A recent study published in BMJ determines whether physical activity assessed over 28 years protects against cognitive decline or the risk of dementia.
Advanced age is a major risk factor for dementia, and the “greying” of the population in many countries across the globe means that greater numbers of older adults will develop the disease. Advanced age is a non-modifiable risk factor, but other risk factors such as a sedentary lifestyle can be modified to delay or prevent the onset of dementia. Physical activity may protect against the development or progression of cognitive decline and dementia. However, studies have shown inconsistent findings, with some suggesting a protective role of physical activity and others showing no effect.
Many studies that show protective effects of physical activity are conducted over the short term, but dementia develops and progresses slowly over many years, predated by slow cognitive decline.This slow progression to dementia is known as the pre-clinical phase. To further complicate matters, reductions in physical activity may be a symptom of impending dementia, making it difficult to determine whether decreased physical activity causes dementia, or whether cognitive decline or mild dementia causes individuals to reduce or cease physical activity. Studies without long-term follow-up are unable to distinguish between these two scenarios.
Relationship Between Physical Activity and Cognitive Decline
In a study published in The British Medical Journal (BMJ), a group of researchers interested in disentangling the association between physical activity and cognitive decline used data collected across three decades from British civil servants based in London. The researchers tested the association between physical activity and cognitive decline when participants were 50, 60, 70, and 80 years old. They also investigated the relationship between physical activity and subsequent 15-year cognitive decline and the association between midlife physical activity and dementia risk. They evaluated changes in physical activity over nearly three decades prior to dementia diagnosis, as well as whether associations varied based on intensity or duration of activity. In all analyses, the researchers accounted for sociodemographic (e.g., age, sex, marital and socioeconomic status) and health-related (e.g., diagnosis of chronic disease, smoking and alcohol use, anxiety, and depression) factors that have been shown to influence participation in physical activity in previous research.
Strong Relationship Found
Overall, the researchers found that the association between physical activity and cognitive decline was stronger at older ages; the association suggested a decline in physical activity among older adults with cognitive decline or preclinical dementia. Fifteen-year cognitive decline was similar in all participants, regardless of intensity or duration of physical activity, and there was no relationship between midlife activity and dementia risk. When the researchers examined physical activity 28 years prior to dementia diagnosis, there were no differences in hours per week of physical activity, regardless of intensity, for those who developed dementia and those who did not 28 and 10 years before diagnosis. They did find that those who developed dementia reduced levels of physical activity about 9 years prior to diagnosis.
More Research Necessary
This study failed to find a protective effect of physical activity on cognitive decline or dementia risk. Instead, it showed that reductions in physical activity occur in the preclinical phase of dementia, years before actual diagnosis. While the study was strengthened by its longitudinal nature, including multiple assessments of physical activity across time, some cases of dementia may have been missed as diagnoses were obtained by electronic health record and not a clinical examination. Given that some longitudinal studies show a protective effect of physical activity on cognitive decline and others, like this study, fail to uncover an effect, more research is needed to determine the reasons for such differences.
Written by Suzanne M. Robertson, Ph.D
Reference: Sabia, Séverine, et al. “Physical activity, cognitive decline, and risk of dementia: 28 year follow-up of Whitehall II cohort study.” BMJ 357 (2017): j2709.