Role of cumulative biological risk in mediating socioeconomic disparities in cognitive function in the elderly: a mediation analysis

OBJECTIVES: To evaluate whether allostatic load (AL), a measure of cumulative biological risk, fully or partially mediates observed socioeconomic status (SES) differences in cognitive function in the elderly. DESIGN: Cross-sectional mediation analysis. SETTING: Community-dwelling US elderly who participated in the National Health and Nutrition Examination Survey (NHANES). PARTICIPANTS: The NHANES uses a complex, multistage, probability sampling design to select a nationally representative sample. Of the 4976 elderly (60 years or older) who were selected, 3234 agreed to participate in the household and medical exam interviews (65% response rate). PRIMARY AND SECONDARY OUTCOME MEASURES: Performance on the Digit Symbol Substitution Test (DSST)-a measure of cognitive function. RESULTS: Relative to participants with the lowest level of education or family income, participants who were college graduates (β=24.4, 95% CI 22 to 26.8, p<0.0001) or in the highest income quartile (β=17.3, 95% CI 15.2 to 19.4, p<0.0001) had the highest DSST scores and the least AL burden (β=-0.72, 95% CI -0.98 to -0.47 and β=-0.82, 95% CI -1 to -0.57; p<0.0001, respectively). Although, AL was significantly negatively associated with cognitive performance (β = -1, 95% CI -1.4 to -0.5, p<0.0001), it mediated at most 4.5% of the SES effect on DSST performance. CONCLUSIONS: The findings suggest that AL, as measured by a summary index of parameters for cardiovascular function, metabolism and chronic inflammation, is not a significant mediator of SES-related differences in cognitive function in the elderly. Further efforts are required to elucidate the exact physiological pathways and mechanisms through which SES impacts cognitive function in late life.