Income, education, and inflammation: differential associations in a national probability sample (The MIDUS study)

Abstract

Objective: To examine the associations between income and education and three markers of inflammation: interleukin-6 (IL-6), C-reactive protein (CRP), and fibrinogen. Socioeconomic status is inversely linked with health outcomes, but the biological processes by which social position "gets under the skin" to affect health are poorly understood.

Method: Cross-sectional analyses involved participants (n = 704) from the second wave of the national population-based Survey of Midlife Development in the United States (MIDUS). Data on pretax household-adjusted income and educational attainment were collected by questionnaire and telephone interview, respectively. Detailed medical history interviews, inventories of medication, and fasting blood samples for assessment of inflammatory proteins were obtained during an overnight clinic stay.

Results: All three inflammatory proteins were inversely associated with both income and education in bivariate analyses. However, multivariate regression models, adjusting for potential confounds, showed that only low income predicted higher levels of inflammatory proteins. Moreover, inclusion of IL-6 in the regression models for CRP and fibrinogen eliminated the associations with income.

Conclusion: These results suggest that income explains the association between education and peripheral inflammation. In short, the reason that higher education is linked to reduced peripheral inflammation is because it reduces the risk for low income status, which is what is directly associated with reduced peripheral inflammation. The findings also suggest that the links between income and both CRP and fibrinogen are mediated by IL-6. These observations help to sharpen our understanding of the relationship between social position and biological markers of illness in the United States.

Figure 1

Biological relationships among interleukin (IL)-6, C-reactive protein (CRP), and fibrinogen. Serum IL-6 has a number of biological sources, most notably adipocytes, which account for as much as 30% of circulating IL-6 levels (71) and immunocompetent cells. CRP and fibrinogen are principally synthesized in and released from the liver (72,73), and IL-6 is one of the most potent stimulators of their production.

Figure 2

Mean (±SE) serum interleukin (IL)-6 (A), serum C-reactive protein (CRP) (B), and serum fibrinogen (C) at each quintile of income. Adjusted means from multivariate analyses are shown. Logged mean values for IL-6 and CRP were back-transformed to facilitate interpretation of results. Results from multivariate regression models showed that the lowest income quintile had levels of IL-6 and CRP that were significantly higher than other quintiles; no other quintile differed significantly from the rest. Levels of fibrinogen in the lowest income quintile were significantly higher than other quintiles in unadjusted analyses, but not in the full regression model.

Figure 3

Graphic representation of meditational analyses. A) Partial correlations of low income status and inflammatory proteins after adjusting for demographic, health status, and health behavior characteristics. B) Partial correlations of low income status and inflammatory proteins after adjusting for demographic, health status, and health behavior characteristics and for associations among inflammatory proteins. The link between low income and interleukin (IL)-6 remained statistically significant in multivariate models that adjusted for C-reactive protein (CRP) and fibrinogen (data not shown), but the association between low income and CRP was reduced to statistical nonsignificance (and the association with fibrinogen reduced by >50%) after inclusion of IL-6 in those respective models (Table 5 and Table 6).