OBJECTIVES: Shift work has been associated with an increased risk of ischaemic heart disease (IHD). Most published studies have had potential problems with confounding by social class. This study explores shift work as a risk factor for IHD after controlling for social class. METHODS: The Copenhagen male study is a prospective cohort study established in 1970-1 comprising 5249 men aged 40-59. Information obtained included working time, social class, and risk factors for IHD. A second baseline was obtained in 1985-6. The cohort was followed up for 22 years through hospital discharge registers for IHD, and cause of death was recovered from death certificates. RESULTS: One fifth of the cohort was shift working at entry with a significantly larger proportion of shift workers in lower social classes. Risk of IHD and all cause mortality over 22 years, adjusted for age only, for age and social class, and finally for age, social class, smoking, fitness, height, weight, and sleep disturbances, did not differ between shift and day workers. The relative risk of IHD, adjusted for age and social class was 1.0 (95% confidence interval (95% CI) 0.9-1.2). Men being shift workers in both 1971 and 1985 had the same risk as ex-shift workers in an 8 years follow up from the 1985-6 baseline. CONCLUSIONS: The present study questions shift work as an independent risk factor for IHD. The results of the study emphasise the importance of controlling adequately for the interplay of shift work and social class.
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