Impact of cardiovascular risk factors and education on the relationship between income and mortality in a Chilean prospective cohort study: Outlining a "pauper rich paradox" in Latin American countries

Background: Chile presents significant inequalities in the distribution of the income (Gini=0.57). In this context, income-related mortality risk can be different to what is observed in societies with less income inequalities.

Methods: We assessed the relationship between income, risk factors, education level and all-cause mortality in a weighted random sample of 11,600 Chilean adults. The cohort was examined in 1997 and followed during 8 years for all-cause mortality events. Data were analyzed using Cox proportional hazard regression models considering 694 cases of death.

Results: Annual income quartiles (Q) were Q1: >US$4,200 (reference group); Q2: US$ 4,200-6,000; Q3: US$ 6,000-10,200 and Q4: >US$ 10,200. Unadjusted risks of all-cause mortality (95%CI) were respectively 1.0, 0.37 (0.31-0.46), 0.34 (0.27-0.42) and 0.38 (0.31-0.46). Relative risks adjusted by age, gender, body mass index, smoking and alcohol use were respectively 1.0, 0.79 (0.65-0.98), 0.96 (0.76-1.21) and 0.93 (0.75-1.16). After further adjusting by hypertension, diabetes, lipids and years of education, the two highest income quartiles showed a slight increment in the mortality risk. The figures were respectively 1.0, 0.98 (0.79-1.21), 1.29 (1.01-1.65) and 1.22 (0.97-1.53).

Conclusions: The highest quartile for income in this cohort corresponds to the poverty level for most developed countries. Our results suggest the existence of a "pauper rich paradox" on the relationship between income and mortality risk in the Chilean society. In societies with significant income inequalities, the protective effect of income for all-cause mortality is not observed. Further research is needed to test the underlying mechanisms that are responsible for this paradox.