For years, scientists have known that regular physical activity can help lower the risk of dementia. But new USC research suggests that it's not only the amount but also the activity's context - for work or for leisure - that makes a difference.
A meta-analysis of more than 4.2 million people has found that leisure-time exercise - activities such as walking, cycling, running, swimming or playing sports - is associated with a substantially lower risk of dementia. In contrast, physical activity performed as part of a job appears to be linked with a higher risk of developing the condition.
Paradoxical results
The findings, published in The Lancet Public Health, are the first to systematically examine whether the so-called "physical activity paradox" extends to dementia. This paradox, well documented in cardiovascular research, describes the surprising observation that physically demanding jobs do not provide the same health benefits as voluntary exercise - and may even increase disease risk.
The research team, led by Professor of Biological Sciences and Anthropology David Raichlen of the USC Dornsife College of Letters, Arts and Sciences, analyzed data from 74 cohort and case-control studies involving more than 4.2 million participants aged 45 to 93 years across 30 countries. Participants were followed for a median of 10 years, during which investigators tracked diagnoses of all-cause dementia, Alzheimer's disease and vascular dementia.
Overall, people who engaged in higher levels of leisure-time physical activity had a 24% lower risk of dementia than those who were least active. Household physical activity also appeared beneficial, although evidence came from only a single study. By contrast, occupational physical activity was associated with a 20% higher dementia risk, while active commuting showed a modest increase in risk, though those findings were based on just two studies.
"The findings challenge the long-held assumption that 'every move counts' for brain health," said Natan Feter, the study's first author, postdoctoral fellow in Raichlen's Evolutionary Biology of Physical Activity Lab at USC and researcher in the Graduate Program of Epidemiology at Universidade Federal do Rio Grande do Sul in Brazil.
Work versus play
"Physical activity" often combines exercise, manual labor, housework and commuting into a single measure. But according to the researchers, these different forms of activity occur under very different circumstances and may have distinct effects on long-term health.
Leisure-time exercise is typically self-directed, can be performed at an enjoyable intensity and is often accompanied by social interaction or stress reduction. Physically demanding jobs, on the other hand, may involve repetitive tasks, prolonged standing, heavy lifting, limited recovery time and exposure to workplace hazards.
The authors argue that the apparent harms associated with occupational activity are unlikely to be caused by movement itself. Instead, they may reflect the broader social and environmental conditions that often accompany physically demanding work.
Manual occupations are more likely to involve psychosocial stress, less job control, hazardous working conditions and greater exposure to air and noise pollution - all factors that have independently been linked to dementia risk. Workers in these jobs may also have fewer opportunities for recreational exercise because of time constraints, fatigue or limited access to safe places for physical activity.
Leisure-time exercise and occupational physical activity exhibited contrasting nonlinear associations with dementia risk. Higher levels of leisure-time exercise were associated with progressively lower dementia risk until the association plateaued, while higher levels of occupational physical activity were associated with progressively greater dementia risk before also plateauing.
Further investigation ahead
The authors note that evidence remains sparse for household and commuting activity, preventing firm conclusions about those domains. Likewise, most of the available research came from high-income countries, even though low- and middle-income countries are expected to experience the largest increases in dementia cases over coming decades.
Another limitation is that nearly all included studies relied on self-reported physical activity, which can introduce recall bias. Still, the consistency of the results across multiple sensitivity analyses gives confidence that the association should be taken seriously, Feter said.
Researchers caution that the results do not mean people should avoid physically active jobs. Because the analysis is based on observational studies, it cannot prove that occupational activity causes dementia. Instead, the findings highlight the importance of considering the context in which physical activity occurs. Leisure-time activity remained protective, while occupational activity remained associated with elevated dementia risk even after accounting for age, education and chronic diseases.
Additionally, research that Raichlen, Feter and colleagues published last month in the journal Alzheimer's & Dementia indicates that the context of sedentary behavior may have an impact on brain structures. In that study, physically sedentary but mentally active behavior, such as working while seated at a desk, was associated with higher volume in regions of the brain's cortex, including areas responsible for executive function, when compared to passive sedentary activity such as watching TV.
The findings indicate the importance of activity context and carry implications for public health, the authors emphasized. Rather than simply encouraging people to "move more," dementia prevention efforts should recognize that not all physical activity occurs under the same conditions.
"In other words, when it comes to protecting the brain, the context of physical activity may matter just as much as movement itself," Raichlen said. "Improving access to safe parks, walking trails and recreational facilities, ensuring people have adequate leisure time for exercise, and reducing harmful workplace exposures may ultimately prove as important as promoting physical activity itself."
About the study: Raichlen and Feter's co-authors include Tiladia Markarian, Dylan M. Luong, Jack Gunnink, Anamika Nanda, Sarah Hourihan and Ashley Ventura of USC; Eduardo Caputo, Jayne Feter, Gustavo S. Silva and Airton José Rombaldi of Universidade Federal de Pelotas, Brazil; Paula Iso-Markku of the University of Helsinki; Daniela Benzano Bumaguin of Fundação Universitária de Cardiologia, Brazil; Yann C. Klimentidis and Gene E. Alexander of the University of Arizona; Daniel Umpierre and Natália Schröder of Universidade Federal do Rio Grande do Sul, Brazil; and Pedro C. Hallal of University of Illinois Urbana-Champaign.
This research was supported by grants P30AG072980, P30AG019610, R56AG067200, R01AG064587 and R01AG072445 from the National Institutes of Health and funding from the state of Arizona, the Arizona Department of Health Services and the McKnight Brain Research Foundation. This study was also financed in part by the Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS) (25/2551-0002834-5) and the National Council for Scientific and Technological Development - CNPq and Department of Science and Technology of Secretariat of Science, Technology, Innovation and Health Complex of Ministry of Health of Brazil - MoH (445538/2023-6).
