Question

A recent review of observational studies concluded that exercise in late life is inversely associated with risk for all-cause dementia and Alzheimer disease (6). Six of 9 studies reviewed showed a reduced incidence of dementia among persons in the highest category of exercise compared with those who were less active. Risk reductions ranged from 20% (7) to 50% (8) over follow-up periods of 5 to 7 years (6). The studies included in the review enrolled healthy adults who were free of dementia or mild cognitive impairment and evaluated the association of baseline physical activity with subsequent development of dementia. These studies had an important potential limitation: reverse causation bias, in which what seems to be an effect actually triggers its putative cause. Although the measurements of physical activity were done before clinically manifest dementia, these studies could not exclude the possibility that lower exercise levels at baseline were a consequence of early, subclinical cognitive deterioration. In the context of such diseases as dementia, in which changes in the brain may develop over decades, reverse causation is difficult to refute even in prospective cohort studies. The study by Larson and colleagues significantly adds to several studies published recently on the association of physical activity with risk for dementia. In the Finnish Cardiovascular Risk Factors, Aging, and Dementia Study (CAIDE), leisure-time physical activity in middle age was inversely associated with risk for dementia in older age. The association with physical activity in middle age is a strong argument against reverse causation bias, because it is extremely unlikely that subclinical dementia was affecting physical activity in middle age (10). Other studies have assessed specific types of physical activity. In the Honolulu-Asia Aging Study, older men who walked more than 2 miles daily had a 40% lower risk for dementia than did those who walked shorter distances (11). In the large Cardiovascular Health Cognition Study, the variety of exercise activity in late life was a stronger inverse predictor of future dementia than energy expenditure in physical activity (12). The evidence connecting physical activity and dementia is not consistent. Other studies have found inverse associations between various cognitive activities and incident dementia but no association with physical activity (13–14). This text is an abstract.

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