Physical activity has long been associated with better brain health and a lower risk of dementia. But new research suggests where and why people are physically active may matter as much as how active they are.
A systematic review and meta-analysis of 74 studies involving more than 4.2 million adults found that leisure-time physical activity was associated with a lower risk of dementia, while physically demanding activity performed as part of a job was associated with a higher risk. The findings suggest that treating all physical activity as equally beneficial may obscure important differences in how activity affects brain health. The analysis was published in Lancet Public Health.
Overall, high levels of physical activity were associated with a 20% lower risk of all-cause dementia, a 21% lower risk of Alzheimer’s disease, and a 29% lower risk of vascular dementia. However, those associations changed when researchers examined different types, or domains, of physical activity.
High levels of leisure-time activity, such as exercise or recreational sports, were associated with a 24% lower risk of all-cause dementia. Household physical activity was associated with a 15% reduction, although that finding was based on only one study.
In contrast, occupational physical activity was associated with a 20% higher risk of dementia, while active commuting was linked to a 10% higher risk. Evidence for commuting and household activity was limited, making those estimates less certain.
“Our findings suggest that the association between physical activity and risk of all-cause dementia is domain-specific,” the authors wrote. “Leisure-time physical activity was associated with a reduced risk of dementia, whereas occupational physical activity was linked with an increased risk of dementia.”
The researchers caution that the results do not necessarily mean physical labor itself causes dementia. Physically demanding jobs can be associated with socioeconomic disadvantage, chronic stress, hazardous exposures, air and noise pollution, and reduced job autonomy—all factors that could affect long-term brain health. Manual occupations may also involve repetitive, lower-complexity tasks that provide fewer opportunities to build cognitive reserve.
“Consequently, the observed associations might partly reflect an accumulation of social, economic, and environmental adversities rather than be fully attributable to occupational physical activity per se,” the authors wrote.
The evidence also has important limitations. Most studies relied on self-reported physical activity, and the certainty of evidence across activity domains ranged from low to very low. Nearly all—93%—of the studies were conducted in high-income countries, despite projections that dementia will rise most rapidly in low- and middle-income countries.
