Worker’s fitness: a key to understanding the physical activity health paradox and exploring potential solutions

This editorial refers to ‘Association between high occupational physical workload and ischaemic heart disease, and the influence of cardiorespiratory fitness in 284 436 Swedish men’, by K. Berglund et al. https://doi.org/10.1093/eurjpc/zwaf344. Physical activity (PA) remains a cornerstone of cardiovascular disease prevention. Yet, the notion that all PA, regardless of context, should be considered equally beneficial has been challenged by the emergent literature suggesting a possible ‘PA health paradox’. Mounting evidence suggests that, while leisure-time physical activity (LTPA) provides well-known cardiovascular health benefits, occupational physical activity (OPA) may not provide equivalent benefits, and in some studies, even appears to be harmful.1,2 This paradox was again brought to light in the recent study by Berglund et al.3 where high OPA was associated with increased incident ischaemic heart disease (IHD) risk, especially among workers with low cardiorespiratory fitness, henceforth referred to as ‘fitness’. We commend the authors for not only their meaningful contribution to the PA health paradox literature, but for including analyses of fitness as a potential effect modifier. The Berglund study and other recent complimentary literature discussed below supports a path forward in research and practice for the PA health paradox: fitness may be a key modifier to reconcile the paradoxical health impacts of OPA. Using a large Swedish cohort of 284 436 middle-aged men, Berglund et al. observed that high OPA was associated with increased risk for incident IHD [hazard ratio (HR) = 1.06 (1.02–1.11)] and IHD mortality [HR = 1.38 (1.23–1.55)] compared to low OPA. Notably, the study included a rarely available direct measurement of worker fitness via cycle ergometer. When comparing associations across fitness levels, the most potent risks were among those with both high OPA and low fitness who had significantly higher incident IHD and IHD mortality risk compared to the reference group with low OPA and high fitness [incident IHD: HR = 1.20 (1.13–1.28); IHD mortality: HR = 1.60 (1.33–1.92)]. Men with high OPA and high fitness did not have increased incident IHD risk [HR = 1.03 (0.96–1.11)] but increased risk for IHD mortality [HR = 1.31 (1.07–1.59)] compared to the low OPA and high fitness reference group.3 These results appear to be stable across multiple cohorts and observations to date. A recent meta-analysis by Ketels et al.4 examined associations between OPA, fitness, and mortality in 9922 male workers from five European cohorts with 25.6 years of follow-up. High OPA was associated with elevated risks for cardiovascular [HR = 1.27 (1.04–1.55)] and all-cause mortality [HR = 1.22 (1.07–1.40)] among men with low fitness but not those with high fitness.4 Given the consistent signals pointing towards low worker fitness as an important modifying factor for the health impacts of OPA, we aim to reflect on the potential biological explanations and implications for research and practice. The overall body of evidence provides interesting support for the previously proposed fitness-related explanatory mechanisms to the paradox and the ‘fit-for-work’ concept, which poses that workers with good fitness are better prepared to safely handle physically demanding OPA.4,5 Holtermann et al.,5 in their 2018 commentary, outlined six possible biological explanations for the paradox. They proposed that the acute cardiovascular demands of OPA may be exaggerated among workers with low fitness due to an imbalance between the cardiovascular demand of OPA and the capacity of the worker.5 Long-term exposure to high relative physical work demands could catalyze inflammation, vascular damage, and disease and offer an explanation for OPA’s paradoxical health impacts.5 This mechanistic explanation is increasingly plausible given the consistent affirmatory epidemiological signals across multiple populations. However, contrary to this explanation, improved fitness among high OPA cleaners resulted in adverse impacts to resting heart rate and blood pressure in a randomized controlled trial.6 As such, future studies should continue to directly test these mechanisms using purpose-built, robust experimental designs or workday simulations with mechanism-driven outcomes. Like Berglund et al., researchers should continue to consider fitness as a potential effect modifier to OPA-health associations whenever possible. The inquiry could be strengthened in the future by considering such interactions across more specific OPA exposures. It appears that fitness modifies the OPA-health association across broad OPA levels.4 However, this interaction may crystalize further if examined across specific components or modalities of OPA. For example, high-intensity OPA (i.e. above the worker’s fitness capacity) may be more harmful to those with low fitness than low-intensity OPA tasks. Similarly, worker fitness may impact OPA-health associations differently across various OPA durations or modalities. To date, these important OPA constructs have been lost in commonly used broad OPA levels (e.g. low, medium, high). As outlined by Pronk,7 OPA researchers can borrow a framework from best practice exercise prescription to quantify OPA exposure in terms of frequency, intensity, time, and type (i.e. F.I.T.T.). These specifics, which have rarely been captured to date, especially in females, could help identify more precise OPA exposures where worker fitness is most crucial for preserving health. The building evidence also points to fitness as a potential intervention target to mitigate the adverse cardiovascular impacts of OPA. Contrary to current international PA guidelines,8 fitness-promoting PA outside of work (e.g. LTPA) may be necessary as supplementary to OPA to preserve health. This may be even more important, for workers who are already achieving recommended PA levels at work. Employers and health promoters can advocate for improved worker fitness in those with high OPA via PA outside of work hours to ensure that they are capable of safely completing the required work tasks. Such efforts could be supported through employer sponsored health promotion programs, financial subsidies for health-promoting expenses, etc. While workplace wellness interventions have a long history, such interventions show limited effectiveness in general and have predominantly catered to workers of higher socio-economic status, leaving workers with high OPA underserved.9 Future studies should address the persistent gap in understanding both the feasibility and cardiovascular health impacts of fitness-promoting interventions for workers in physically demanding jobs. As the PA health paradox continues to unfold, worker fitness stands out as a reproducible, biologically plausible, and actionable modifier to explore further. With this foundation, researchers have an opportunity to develop purpose-built studies examining the nuanced health impacts of balancing worker fitness with specific OPA F.I.T.T. characteristics. Researchers and practitioners have an opportunity to further leverage worker fitness as a potentially high-impact intervention target to mitigate the adverse health impacts of OPA. Although we should continue to strive for healthy OPA levels, steps towards fitness improvements offer not only a path toward cardiovascular risk reduction but also a tangible strategy to support health equity in physically demanding occupations. TDQ received salary support by the National Institute of General Medical Sciences (5U54GM104942-07). No new data were generated or analysed in support of this research.