Leisure-time physical activity lowers the risk of dementia, but physically demanding labour may instead raise the risk, a study found.

On Aug. 19, Medical News Today (MNT) reported that an international study led by researchers at the University of Southern California found that not only the amount of physical activity, but also its timing and purpose, could affect dementia risk.

The researchers analysed 74 studies involving more than 4.2 million participants in 30 countries. Participants were aged 45 to 93 at the start of follow-up, and the median follow-up period was 10 years. The team also examined diagnoses of all-cause dementia, Alzheimer's disease and vascular dementia over the period.

The key point was that results differed by the type of activity. People who took part in leisure-time physical activity had a 24 percent lower risk of dementia than those with the least activity. By contrast, occupational physical activity was linked to a 20 percent higher dementia risk. Occupational activity data were compiled based on people who were employed.

Neurologist Steven Adler (스티븐 알더), who did not take part in the research, said it was meaningful as an analysis tracking more than 4 million people over a long period, but drew a line that, as an observational study, an association does not mean causation. It cannot be concluded that occupational physical activity causes dementia or that leisure exercise directly prevents dementia, he explained.

Improved cardiovascular health and increased blood flow were cited as reasons leisure-time exercise may be beneficial. Adler said regular exercise helps control blood pressure, blood sugar and cholesterol, and could also contribute to maintaining healthy connections between nerve cells. Autonomy and enjoyment were also presented as variables. He explained that activities chosen by an individual can provide social interaction and mental stimulation, and these factors could reduce stress and social isolation and increase brain resilience.

By contrast, the association between physical labour and increased dementia risk should be interpreted together with the overall work environment, the report said. The researchers said occupational physical activity was consistently linked to increased dementia risk even in analyses adjusted for age, education and chronic disease. They added the cause may lie in the job environment rather than movement itself. Factors such as psychosocial stress, harmful workplace exposure, and exposure to air pollution and noise could act together, they said.

Adler said people in physically demanding jobs often perform the same actions for long periods, such as standing or carrying items, and it is difficult for them to control pace and intensity on their own. He also said other workplace factors such as insufficient recovery time, shift work, noise and air pollution could overlap. He added that after hard labour, people may have less time and energy for leisure exercise, social activities and other behaviours that support healthy ageing.

Lead author Nathan Petter (네이선 페터) stressed the need for caution in interpreting the results. He said it cannot be ruled out that multiple factors not sufficiently reflected in the analysis influenced the findings, and explained that physical labour in particular is often intertwined with various social and environmental factors.

The researchers said exercise recommendations for dementia prevention should also change. They said it should go beyond simply advising people to move more and should ensure that everyone can access physical activity that is safe, enjoyable and self-chosen.

The study shows that the context of activity may be a bigger variable for brain health than the amount of exercise. It also confirmed that dementia prevention should be approached by managing cardiovascular health, sleep, social and mental activities and other controllable risk factors together, rather than being determined by regular exercise alone.