In the pursuit of optimal health, regular physical activity (PA) is recommended to protect against dementia, cardiovascular disease (CVD), cancer, and other noncommunicable diseases. A significant body of research suggests the benefits of PA are positively correlated with higher frequency and intensity – with more often deemed better. This research has spawned a focus on increasing step counts and investing in standing desks and other interventions aimed at keeping people active.
But , but they also actually experience an increased risk for the very conditions that PA is intended to prevent.
A study published recently in The Lancet Regional Health – Europe used registry data from more than 7,000 adults in Norway, following them from age 33 to 65 years, to assess PA trajectories and risks for later-life mild cognitive impairment (MCI) and dementia at age 70 or older.
“Incorporating a life-course perspective gives a broader picture of how participants’ occupational histories relate to cognitive impairment later in life,” principal investigator Vegard Skirbekk, PhD, Columbia University Mailman School of Public Health, New York, said in an interview. Other studies typically have assessed occupational PA at a single time point, often close to the end of an individual’s career, and largely relied on self-report, he said.
Study participants worked in more than 300 different occupations. General physical activities performed on the included jobs required “considerable” use of arms and legs and moving the whole body, such as climbing, lifting, balancing, walking, stooping, and handling of materials.
Dr. Skirbekk and colleagues grouped participants into four PA trajectories over the 44-year study period: stable low, increasing then decreasing, stable intermediate, and stable high.
A total of 902 individuals were diagnosed with dementia and 2,407 with MCI at age 70 years or older. After adjustment, risks for MCI and dementia were 15.5% for those with higher occupational PA scores in the latter part of their working life and 9% for those with lower physical demands. The researchers concluded that “consistently working in an occupation with intermediate or high occupational PA was linked to an increased risk of cognitive impairment.”
The findings support those of the Copenhagen Male Study. Published in 2020, this longitudinal study compared leisure-time and occupational PA among more than 4,000 men in Denmark aged 40-59 at baseline in 1970-1971 and followed them until they turned 60. After adjustment, participants with high occupational PA had a 55% greater risk of developing dementia compared with those doing sedentary work.
Good vs. bad PA
“[T]he WHO [World Health Organization] guide to preventing dementia and disease on the whole mentions physical activity as an important factor. But our study suggests that it must be a ‘good’ form of physical activity, which hard physical work is not,” said Kirsten Nabe-Nielsen, PhD, lead author of this study
Beyond dementia, another recent study adds to a wealth of data on associations between occupational PA and cardiovascular risks. The cross-sectional analysis of U.S. data from the National Institute for Occupational Safety and Health showed that odds of CVD were higher when participants were “always” performing total occupational activity (odds ratio [OR], 1.99), occupational exertion (OR, 2.15), or occupational standing and walking around (OR, 1.84) compared with “never” engaging in these activities.
The contrasting effects of leisure-time vs. occupational PA constitute the “physical activity paradox” hypothesis. Starting in 2011, multiple studies by Andreas Holtermann, PhD, of the National Research Centre for the Working Environment lend support to the PA paradox theory, as do subsequent studies by others.
Although only “marginally considered” until a few years ago, recent large cohort studies seem to confirm the paradox, Pier Luigi Temporelli, MD wrote in a recent editorial.
In separate interviews, Dr. Skirbekk and lead author Tyler Quinn, PhD, MS, West Virginia University, Morgantown, pointed to the PA paradox as an explanation for their own recent findings, suggesting that the mechanisms that underlie it probably are responsible for the associated deleterious effects of occupational PA on the brain and heart, and even mortality.
“It’s well established that PA in your leisure time can be positive, but in the workplace, the results are quite the opposite,” Dr. Skirbekk said. “The specific mechanisms for why occupational PA is associated with elevated dementia risk are still not well understood and we need more knowledge. But we know that higher occupational physical demands have been linked to smaller hippocampal volume and poorer memory performance.”
Furthermore, he said, individuals working in jobs with high demands, both psychological and physical, combined with low job control perform more poorly on cognitive testing later in life.
“We looked mainly at professions where people have heavy workloads and you have much less autonomy, such as nursing assistants, office cleaners, childcare workers, and other personal care workers,” he said. “You cannot sit. You have somebody relying on you. It’s not all pleasure, and it can be very hard. That’s where we find the associations.”