline.

"This suggests that people in their 80s who can't participate in formal exercise still get a benefit by leading a more active lifestyle," says lead author Aron S. Buchman, MD, associate professor of neurological sciences at Rush University Medical Center, in Chicago. "You don't have to get a membership in the local YMCA. If you walk up some more steps, stand up and do the dishes more, you stand to benefit because it's incremental and adds up over the course of a full day."

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Most previous research on physical activity and dementia risk has relied on questionnaires that ask participants to remember how much exercise they got in recent days—a potentially iffy method with people of any age, let alone older adults whose memory may be waning. The new study, by contrast, is among the first to use an objective measure of physical activity.

All of the participants wore a motion-sensitive, wristwatch-like device 24 hours a day for up to 10 days. These devices, known as actigraphs, have been shown to provide an accurate snapshot of a person's total everyday activity, including mild activity. "It doesn't make a difference if you're chopping onions, or walking up and down stairs, or on an exercise machine," Buchman says.

Over the next four years, the participants underwent annual cognitive tests and were asked to report how often they engaged in physical activities such as gardening, walking, and swimming, as well as social and brain-stimulating activities. (The researchers took all of these activities into account, along with other variables such as age, sex, education, overall health, depression, and genetic factors.)

Roughly 10% of the participants received an Alzheimer's diagnosis during the follow-up period. The higher a person's activity level, the lower his or her risk of Alzheimer's tended to be. The participants who were least active at the beginning of the study—those with actigraph readings in the bottom 10th percentile—were 2.3 times more likely to receive a diagnosis than those in the 90th percentile.

The findings show only an association, and do not establish that physical activity directly prevents Alzheimer's. That said, Buchman and his colleagues assessed the participants' cognitive health and prior physical activity in detail at the start of the study, which allowed the researchers to all but rule out the possibility that undiagnosed or early-stage dementia was leading to low physical activity, says Michal Schnaider Beeri, PhD, an associate professor of psychiatry at the Mount Sinai School of Medicine, in New York City.

The big picture provided by actigraphs is the study's main selling point, but it also leaves some questions unanswered. Because actigraphs don't differentiate between the type or intensity of activity, it's difficult to determine whether some types of physical activity protect against Alzheimer's more than others.

The physical activity in the study "was for the most part heavily weighted toward non-exercise activity," Buchman says. This non-exercise activity appears to be beneficial, but the study findings suggest that exercise might be even better.

Buchman and his colleagues tried to estimate the intensity of the participants' activity by looking at whether their movements were spread out evenly throughout the day or clustered in short bursts suggestive of vigorous exercise. By this measure, the people whose physical activity was least intense were nearly three times more likely to develop Alzheimer's compared to those whose activity was most intense.

In general, the evidence to date suggests that more vigorous activity appears to be better when it comes to staving off Alzheimer's, says Richard S. Isaacson, M.D., an associate professor of clinical neurology at the University of Miami Miller School of Medicine.

"Do the exercise—push it," Isaacson says. "This is one part of the puzzle."