Socioeconomic status and ischaemic heart disease mortality in middle-aged men: importance of the duration of follow-up. The Copenhagen Male Study

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Abstract
Objectives The predictive value of some risk factors may diminish with increasing duration of follow-up. This study was performed to elucidate the role of socioeconomic status as a risk factor for ischaemic heart disease (IHD) mortality in middle-aged men, testing the hypothesis that the role of mediators of the association of socioeconomic status with risk of IHD would diminish with increasing length of follow-up. Methods A cohort of 5249 men aged 40–59 was established in 1971. Baseline data on social class and other confounder variables were collected, and the cohort was followed through registers for 8, 15, and 22 years. In all, 5028 without a history of myocardial infarction or angina pectoris were included in the follow-up. Four factors associated with either occupation or lifestyle were strong mediators of the association found between social class and risk of fatal IHD, and were more common in the lower social classes (classes IV and V): occasional demand for vigorous activity at work, low leisure time physical activity level, high alcohol consumption, and smoking. Results After the first 8 years, 78 men had died due to IHD, after 15 years: 222, and after 22 years: 411. Compared with social classes I, II, and III, the age-adjusted relative risk (RR) with 95% CI for classes IV and V was 1.69, P < 0.05 after the first 8 years; adjusted for the above potential risk factors the RR dropped to 1.09, P = NS. Corresponding RR after 15 years were 1.67, P < 0.001 and 1.33, P = NS; and after 22 years, 1.59, P < 0.001 and 1.36, P < 0.05. Conclusions Risk factors with an uneven social distribution related to occupation and lifestyle were strong mediators of the association of socioeconomic status with risk of IHD. A quite strong explanatory potential persisted but diminished with length of follow-up.