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Income, Education, and Inflammation: Differential Associations in a National Probability Sample (The MIDUS Study)

Friedman, Elliot M. PhD; Herd, Pamela PhD

doi: 10.1097/PSY.0b013e3181cfe4c2
Original Articles

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.

IL-6 = interleukin-6; CRP = C-reactive protein; BMI = body mass index; SES = socioeconomic status; MIDUS = Survey of Midlife in the United States; GCRC = General Clinical Research Center.

From the Institute on Aging (E.M.F.) and the Department of Sociology (P.H.), University of Wisconsin-Madison, Madison, Wisconsin.

Address correspondence and reprint requests to Elliot M. Friedman, PhD, University of Wisconsin-Madison, 624 WARF Office Bldg., 610 N Walnut Street, Madison, WI 53726. E-mail: friedman1@wisc.edu

Received for publication April 23, 2009; revision received October 29, 2009.

The present analyses were supported, in part, by Grant K01-AG029381 (E.M.F.) and the longitudinal follow-up of the MIDUS investigation was supported, in part, by Grant P01-AG020166 from the National Institute on Aging. The original study was supported, in part, by the John D. and Catherine T. MacArthur Foundation Research Network on Successful Midlife Development.

Copyright © 2010 by American Psychosomatic Society
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