January 24, 2006

Does Exercise Prevent Dementia?

We would all like to believe that if only we eat well and exercise regularly, we will remain vigorous well into our eighties and perhaps our nineties. But is it true? A new study published this week argues that exercise can help fend off dementia, one of the major plagues of old age.

Writing in the Annals of Internal Medicine, a leading medical journal, Dr. Eric Larson and his colleagues in Seattle, Washington, describe a study in which they monitored a group of 1740 people over age 65 for an average of 6 years to see whether they developed dementia. To be part of the study, you had to be free of any evidence of dementia. You were asked questions about your medical conditions, whether you smoked, whether you used alcohol, and whether you rated your health as excellent, very good, fair, or poor. You were also asked one question about exercise: over the past year, how many times per week did you spend at least 15 minutes on an activity such as walking, hiking, bicycling, aerobics, or swimming. All the people who agreed to be in the study were then tested for dementia twice a year. By the end of 6 years, 1185 of them were still free of dementia, 158 had been diagnosed with dementia, and another 397 either died or withdrew from the study. The authors then compared the risk of developing dementia among those who had said they exercised at least 3 times a week with the risk of developing dementia among those who said they exercised less than 3 times a week. What they found was that when they made adjustments to account for factors such as age and gender, the exercise group was 32% less likely than the low-exercise group to be diagnosed with dementia. (See Eric Larson, Li Wang, James Brown et al, “Exercise is Associated with Reduced Risk for Incident Dementia among Persons 65 Years of Age and Older,” Annals of Internal Medicine 2006; 144: 73-81).

Physical activity in both middle age and old age is a good idea for a variety of reasons. In addition to its well-known effects on preventing heart disease, the leading cause of death in older adults, exercise can prevent or at least delay the onset of frailty, a syndrome in which people typically have multiple medical problems which together cause impairments in their ability to function in their daily lives (see my book, Lifelines: Living Longer, Growing Frail, Taking Heart, NY: Norton, 2001.) Exercise can help prevent obesity, which is currently epidemic in the U.S. But it’s important to remember that while exercise can decrease the risk of a variety of conditions that afflict older people, it does not prevent them. In the study of exercise and dementia, for instance, fully two-thirds of the people who developed dementia had exercised at least 3 times a week. Among those who did not get dementia, there were more regular exercisers (77% instead of 67%), but it was hardly the case that the couch potatoes got dementia and the exercisers did not. In fact, in this study of relatively young, educated, middle class people, dementia didn’t develop very often either in exercisers (10.38%) or non-exercisers (16.4%). Among the general population over age 85, by contrast, nearly 50% have dementia.

It’s also important to realize that the new study doesn’t definitively show a causal connection between lack of exercise and dementia. Because it is an observational study and not a randomized trial—the people in the study were watched and tested rather than being assigned to either an exercise group or a sedentary group—we don’t know for sure whether it was exercise that was responsible for the difference in outcomes. The authors did their best to make statistical corrections to account for factors such as age, cigarette smoking, and alcohol use that might affect the likelihood of getting dementia. But it’s possible that people who exercised (or more precisely who said they exercised) were also more socially active or intellectually engaged and it was those behaviors that helped protect them from dementia. It’s even possible that the people who didn’t exercise were already in the early stages of dementia—so early they still did well on the tests of cognitive function they were given—and that it was the dementia that prevented them from exercising rather than the other way around.

The answer to whether you should exercise, whether you are 60, 70, or 80, is a resounding yes. But will exercise save you from dementia? Maybe, but don’t count on it.

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