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Physical activity in prevention of Alzheimer’s - very interesting, but not conclusive
(June 2006)

Most studies find that physical activity lessens the thinking decline that accompanies aging and some suggest that it reduces the risk of Alzheimer’s. However, a sizeable minority of studies show no such benefit from physical activity, especially in regard to Alzheimer’s. Another problem is that there are considerable inconsistences in the positive studies relating to Alzheimer’s. A look at four of the studies will highlight the issues.

1. The Honolulu Heart Study included 3,734 men ages 70 to 93 years. The risk of dementia was less among men who walked one mile a day and even lower among those who walked at least two miles a day. There was some suggestion that those walking at a faster speed had less risk of dementia than those walking at a slower pace. The role of high blood pressure in the dementias could not be evaluated because the definition of high was 160 millimeters of mercury systolic, 95 millimeters diastolic; most experts would agree that a blood pressure of 140 systolic and over or a diastolic of 90 and over defines hypertension. That makes it a little harder to interpret the study because there is inadequate adjusting for high blood pressure.

2. A Canadian study, reported in the Archives of Neurology in 2001, looked at dementia in 4,615 community-dwelling men and women ages 65 and older who were followed for five years, during which time 285 developed dementia. The assessment of the role of physical activity was based on only two questions. One question focused on frequency - three or more times a week, weekly, or less than weekly. The other question related to intensity - more vigorous than, equal to, or less vigorous than walking. Men who exercised most frequently at the highest intensity showed a 27 percent reduction in risk of Alzheimer’s that was not statistically significant. Women who exercised at least three times a week at an intensity more vigorous than walking showed a 73 percent reduced risk that was statistically significant. An obvious problem is that the exercise questions were not at all sophisticated and were quite limited. Additionally, there was no attempt to adjust for hypertension or pre-hypertension. If they were less frequent in the group with high physical activity, that rather than the physical activity could account for the reduced risk of Alzheimer’s.

3. A four community study of 3,375 men and women, average age 75 years, followed for five years for dementia occurrence was published in the American Journal of Epidemiology in 2005. During that five-year period, 480 participants developed dementia. Physical activities were measured quite accurately according to intensities, but then a composite score was created that admixed more vigorous and less vigorous activities. Those with the greatest amount of physical activity had a 30 percent reduced risk of developing Alzheimer’s, but the reduction was not statistically significant. However, when physical activity was analyzed by number of activities, those with more different types of physical activities in a given time period had a 45 percent reduction in risk that was statistically significant. Interestingly, a clear risk factor for Alzheimer’s is a certain specific gene pattern. For those with this gene pattern, high physical activity was not protective; it was protective for those without this genetic marker for both those with Alzheimer’s and those with dementia due to insufficient blood supply to the brain.

This is a good study with some real problems. Those who developed dementia had many more chronic conditions; some were adjusted for, but not high blood pressure. A lower blood pressure, not the exercise, might explain the apparent benefit of exercise. It is also of some concern that one of the two measurements of physical activity, the better one, showed no significant protection. The finding of protection only for those without the genetic marker is also of some concern.

4. One thousand three hundred forty-three men and women, ages 65 years and older from Seattle, Washington were followed for six years for development of dementia; 107 developed Alzheimer’s. Those who exercised at least three times a week had a 32 percent reduced risk of developing Alzheimer’s during the followup period Exercises included: walking, swimming, stretching, bicycling, and “other exercise”. At baseline, those who later developed dementia had lower physical function, took less vitamins, and had more depression. In this study, published in the prestigious Annals of Internal Medicine in January 2006, there was no measurement of intensity or duration of exercise, just frequency. Additionally, there was no dose-response; exercising three, four, five, or seven times a week had the same reduced risk; indeed, surprisingly, those who exercised more than seven times a week had, if anything, less benefit from the exercise. The authors concluded that their results suggest a delay in onset of Alzheimer’s (rather than full prevention).

The four cited are among the best studies, but they either have major flaws or difficulties in interpretation, and there are about on equal number of studies showing no benefit from exercise in regard to dementia.

To summarize the problems in these studies:

- lack of information on intensity or duration of exercise

- co-mingling more and less vigorous activities

- huge differences at baseline between those who did and those who did not develop dementia

- failure to adjust for pre-hypertension or hypertension

- inconsistencies within studies (eg, protection in women, not men; benefit from one measurement of physical activity, but not from another).

What, then, can we conclude from these and other studies about exercise and the mind?

- Exercise probably does help keep the mind sharp and reduce or delay some of the thinking problems that accompany aging.

- Physical activity may or may not delay onset of or prevent Alzheimer’s. The studies available are just not good enough to allow a judgement.

- If physical activity does delay or reduce the risk of Alzheimer’s, we do not know the intensity or duration required for the benefit.

- If physical activity is beneficial, it could act in part by lowering elevated blood pressure (a known benefit of exercise) which is a likely risk factor for Alzheimer’s (and a definite risk factor for vascular dementia).

So, there is a lot we do not know about physical activity, exercise, and Alzheimer’s. Still, physical activity has been shown to have multiple benefits and should be encouraged for a lot of reasons. Physical activity and exercise probably do benefit the mind, and if they delay the onset of or reduce the risk of Alzheimer’s, so much the better – but do not bet on it just yet.

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