Not all physical activity is equal when it comes to protecting the brain from dementia. Where and why we move may matter more than most people realise.
A large new review of studies published in The Lancet Public Health found that leisure-time physical activity – such as sport, dancing or going to the gym – was linked to a 24% lower risk of dementia. Housework was linked to a 15% lower risk, although this finding came from just one study.
Occupational physical activity, on the other hand, was linked to a 20% increased risk of dementia.
The study compiled data from 74 studies involving more than 4.2 million people in 30 countries, making it one of the largest studies of its kind.
Brain protection is well established
Physical activity may protect the brain in several ways. It improves blood flow, helping to deliver oxygen and nutrients to the brain. It also causes muscles to release molecules that can reduce inflammation, help the brain use energy and encourage the growth of new brain cells.
That is one reason why reducing physical inactivity is a key part of the World Health Organization’s recently updated dementia prevention guidelines.
There is plenty of evidence that being active can help protect against dementia. A UK Biobank study tracked people’s activity using wearable devices for four years. Even less than 35 minutes of moderate-to-vigorous activity a week was linked to a 41% lower risk of dementia compared with no such activity.
More vigorous exercise – the kind that leaves you too breathless to talk – was linked to a 63% lower risk of dementia compared with being inactive.
Meanwhile, the English Longitudinal Study of Ageing followed people for 17 years and found that the more active people were, the lower their risk of dementia. The most active group had about half the risk of the least active group.
Walking outdoors – even as little as two miles a day – and gardening have also both been linked to a 25 to 36% lower risk of dementia.
But the new Lancet study suggests that physical activity at work or while travelling to work may not offer the same protection. It may even be linked to a higher risk of developing dementia.
Physical activity paradox
Physical activity at work, based on the combined data from five studies, was linked to a 20% higher risk. And active commuting, such as walking or cycling, based on just two studies, was linked to a 10% higher risk.
Researchers call this the “physical activity paradox”. It is well documented in heart disease research: physical activity at work fails to deliver the same protective effect as the same activity done during leisure time, and can sometimes increase risk instead. Several explanations have been proposed.
People in physically demanding jobs may have had fewer opportunities for education than people in office jobs. Education and mentally stimulating activities throughout life are linked to greater reserve capacity in the brain – its ability to cope with damage and keep working normally.
Manual workers may also be more likely to work in polluted environments, such as factories or building sites. And commuting, particularly by bicycle or on foot, can mean greater exposure to air pollution. Air pollution has separately been linked to a higher risk of Alzheimer’s disease.
Recreational exercise accounts for just 4% of active time in low-income countries, but 28% in high- and middle-income countries. So differences in income may shape the factors that influence dementia risk around the world.
The researchers say more research is needed, particularly in low-income countries, to understand what is behind the link between physical activity at work and dementia.
Not all physically demanding jobs are the same. Two physically demanding jobs can involve similar amounts of activity but very different exposures to noise, pollution, stress, heat and other hazards.
These findings point to a deeper inequality: leisure time itself is a resource – one that isn’t distributed evenly. People in low-income countries and those who do physically demanding jobs often have the least opportunity for the kind of activity that seems to protect the brain, while facing the working conditions that may undermine it. Understanding and addressing that imbalance may matter as much for dementia prevention as encouraging people to move more.
Eef Hogervorst works for Loughborough University National Centre for Sports and Exercise Medicine. She has received funding from many funders including NIHR, ESRC, UKRI, British Council Newton Trust to investigate exercise and physical activity to reduce dementia risk.
* This article was originally published at The Conversation
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