Do Cardiovascular Risk Factors Explain the Relation between Socioeconomic Status, Risk of All-Cause Mortality, Cardiovascular Mortality, and Acute Myocardial Infarction?

American Journal of Epidemiology Copyright © 1996 by The Johns Hopkins University School of Hygiene and Public Health All rights reserved Vol. 144, No. 10 Printed in U.S.A. Do Cardiovascular Risk Factors Explain the Relation between Socioeconomic Status, Risk of All-Cause Mortality, Cardiovascular Mortality, and Acute Myocardial Infarction? John W. Lynch,1 George A. Kaplan,1 Richard D. Cohen,1 Jaakko Tuomilehto,2 and Jukka T. Salonen3 mortality; myocardial infarction; risk factors; socioeconomic factors The inverse relation between socioeconomic status (SES) and health has been observed for centuries (1). With few exceptions, this association exists regardless of the measure of SES that is employed or the health outcome studied (2-5). Even though the association between SES and health is a strong and consistent finding, is inversely graded across levels of SES, and has been noted in many countries across varying time periods, much remains to be understood about the ways in which SES affects health. In 1981, Rose and Marmot (6) showed that statistical adjustment for age, smoking, height, body mass index, systolic blood pressure, cholesterol, and blood glucose had only a moderate impact on reducing the magnitude of the inverse association between social class, as measured by occupational grade, and coronary heart disease risk in British civil servants. Since then, many studies have found that the elevated risks of disease associated with lower SES are not greatly attenuated by adjustment for traditional biologic and behavioral risk factors (7-13). In attempts to explain the persistent association between SES and health, a number of literature reviews on the subject have also pointed to an even wider variety of possible risk factors and have argued their potential importance in helping to understand the relation between lower SES and poorer health (14-17). The proposition that SES impacts health in some way independent of known risk factors, if true, has important implications for both research and public health policy. We investigated the association between income, all-cause mortality, cardiovascular mortality, and acute myocardial infarction (AMI) in a prospective study of a population-based sample of eastern Finnish men. Extensive information on medical, biologic, behavioral, psychologic, and social risk factors allowed the most comprehensive examination to date of the potential pathways that mediate the relation between SES and cardiovascular mortality, all-cause mortality, and AMI. Received for publication October 10, 1995, and in final form August 12, 1996. Abbreviations: AMI, acute myocardial infarction; Cl, confidence interval; RH, relative hazard; SES, socioeconomic status. 1 Human Population Laboratory, California Public Health Foundation, Berkeley, CA. 2 National Public Health Institute, Helsinki, Finland. 3 Department of Community Health and General Practice, University of Kuopio, Kuopio, Finland. Reprint requests to Dr. John W. Lynch, Human Population Laboratory, 2151 Berkeley Way, Annex 2, Berkeley CA 94704. 934 Much remains to be understood about how low socioeconomic status (SES) increases cardiovascular disease and mortality risk. Data from the Kuopio Ischemic Heart Disease Risk Factor Study (1984-1993) were used to estimate the associations between acute myocardial infarction and income, all-cause mortality, and cardiovascular mortality in a population-based sample of 2,272 Finnish men, with adjustment for 23 biologic, behavioral, psychologic, and social risk factors. Compared with the highest income quintile, those in the bottom quintile had age-adjusted relative hazards of 3.14 (95% confidence interval (Cl) 1.77-5.56), 2.66 (95% Cl 1.25-5.66), and 4.34 (95% Cl 1.95-9.66) for all-cause mortality, cardiovascular mortality, and AMI, respectively. After adjustment for risk factors, the relative hazards for the same comparisons were 1.32 (95% Cl 0.70-2.49), 0.70 (95% Cl 0.29-1.69), and 2.83 (95% Cl 1.14-7.00). In the lowest income quintile, adjustment for risk factors reduced the excess relative risk of all-cause mortality by 85%, that of cardiovascular mortality by 118%, and that of acute myocardial infarction by 45%. These data show how the association between SES and cardiovascular mortality and all-cause mortality is mediated by known risk factor pathways, but full "explanations" for these associations will need to encompass why these biologic, behavioral, psychologic, and social risk factors are differentially distributed by SES. Am J Epidemiol 1996;144:934-42. SES, Risk Factors, Mortality, and AMI MATERIALS AND METHODS Study population Assessment of socioeconomic status At the baseline examinations, participants completed detailed questionnaires that included items on personal and household income, education, lifetime occupation, housing tenure, and material living conditions. We report results based on personal income as the measure of SES, since previous analyses revealed that it was the strongest predictor of mortality and AMI in this group of men (21). Furthermore, similar findings were obtained for the other indicators of SES, such as education and occupation. The income distribution was divided into quintiles, and four indicator variables were used in the analyses, with those in the highest 20 percent of income as the reference group. Assessment of follow-up events Participants were followed until the end of December 1993 for the mortality analyses, with a mean follow-up of 7.2 years (range, 4.05-9.8 years). For the AMI analyses, men were followed until the end of December 1992, for an average of 6.2 years (range, 3.1-8.8 years). All-cause mortality and cardiovascular mortality were ascertained by linkage to the National Death Registry, which is maintained for all Finnish citizens. Classification of death was based on the underlying cause, reviewed at the National Center of Am J Epidemiol Vol. 144, No. 10, 1996 Statistics of Finland. Cardiovascular deaths were classified according to International Classification of Diseases, Ninth Revision, codes 390-459. There were 156 deaths, 76 of which were from cardiovascular causes. Nonfatal AMIs and coronary deaths were ascertained by linkage to an AMI register established under the World Health Organization Monitoring of Trends and Determinants of Cardiovascular Diseases project (22). If multiple events occurred during follow-up, only the first event for each subject was considered. There were 88 fatal or nonfatal incident AMIs recorded in this group of men. Assessment of risk factors Extensive risk factor information was collected as part of the baseline examinations. Risk factors were included in the analyses if they had previously been shown to be associated with mortality or AMI or if they were candidates on theoretical grounds. Biologic risk factors. Blood samples were drawn after fasting and abstinence from smoking for 12 hours, abstinence from alcohol for 3 days, and abstinen (...truncated)