8239169 Association between high occupational physical workload and ischemic heart disease, and influence of cardiorespiratory fitness

<h3>Objective</h3> High occupational physical workload may increase the risk of cardiovascular disease, while cardiorespiratory fitness could help prevent negative health effects. This study aimed to examine the association between high occupational physical workload in mid-life and later ischemic heart disease (IHD), and whether this relationship is influenced by cardiorespiratory fitness in youth. <h3>Material and Methods</h3> This study included 284,436 men born between 1951–1961. Occupational physical workload was assessed using a job exposure matrix in 2005 (age 44–54) and participants were followed for IHD incidence and mortality between 2006–2020 (age 45–69). Cardiorespiratory fitness in youth was assessed during military conscription using a maximal cycle test. Cox regression and additive interaction modelling, using the synergy index (SI), were applied. <h3>Results</h3> High occupational physical workload in mid-life was associated with increased risk of incident IHD (HR 1.34, 95% CI 1.29–1.39) and IHD mortality (HR 1.93, 95% CI 1.75–2.14), compared to low occupational physical workload. After adjusting for confounders including childhood socioeconomic position, cognitive ability, body mass index, blood pressure, and education, the associations were attenuated but remained significant: HR 1.06 (95% CI 1.02–1.11) for incident IHD and HR 1.38 (95% CI 1.23–1.55) for IHD mortality. The combination of low cardiorespiratory fitness in youth and later high occupational physical workload showed the highest IHD risk, indicating an additive interaction, though the SI was non-significant. <h3>Conclusions</h3> High occupational physical workload in mid-life was associated with increased risks of IHD incidence and, more strongly, IHD mortality. The highest risk was observed in those with the combination of low fitness and high workload. These findings support workplace and public health strategies aimed at reducing physical workload and enhancing cardiorespiratory fitness, particularly for youths entering physically demanding occupations.