We’ve known for quite some time that regular physical activity is associated with a reduced risk of dementia. But according to a new meta-review published today in The Lancet Public Health, that association comes with a key caveat—it depends on the context of the activity.
Combing through research into activity and dementia involving more than 4.2 million adults, a team of researchers found stark differences between work and play. Leisure time activities (running, cycling, playing sports, and so on) showed a reduced risk of dementia, but physically demanding jobs showed an increased risk. In general, those who exercised more during their free time saw their risk of dementia cut by 24 percent, while those who worked tough jobs saw theirs rise 20 percent.
“When it comes to protecting the brain, the context of physical activity may matter just as much as movement itself,” study author David Raichlen of the University of Southern California said in a statement. “Improving access to safe parks, walking trails, and recreational facilities, ensuring people have adequate leisure time for exercise, and reducing harmful workplace exposures may ultimately prove as important as promoting physical activity itself.”
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Called the “physical activity paradox,” this dynamic was already known for risks of cardiovascular disease, but now it extends to dementia, as well. But why?
The reasons are murky; the team, though, has some ideas. For starters, leisure activity is usually, well, relaxing. It reduces stress, another contributor to the risk of dementia, and is often combined with socializing, something that seems to provide protection against the disease as well.
As anyone who’s held a physically demanding job can tell you, they’re quite the opposite. They also usually involve a lot of repetitive tasks, limited recovery time, and exposure to hazards like noise and air pollution—all of which could contribute to dementia, too. Additionally, they can leave you with little time or desire to exercise recreationally.
Of course, because this is just an observational study, the researchers can’t say for sure, but it’s another finding that points to the importance of context. Two of the authors conducted a similar study on the context of sedentary activity that was published last month. They found that people who remained mentally active while sitting, such as working at a desk from home, showed larger volumes in their reasoning centers and other neural regions compared to those who spent their chill time doing something passive, like watching TV.
Whether you’re moving or not, context, it turns out, is king. 
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