Physical Fitness Can Fight Off Dementia

Summarized by Robert W. Griffith, MD
February 13, 2006

Introduction

We know that keeping the mind nimble will help reduce the chances of developing Alzheimer's, but what about physical activity? There have been a number of studies in the past that suggest it may be as important as mental activity, but most of these have been so-called 'cross-sectional' studies, where the lifestyles of people with Alzheimer's are compared with those of a group of similar people but who don't have dementia. In a cross-sectional study an apparent link between a specified factor (exercise) and a disease (Alzheimer's) may be suggested, but it only represents a snapshot in time, and cannot prove a particular cause-and-effect. This sort of information is best obtained from a prospective study, in which a group of subjects (called a 'cohort') is studied from a baseline time point for a number of years.

One relevant prospective cohort study was reported from Canada in 2002. Now another has been published, in the Annals of Internal Medicine, making it even more imperative for older people to embark on an exercise regime to try to avoid this distressing disease.

What was done

Seattle scientists enrolled 1740 people over 65 who didn't have mental impairment. In fact, they scored highly on tests of mental functioning. Measurements at baseline included an examination of physical performance (a timed walk, time to stand up from seated position 5 times, standing balance, and grip strength), a test for depression, general health status, and lifestyle characteristics. They were asked to report on the number of days a week they took part in at least 15 minutes of exercise - walking, hiking, bicycling, aerobics, weight training, or equivalents.

The subjects were examined every 2 years to see if they had developed any evidence of dementia, and to assess the status of their exercise program.

What was found

On average, the follow-up period was 6.2 years. During this time 158 participants had developed dementia (107 cases of Alzheimer's, 51 cases of non-Alzheimer's dementia, chiefly vascular dementia).

The participants were classified into two groups, according to whether they exercised three or more times a week, or fewer than three times a week. For those who exercised less than three times weekly the rate of dementia was 19.7 cases per 1000 person-years. For those exercising three or more times weekly the rate was 13 per 1000 person-years. This represented a 32% reduction in risk of dementia for the more physically active participants.

After adjustments for the age and gender of the participants, the numbers were only slightly altered. They showed a 38% reduction in the rate of dementia in the group who exercised more, compared with those who exercised less. Interestingly, those who started with the lowest exercise levels at baseline had the greatest risk reduction with increased exercise.

What this study means

Previous studies of this nature have been criticized for the possibility of 'reverse causation bias'. This is a complicated term for the concept that pre-existing very early Alzheimer's before symptoms develop would cause those affected to exercise less, thus producing the final study numbers when their Alzheimer's had been diagnosed. Fortunately, the cohort in the present study was selected to have a high cognitive ability, effectively discounting the likelihood of any reverse causation bias.

There are lab studies that support the concept that physical activity will reduce the risk of dementia. Active adult mice have reduced accumulation in the brain of evidence of nerve cell degeneration, such as amyloid plaque or products of oxidative stress, when compared with inactive mice. And the increased blood flow that exercise can produce may prevent damage due to poor blood flow that occurs in some people as they age.

The totality of work so far strongly suggests that physical activity is able to reduce the risk of developing dementia. Exercise has been proved beneficial for numerous conditions associated with aging - diabetes, cancer, heart attack, stroke, osteoporosis, and overweight. Why shouldn't it be helpful for preventing dementia, too?

Source

• Exercise is associated with reduced risk for incident dementia among persons 65 years of age and older. LEB. Larson , WL. Wang , BJD. Bowen ,  al., , 2006, vol. Ann Intern Med, pp. 73--81

Related Links
Can Exercise Help Protect You From Alzheimer's Disease?
NIH SeniorHealth: Exercise
AARP: Physical Activity
CDC: Growing Stronger - Strength Training for Older Adults

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