A person who spends eight hours lifting, carrying and standing at work may move far more than someone who sits at a desk all day.
It would seem logical that the more active worker should receive greater protection against dementia, but a huge international study suggests the picture is not that simple.
Researchers at the University of Southern California found that physical activity during free time was associated with a lower risk of dementia. In contrast, high levels of physical activity at work were associated with a higher risk.
The researchers combined results from 74 studies involving more than 4.2 million people in 30 countries. Participants ranged in age from 45 to 93 and were generally followed for about a decade.
During that time, researchers recorded diagnoses of dementia. They examined dementia overall as well as Alzheimer’s disease and vascular dementia.
Dementia develops when diseases or damage in the brain become serious enough to interfere with memory, reasoning and normal daily life. Age is the strongest risk factor, but it is not the only one.
Heart health, blood pressure, diabetes, smoking, education, social activity and exercise have all been connected with dementia risk. This has led health experts to encourage people to remain physically active as they get older.
The new research does not challenge the importance of exercise. Instead, it asks a more detailed question: Does the reason a person is moving make a difference?
Researchers separated physical activity into different parts of everyday life. These included exercise during leisure time, activity at work, household activity and active travel such as walking or cycling to get somewhere.
The clearest benefit appeared in leisure-time exercise. People who were most active during their free time had a 24% lower risk of dementia than people who were least active.
These activities could include going for walks, jogging, cycling, swimming or participating in sports. Greater amounts of leisure activity were generally associated with lower dementia risk until the benefit eventually appeared to level off.
The researchers found almost the opposite pattern with physically demanding work. Higher occupational activity was associated with a 20% greater risk of dementia.
That finding does not mean lifting boxes, standing or walking at work directly damages the brain. The researchers believe the circumstances surrounding physical work could be crucial.
Imagine two people who are physically active for several hours. One spends Saturday morning walking through a park with friends, while the other spends a work shift repeatedly lifting heavy objects under time pressure.
Both are moving, but the experiences are very different. The first person can choose the speed, stop for a rest and end the activity when tired.
The worker may have much less control. Repetitive movement, long periods of standing, heavy lifting and insufficient recovery can place continuing demands on the body.
Physically demanding jobs can also come with other disadvantages that have little to do with exercise itself. Some workers experience high stress, low control over their work, noise, polluted air or exposure to other workplace hazards.
Social and economic differences may matter as well. People doing heavy manual work may have different levels of education, income and access to health care, all of which can be related to long-term brain health.
They may also finish work exhausted. This could leave less time and energy for the kind of voluntary exercise that research has repeatedly associated with better health.
The unusual difference has already been observed in studies of heart disease. Scientists call it the “physical activity paradox” because recreational exercise appears beneficial while large amounts of physical activity at work may not produce the same advantages.
The USC research is the first systematic review to examine whether this paradox also appears in dementia. The results suggest that it does.
The study also raises questions about the popular health message that “every move counts.” Encouraging movement remains useful, but the researchers argue that public health advice may need to consider the circumstances in which people are active.
For example, giving workers enough time to recover from physically demanding jobs could matter. Reducing workplace stress and harmful environmental exposures could also have long-term health benefits.
Communities can also make leisure exercise easier. Safe parks, footpaths, walking trails and affordable recreational facilities give more people opportunities to exercise by choice rather than simply accumulating physical activity through work.
There are important reasons to be cautious about the findings. The research cannot prove that physically demanding work directly increases dementia risk because the studies mainly observed people’s existing lifestyles.
Another weakness is that most people reported their own physical activity. Memories about how often or how intensely someone exercised or worked can be inaccurate.
Evidence for housework and active commuting was also limited. Household activity appeared to be associated with lower dementia risk, but that result came from only one study, while the commuting findings were based on only two.
Most available evidence also came from high-income countries. This is an important gap because dementia is expected to become an increasingly serious health problem in low- and middle-income countries as populations age.
Despite these limitations, the enormous number of people included and the generally consistent pattern make the findings difficult to ignore. Leisure-time exercise continued to be associated with lower dementia risk even when researchers considered several other health and personal factors.
The study was led by Professor David Raichlen of the USC Dornsife College of Letters, Arts and Sciences, with Natan Feter as first author. The findings were published in The Lancet Public Health.
The research does not overturn the advice to stay active. Instead, it adds an important detail: exercise chosen for recreation may offer benefits that a tiring and stressful physical job does not automatically provide.
The 24% lower risk and 20% higher risk should also be interpreted carefully because they describe associations across large groups, not what will happen to a particular individual. More research is needed to separate the effects of movement itself from income, education, stress, working conditions and other differences between occupations.
For now, the strongest practical message is that being active at work should not necessarily be considered a substitute for enjoyable exercise. When it comes to protecting the aging brain, how, why and under what conditions we move may matter almost as much as the movement itself.
Source: University of Southern California.
