Does Socioeconomic Status Moderate the Association Between Metabolic Syndrome and Cognitive Decline? A Moderated Mediation Analysis Using the China Health and Retirement Longitudinal Study (CHARLS)

Background Metabolic syndrome (MetS) has been associated with cognitive decline, yet the role of socioeconomic status (SES) in modifying this association through inflammatory pathways remains unclear. This study investigated whether SES moderates the mediating effect of C-reactive protein (CRP) in the MetS–cognitive decline pathway. Methods We used longitudinal data from the China Health and Retirement Longitudinal Study (CHARLS) spanning 2011–2018. The final analytic sample comprised 2,357 participants (aged ≥ 45 years, baseline stroke-free, with complete MetS, cognitive function, SES, and 2015 CRP data). MetS was defined using the IDF Asian criteria. Cognitive decline was operationalized as the difference between 2011 and 2018 total cognitive scores. An SES index was constructed from standardized education years and household income. A moderated mediation model with log(CRP + 1) at 2015 as the mediator was tested using 5,000 bootstrap resamples. Sensitivity analyses included continuous MetS component count, education-only SES, and exclusion of CRP > 10 mg/L. Results Among 2,357 participants (29.6% with MetS, mean age 58.2 years), MetS significantly predicted higher CRP levels (β = 0.120, p  < 0.001), but CRP was not significantly associated with cognitive decline (β = 0.041, p  = 0.798). The Index of Moderated Mediation was non-significant (IMM = − 0.0003, 95% CI [− 0.014, 0.013]). Stratified analyses revealed no significant indirect effects through CRP in any SES stratum. All sensitivity analyses yielded consistent null findings. Notably, SES emerged as the strongest independent predictor of cognitive decline (β = −1.89, p  < 0.001). Conclusions Although MetS elevates systemic inflammation as indexed by CRP, the CRP pathway does not significantly mediate the effect of MetS on cognitive decline, and this indirect effect is not moderated by SES. SES independently and strongly predicts cognitive decline trajectories, suggesting that socioeconomic factors may influence cognitive aging through pathways beyond metabolic inflammation.