High occupational physical activity (OPA) seems to increase risk of cardiovascular diseases among men. However, findings are mixed, and it is not known if women are differently affected. Therefore, the aim of this study is to investigate the relationship between OPA and risk for ischaemic heart disease (IHD), and whether it differs across sex.

Methods and results
This prospective cohort study was based on 1399 women and 1706 men, aged 30–61 years, participating in the Danish Monica 1 study in 1982–84, actively employed, without prior IHD and answering an OPA question. The information on incidence of IHD, before and during the 34-years follow-up, was retrieved by individual linkage to the Danish National Patient Registry. Cox proportional hazards models were used to investigate the association between OPA and IHD. Compared to women with sedentary work, women in all other OPA categories had lower hazard ratio (HR) for IHD. Among men, the risk of IHD was 22% higher among those with light OPA, and 42% and 46% higher among those with moderate OPA with some lifting or strenuous work with heavy lifting, respectively, compared to men with sedentary OPA. Compared to women with sedentary work, HR for IHD was higher among men in all OPA categories. There was statistically significant interaction between OPA and sex.

Conclusion
Demanding or strenuous OPA seems to be a risk factor for IHD among men, whereas a higher level of OPA seems to protect women from IHD. This emphasizes the importance of taking sex differences into account in studies of health effects of OPA.