The rate of dementia in men who once had jobs that involved tough physical work is almost one and a half times greater than in those whose work was sedentary, a study has found.
Dementia affects memory, thinking, behavior, and the ability to perform everyday activities.
According to the World Health Organization (WHO), around 50 million people worldwide are living with dementia, and the estimates suggest that the number will triple by 2050.
There is some evidence that physical exercise during leisure time, such as working out, cycling, jogging, and competitive sports, may protect against dementia in later life.
However, the latest research suggests that hard physical labor, such as construction work or house removals, has the opposite effect.
The explanation may lie in the different physiological effects of the two kinds of physical activity.
The scientists, led by the University of Copenhagen in Denmark, say physically demanding work often involves standing for long periods, maintaining “static” postures, lifting heavy objects, and working in repetitive or physically awkward conditions.
This kind of work may involve almost continuous exertion — for example, moving heavy loads or wielding power tools — with insufficient time between bouts for the body to recover.
By contrast, physical exercise during leisure time is often of high intensity but short duration, with plenty of recovery time. It also tends to involve more “dynamic” postures, like those involved in competitive sports, such as squash or basketball.
There is ample evidence that recreational exercise improves cardiovascular fitness and reduces inflammation around the body, the scientists write. By improving blood flow to the brain, boosting nerve growth, and preserving the hippocampus — the brain region where memories are encoded — this may help stave off dementia.
Sustained and repetitive physical labor, on the other hand, may impair cardiovascular fitness and increase inflammation. Previous research has found an association between high levels of physical labor and a higher risk of cardiovascular disease and mortality.
For people whose work involves a lot of physical exertion, poor cardiovascular function may also increase the likelihood that they will develop dementia.
“[This] is something other studies have tried to prove, but ours is the first to connect the two things convincingly,” says Kirsten Nabe-Nielsen, who led the research in collaboration with Denmark’s National Research Centre for the Working Environment and Bispebjerg and Frederiksberg Hospital in Copenhagen.
In public health advice on preventing dementia, agencies such as the WHO recommend physical activity without differentiating between the different types, says Nabe-Nielsen.
“But our study suggests that it must be a ‘good’ form of physical activity, which hard physical work is not,” she says. “Guides from the health authorities should therefore differentiate between physical activity in your spare time and physical activity at work, as there is reason to believe that the two forms of physical activity have opposite effects.”
The research has been published in the Scandinavian Journal of Medicine & Science in Sports.
For their analysis, the scientists drew upon data from the Copenhagen Male Study. This was a longitudinal study that began in 1970-71 and in which male employees of 14 large public and private companies filled out questionnaires about their work and lifestyle.
The participants were 40–59 years old at the start of the study. A total of 4,721 men were each followed up from when they turned 60 years of age until 2016.
The researchers used national registers to identify a total of 697 cases of dementia among the participants in the course of the study.
For their analysis, the researchers took into account other factors that could contribute to the risk of dementia, such as the participants’ age, socioeconomic status, marital status, and psychological stress. They also made adjustments for whether the participants smoked, how much alcohol they drank, and what their body mass index and blood pressure were.
Even after accounting for these factors, by the end of the study, the participants who reported high levels of physical labor had almost 1.5 times the rate of dementia than people who said their work was sedentary.
The authors note that one of the strengths of their study was its very long follow-up period. This is important, because pathological changes associated with dementia can start decades before the condition becomes apparent.
They also note several limitations. In particular, the amount of physical activity at work was reported only at one point in time, so the scientists’ analysis could not account for any changes in occupation either before or during the course of the study.
In addition, their analysis relied upon the dementia diagnoses given on death certificates. They write that even today Danish registers substantially underreport dementia.
This tendency to underreport dementia may have biased the results if it was more likely to occur in one study group than the other.
The lack of female participants is another major limitation. Physical labor and recreational exercise may not affect the sexes equally.