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HomeHealth & FitnessWhy all physical activity is not equally beneficial

Why all physical activity is not equally beneficial

For optimal health, regular physical activity (PA) is recommended to protect against dementia, cardiovascular disease, cancer and other non-communicable diseases – yet experts suggest some people might actually run an increased risk for the very conditions PA is intended to prevent.

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.

However, for many people, PA is a work requirement over which they have little control, and emerging evidence suggests these workers not only do not reap the benefits associated with leisure-time PA, but they also experience a greater risk for various conditions which 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 the age of 33 to 65, to assess PA trajectories and risks for later-life mild cognitive impairment (MCI) and dementia at 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, told Medscape Medical News.

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, like climbing, lifting, balancing, walking, stooping and handling of materials.

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 70 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 conclude 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

Kirsten Nabe-Nielsen, PhD, lead author of this study, said: “The WHO guide to preventing dementia and disease … mentions physical activity as an important factor. But our study suggests it must be a ‘good’ form of physical activity, which hard physical work is not.”

Beyond dementia, another recent study adds to a wealth of data on associations between occupational PA and cardiovascular risks.

The cross-sectional analysis of US 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.

“Our national guidelines for PA include occupational activity,” lead author Tyler Quinn, PhD, MS, West Virginia University, Morgantown, told Medscape. “But it’s clear many people who are getting a lot of occupational PA, particularly socioeconomic and racial/ethnic minorities, are not deriving benefits from it.”

The contrasting effects of leisure-time vs occupational PA constitute the “physical activity paradox” hypothesis.

Starting in 2011, multiple studies by Andreas Holtermann 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, said Pier Luigi Temporelli, MD, of Istituti Clinici Scientifici Maugeri, Italy, in a recent editorial.

Skirbekk and Quinn pointed to the PA paradox as an explanation for their own recent findings, suggesting the mechanisms that underlie it probably are responsible for the associated deleterious effects of occupational PA on the brain and heart, and even mortality.

“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, 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 much less autonomy, like nursing assistants, office cleaners, childcare workers, and other personal care workers. You cannot sit. You have somebody relying on you … it can be very hard. That’s where we find the associations.”

Lack of autonomy

Specific characteristics indirectly associated with high occupational PA jobs – low cognitive stimuli, lifestyle factors, and socioeconomic influences – as well as factors directly related to high occupational PA, like long hours, repetitive tasks, low levels of control, and stress, could also adversely affect cognitive trajectories, Skirbekk said.

“By contrast, leisure-time physical activities are of much shorter duration; are associated with socialisation, play, (and) positive emotions; and (include) the opportunity to take breaks or shift to other types of activities if one prefers,” he said.

“It may also be that too little or too much PA could be adversely related to cognitive outcomes, hence, moderate activity levels, for example 10 000 steps a day, are still probably beneficial for cognitive functioning.”

Quinn said most of the CVD risk linked to occupational PA has to do with long periods of exertion, such as lifting and carrying objects.

While occupational standing and walking all day are also linked to CVD risk, they’re not as risky as lifting and carrying, he said.

Like Skirbekk, Quinn noted that individuals can take a break from leisure-time PA when they are tired, but occupational PA doesn’t have that same autonomy to allow for recovery.

“So, in many cases, people are not getting the recovery their body needs to actually experience PA benefits, because those benefits come during rest.

“We’ve shown that PA at work raises acute cardiovascular responses – related to cardiovascular risk. For example, 24-hour and waking heart rate and diastolic blood pressure, as well as non-work diastolic blood pressure, all were significantly higher on workdays versus non-workdays,” he said.

Research gaps

Skirbekk said there are strategies that can reduce the risk for MCI and dementia despite high levels of occupational PA.

“It is often difficult to change professions, and even if you do, it won’t immediately affect cognition. But altering your lifestyle is likely to have effects on cognitive development across the life cycle.

“Many clinicians say they always advise lifestyle changes, but nothing happens. Yet it makes sense to emphasise that these changes – stopping smoking, eating well, getting proper sleep, etc. – affect not only cardiovascular risk but also cognition. And I think clinicians should also take a patient’s occupation into account during any evaluation."

Quinn said it isn’t realistic to expect workers to come up with solutions to the PA paradox because many don’t have the autonomy to be able to mitigate their occupational risk.

“I think administrative controls and policy changes eventually will be the levers of change … those are the types of things we should do when we’re trying to reduce loads in some way, or reduce the time people spend doing certain tasks we know are potentially bad.”

However, not everyone agrees that occupational PA doesn't confer the same benefits of leisure-time PA, at least with respect to cardiovascular risk.

For example, the Prospective Urban Rural Epidemiology (PURE) study, which includes a cohort of 130 000 people from 17 high-income, middle-income, and low-income countries, concluded that both higher recreational and non-recreational PA were associated with a lower risk for mortality and CVD events.

What additional research is needed to clarify the effects of occupational and leisure-time activity and to address conflicting findings?

“Even studies coming out now on the effects of occupational PA have mainly used older data,” Skirbekk said. “Labour markets and job demands have changed over time. There are different types of tasks and skills now from 20 or 40 years ago.

“And working from home is a recent phenomenon that’s happened on a large scale and might affect daily routines, sleep patterns, and also cognition. We need a better understanding of what the consequences might be.”

Health inequity issue

More research is also necessary to understand the social determinants of cognitive decline, impairment and dementia, he said.

“Many of today’s studies are based on self-report of what someone has done in the past, which is problematic for individuals who are impaired or who give interviews with others, which can induce biases.”

Quinn suggests that PA guidelines may need to differentiate between occupational and leisure-time PA to better reflect current research findings.

Meanwhile, Skirbekk and Quinn both point out the toll of occupational PA on the brain and body in lower-income workers as an important health equity issue.

“Our national guidelines for PA include occupational activity,” said Quinn. “But it's clear many people who are getting a lot of occupational PA, particularly socioeconomic and racial/ethnic minorities, are not benefitting from it.”

Holtermann, who has arguably done the most research to date on the PA paradox, said most workers with high occupational PA have a low socioeconomic position, so “improving our understanding of the underlying mechanisms behind the PA health paradox and identifying new intervention targets along those pathways will be an important step to reduce socioeconomic health inequalities across the globe”.

Study details

Trajectories of occupational physical activity and risk of later-life mild cognitive impairment and dementia: the HUNT4 70+ study

Ekaterina Zotcheva, Bernt Bratsberg, Bjørn Heine Strand, et al.

Published in The Lancet in November 2023

Summary

Background
High levels of occupational physical activity (PA) have been linked to an increased risk of dementia. We assessed the association of trajectories of occupational PA at ages 33–65 with risk of dementia and mild cognitive impairment (MCI) at ages 70+.

Methods
We included 7005 participants (49.8% were women, 3488/7005) from the HUNT4 70+ Study. Group-based trajectory modelling was used to identify four trajectories of occupational PA based on national registry data from 1960 to 2014: stable low (30.9%, 2162/7005), increasing then decreasing (8.9%, 625/7005), stable intermediate (25.1%, 1755/7005), and stable high (35.2%, 2463/7005). Dementia and MCI were clinically assessed in 2017–2019. We performed adjusted multinomial regression to estimate relative risk ratios (RRR) with 95% confidence intervals (CI) for dementia and MCI.

Findings
902 participants were diagnosed with dementia and 2407 were diagnosed with MCI. Absolute unadjusted risks for dementia and MCI were 8.8% (95% CI: 7.6–10.0) and 27.4% (25.5–29.3), respectively, for those with a stable low PA trajectory, 8.2% (6.0–10.4) and 33.3% (29.6–37.0) for those with increasing, then decreasing PA; while they were 16.0% (14.3–17.7) and 35% (32.8–37.2) for those with stable intermediate, and 15.4% (14.0–16.8) and 40.2% (38.3–42.1) for those with stable high PA trajectories. In the adjusted model, participants with a stable high trajectory had a higher risk of dementia (RRR 1.34, 1.04–1.73) and MCI (1.80, 1.54–2.11), whereas participants with a stable intermediate trajectory had a higher risk of MCI (1.36, 1.15–1.61) compared to the stable low trajectory. While not statistically significant, participants with increasing then decreasing occupational PA had a 24% lower risk of dementia and 18% higher risk of MCI than the stable low PA group.

Interpretation
Consistently working in an occupation with intermediate or high occupational PA was linked to an increased risk of cognitive impairment, indicating the importance of developing strategies for individuals in physically demanding occupations to prevent cognitive impairment.

 

The Lancet Regional Health article – Trajectories of occupational physical activity and risk of later-life mild cognitive impairment and dementia: the HUNT4 70+ study (Creative Commons Licence)

 

Scandinavian Journal of Medicine & Science in Sport article – The effect of occupational physical activity on dementia: Results from the Copenhagen Male Study (Open access)

 

AM Heart article – The Prospective Urban Rural Epidemiology (PURE) study: examining the impact of societal influences on chronic non-communicable diseases in low-, middle-, and high-income countries (Open access)

 

European Heart Journal – Is physical activity always good for you? The physical activity paradox (Creative Commons Licence)

 

Medscape article – Not All Exercise Is Beneficial: The Physical Activity Paradox Explained (Open access)

 

See more from MedicalBrief archives:

 

No limit to benefits of exercise in reducing risk of CVD — UK Biobank cohort study

 

Weekend exercise enough to stay fit – decade-long US cohort study

 

Leisure physical activity linked to health benefits but not work activity

 

Exercise test predicts CVD mortality risk

 

Sitting for hours can increase CVD risk, hasten death – cohort study

 

 

 

 

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