Moving your body is generally good for your health, but new research suggests that all physical activity may not have the same relationship with the aging brain.
A huge analysis found that exercise chosen for fun was linked to less dementia, while heavy physical activity at work was linked to more.
The research came from scientists at the University of Southern California and included information from more than 4.2 million people. It provides new evidence that the setting in which physical activity happens may be important for long-term brain health.
Dementia affects memory, reasoning and the ability to manage normal daily life. Alzheimer’s disease accounts for most cases, although several diseases and types of brain damage can cause dementia.
There is currently no guaranteed way to prevent dementia. However, researchers have identified several factors that may influence risk, including smoking, high blood pressure, diabetes, social isolation and lack of physical activity.
Exercise is thought to help in several ways. A healthier heart and blood vessels can improve blood flow to the brain, while regular activity can also help control weight, blood pressure and blood sugar.
But saying that physical activity is healthy leaves an important question unanswered. Is spending hours doing hard physical work equivalent to taking a brisk walk, going swimming or playing a sport after work?
To investigate, researchers reviewed 74 cohort and case-control studies from 30 countries. Together, these studies included adults from age 45 to 93 and followed them for a typical period of around 10 years.
Instead of combining all movement into a single number, the team examined activity according to where it occurred. They looked separately at leisure exercise, work-related activity, household activity and active commuting.
The strongest finding involved leisure time. People who reported higher levels of recreational exercise had a 24% lower risk of dementia than people who were least active during their free time.
The relationship was not unlimited. As leisure exercise increased, dementia risk generally became lower until the apparent benefit eventually reached a point where additional activity was no longer linked to much further reduction.
The results for physically demanding work went in the opposite direction. People with higher occupational physical activity had a 20% higher risk of dementia.
That does not necessarily mean that movement during work is harmful. A physically active job usually comes with a very different set of conditions from a voluntary exercise session.
Consider someone who chooses to cycle for an hour on a quiet morning. That person controls the route, speed, length of the ride and when to rest.
Now consider someone who spends an entire shift lifting, carrying or standing. The worker may have to continue despite tiredness, repeat the same movements and work at a pace set by someone else.
Leisure exercise may also lower stress and provide enjoyment. Walking with friends or playing a sport can add social contact, while outdoor activity may provide a break from the pressures of daily life.
Some physically demanding jobs bring extra risks that have nothing to do with movement. Workers may experience job stress, low control, noise, polluted air or other harmful workplace conditions.
These factors could partly explain the higher dementia risk seen in the study. Education, income and access to health care may also differ between people in physically demanding and less demanding occupations.
Another possibility is that hard physical work leaves people exhausted. Someone who is tired after a long shift may have little time or energy for recreational exercise, social activities or other healthy habits.
Scientists have previously noticed a similar puzzle in studies of heart health. This has become known as the physical activity paradox: exercise during free time is consistently associated with better health, while high levels of occupational activity may not offer the same protection.
The USC researchers say their analysis is the first systematic investigation of whether this paradox also applies to dementia. Their results suggest that it may.
There were hints about other types of movement, but the evidence was much weaker. Household activity appeared beneficial, yet only one study provided enough information to examine it.
Active commuting showed a modest increase in dementia risk, but this result was based on just two studies. The researchers said there is not enough evidence to draw strong conclusions about either household activity or commuting.
The study has other limitations. Most participants described their own physical activity, which means the information may be affected by imperfect memory or differences in how people judge activity levels.
The majority of studies also came from high-income countries. This leaves an important gap because some of the largest future increases in dementia are expected in countries with fewer economic resources.
Most importantly, the research cannot establish cause and effect. People are not randomly assigned to spend decades in physically demanding or sedentary jobs, so many differences between these groups could influence the results.
The researchers therefore do not recommend avoiding active work. Instead, the findings suggest that occupational movement should not automatically be assumed to provide all the same benefits as voluntary exercise.
The research was led by Professor David Raichlen and first author Natan Feter at USC and was published in The Lancet Public Health. It included more than 4.2 million participants, giving researchers an unusually large amount of evidence to examine.
Taken together, the study provides strong evidence of an association but not proof that work-related activity raises dementia risk. The size of the analysis is a major strength, while self-reported activity and differences between occupations remain important weaknesses.
For individuals, the practical message is not to fear movement at work. Rather, people with physically demanding jobs may still benefit from opportunities for enjoyable recreational activity, adequate rest and healthier working conditions.
For governments and communities, the findings suggest that dementia prevention may involve more than simply telling people to exercise. Providing safe parks and walking paths, protecting workers from harmful conditions and giving people enough free time to be active by choice may also be part of protecting brain health.
Source: University of Southern California.
