Socioeconomic inequalities in health : epidemiological studies of disease burden, mechanisms, and gender differences

Abstract

The overall aim of this thesis was to study inequalities in health, in particular, focusing on the risk of myocardial infarction and etiologic mechanisms from a life course perspective and on epidemiological measures and methodology.

Paper I uses the Swedish Burden of Disease Study together with the Social Database managed by the Swedish National Board of Health and Welfare. The analyses in Paper II-IV are based on the Stockholm Heart Epidemiology Program (SHEEP), a population-based case-control study of first myocardial infarction, in total 2,246 cases and 3,206 controls, among all Swedish citizens aged 45-70 living in Stockholm County 1992-1994.

Ischemic heart disease, depression and neurosis, and stroke are the three diseases with the largest contribution to the burden of disease, measured as disability-adjusted life years (DALY), in Sweden. Approximately one third of the burden of disease is unequally distributed, to a large extent this burden is put on the manual workers. The diseases with the largest contribution of DALYs to the total burden are also the diseases that stand for the largest part of the unequally distributed burden.

Misclassification of socioeconomic position among women has been proposed to affect the gender comparison of socioeconomic inequalities in health. In Paper II we find women to be categorized into fewer occupational groups and more often into unclassifiable subgroups than men. However, male occupational groups are nevertheless socioeconomically more heterogeneous. Furthermore, regarding the gender comparison of socioeconomic inequalities; empirical simulation of varying degrees of misclassification of men and women show that the dilution of the socioeconomic gradient among women, due to random misclassification of socioeconomic position, may be compensated by less misclassification among men.

The accumulation model in life course epidemiology hypothesizes that the longer time spent in socioeconomic adversity the greater is the risk of disease. In Paper III each year from birth till inclusion in manual position for men and women has been defined as in socioeconomic adversity. The number of years in adversity has been used to calculate the proportion in life spent in adversity. Men always in adversity have a relative risk of 2.36 (95% CI: 1.79 -3.11) for myocardial infarction and women in always adversity have a relative risk of 2.54 (95% CI: 1.70 - 3.78) compared to the reference ups of those never in adversity. Adjustment for unhealthy behavior, social factors, and social trajectories decreased the observed accumulation effect. The increased risk of myocardial infarction is present even after just a few years in socioeconomic adversity.

Men in non-manual socioeconomic position in both childhood and adulthood have the lowest prevalence of seven cardiovascular risk factors, i.e. diabetes, hypertension, low life control, low social network, obesity, physical inactivity, and smoking. For women the pattern is similar. Those with adulthood manual experience have the highest prevalence of exposure to three or more risk factors, whereas those with non-manual adult socioeconomic position have the highest prevalence of null-exposed. Men born in non-manual position but who end up in manual position have high prevalence of exposed to three or more risk factors and a strong tendency to cluster risk factors on the individual level. Though it is notable that the largest observed-to-expected ratio for both men and women is in the always non-manual group exposed to three or more risk factors, indicating that individual clustering is most common in this socioeconomic trajectory.

We do not find that risk clustering plays an important role in the accumulation process leading to higher prevalence of multi-exposed in the most adverse socioecononmic groups. Together the findings in Paper III and IV question an overly simplistic interpretation of the accumulation hypothesis, i.e. the longer time spent in socioeconomic adversity the higher the morbidity, as they indicate that it also depends on how (social mobility) and when (critical period) the accumulation occurs.