“The physical activity context is something we need to pay more attention to,” said I-Min Lee, an epidemiologist at the T.H. Chan School of Public Health at Harvard University. She researches exercise and health but was not involved in the study.
The findings suggest that why we’re moving might influence how that movement affects us.
They also raise questions with implications for everyone. Could being required to move somehow be less healthy than deciding to move? Is that because of stress, if so, or something else? And what, if anything, do those possibilities mean for those of us who sometimes feel we have to force ourselves to exercise?
Is all physical activity the same?
Scientists have suspected for some time that strenuous manual labour can influence health as much as leisure-time exercise does, but in the opposite direction – an effect known as the “physical activity health paradox”.
The paradox’s effects have been especially evident in longevity and heart health. In a 2018 review of previous studies, men who worked in jobs with heavy physical demands were about 18% more likely to die prematurely than men whose jobs didn’t require much labour.
Similarly, in a 2021 study of 16,974 male and female workers, those whose work involved “repeated lifting, pushing, pulling or bending” or frequent standing or walking were more likely to develop heart disease than other workers, even when the scientists controlled for sex, smoking, education and many other factors.
But these studies hadn’t considered brain health and people’s long-term likelihoods for dementia.
Looking at cognitive risks
So, for the new study, published in Lancet Public Health, researchers at the University of Southern California in Los Angeles appraised 74 past studies involving more than 4.2 million participants that examined the relationships between physical activity and dementia.
Some focused on voluntary exercise. Others on work-related physical activity. A few examined household activities, like cooking and cleaning, or active commuting, which are other ways people often accumulate physical activity. But the studies were too small to allow for meaningful analysis.
The studies included information, mostly from questionnaires, about how people moved during their work and home lives and whether they developed any type of dementia, including Alzheimer’s disease, within about a decade of joining the original studies.
The link between dementia and manual labour
When the scientists cross-checked activity and cognition, the associations they found were strong and a bit worrying.
Unsurprisingly, people who were the most active on their own time and initiative, who decided to exercise, were about 24% less likely to develop any kind of dementia than people who rarely moved much.
But men and women who spent the most time labouring at work were about 20% more likely to develop dementia than those whose jobs required little physical effort.
The risk of dementia dropped slightly if people were highly active both at work and outside of it, but stayed heightened. It also remained after the scientists accounted for sex, age, education and many socioeconomic factors.
The data “fit with the idea that not every piece of physical activity functions the same way,” said David Raichlen, a professor of human and evolutionary biology at USC and the study’s senior author.
Stress may be a factor
The results don’t explain, though, why or how occupational physical activity might contribute to dementia later.
But stress and lack of agency are probably involved, said Natan Feter, a postdoctoral research associate in Raichlen’s lab and the study’s lead author. Having to move, whether you really plan or wish to, might increase stress hormones or cause other physiological reactions in the body that undermine the usual benefits of activity.
Studies with lab mice have shown their bodies respond differently when they run on wheels, of their own volition, rather than on treadmills, which are controlled by scientists, Feter said.
But other factors also could be at play, Raichlen said, including repeated physical fatigue at work or occupational exposure to toxins such as air pollutants or chemicals.
The contributors might also be unrelated to the physical activity itself, said Lee, and instead be tied to lower incomes, less medical care or other “associated phenomena,” she said, “and not the occupational physical activity, per se”.
Prioritise exercise during your free time
None of this data changes the broader point that moving is still healthy, Raichlen said, in most contexts and for most of us. If you mostly sit on the job, getting up and walking around is still almost certain to improve your health, even though the activity takes place at work.
Ditto for dragging yourself reluctantly to the gym or onto the running path. At base, those workouts remain a choice – and often a pleasure and relief. “We all know how often we didn’t want to work out and afterward felt so much better,” Raichlen said.
But for people whose jobs demand frequent manual labour, and possible related health risks, any solutions are unlikely to be personal. “The individual decision-making isn’t what we should be relying on,” Raichlen said. Better monitoring of workplace conditions might help but isn’t something people can do themselves. “These are public health problems that require public health solutions,” he said.
The good news embedded in the study is that if people aren’t too exhausted by their occupational labour, leisure-time physical activity – going for a walk or playing with the children at dusk – might help to improve health and protect brains over time. “Trying to create opportunities for physical activity during leisure time is a good idea,” Feter said, “so the decision of being active or inactive is your choice.”