Income as mediator of the effect of occupation on the risk of myocardial infarction: does the income measurement matter?

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Abstract
Aim: To investigate whether the effect of occupational grade on the risk of myocardial infarction (MI) is mediated by income with different aspects of income taken into account. Methods: Data were used from three prospective population studies conducted in Copenhagen. A total of 16 665 employees, 43% women, aged 20–75 years, with an initial examination between 1974 and 1992 were followed up until 1999 for incident (hospital admission or fatal) MI. Register based information on job categories and income was used. Results: During follow up, 855 subjects were diagnosed with a MI: 708 men and 147 women (in total 47% fatal). The hazards by household and individual income showed a graded effect with a hazard ratio (HR) of 1.43 (95% CI 1.12 to1.83) for the lowest household income group compared with the highest, whereas equivalent income showed an inverse “J shape” effect with a HR of 1.55 (95% CI 1.25 to1.82) for the third income group compared with the highest. HR for unskilled workers as compared with executive managers was reduced from 1.55 (95% CI 1.24 to1.93) to 1.42 (95% CI 1.12 to1.81) after adjustment for household income. Conclusions: Occupation and income are not mutually exclusive, but at least partly explained by or mediated through the other on the risk of MI. The mediating effect of income is independent of the choice of an income indicator. Income is not a big contributor to inequality in MI; probably because of the rather even income distribution in Denmark.