Mediation Effect of Metal Mixtures in the Association Between Socioeconomic Status and Self-rated Health Among US Adults: A Weighted Quantile Sum Mediation Approach

This study aimed to examine the association between socioeconomic status (SES) and self-rated health (SRH) among US adults and the extent to which blood and urinary metal mixtures explain this association. We used 14 years of repeated cross-sectional data that consists of seven consecutive NHANES cycles from 2003 to 2016 (n = 9497). SRH was measured using a 5-point Likert scale, and SES was measured by family income to poverty ratio (FMPIR), levels of education, and employment status. Blood concentration of lead, mercury, and cadmium, and urinary concentrations of ten heavy metals (arsenic, barium, cadmium, cesium, cobalt, lead, mercury, molybdenum, thallium, tungsten) were used as metal mixtures. The total effect of SES on SRH was examined by linear regression model. The direct effect of SES on blood and urinary metal mixtures was examined by the weighted quantile sum (WQS) regression with repeated holdout validation method, and the average causal mediation effects of blood and urinary metal mixtures were examined by model-based causal mediation technique. Results showed that SES indicators [education β: 0.17; 95% Confidence Interval (CI): 0.15, 0.18; employment β: 0.16; 95% CI: 0.12, 0.21; and FMPIR β: 0.09; 95% CI: 0.08, 0.11] were significantly positively, and the WQS indices of blood and urine metal mixtures (blood β: −0.04; 95% CI: − 0.05, − 0.03, urine β: − 0.07; 95% CI: − 0.13, − 0.004) were significantly inversely associated with SRH in the US adults. The novel finding was the mechanism between SES and SRH that exposure to heavy metals may explain socioeconomic inequalities in SRH in the US general population. Longitudinal studies are needed to corroborate this study results.