The Mediating Role of Atherogenic Index of Plasma in the Association Between Dietary Inflammatory Index and Metabolic Syndrome: Evidence from NHANES and CHARLS

Aims: Metabolic syndrome (MetS) is a growing public health burden driven by chronic low-grade inflammation and lipid metabolic dysregulation, with diet as a major modifiable determinant. Proinflammatory diets [assessed by the dietary inflammatory index (DII)] are linked to MetS risk, but the underlying metabolic pathways remain unclear. This study investigated the independent and joint associations of DII and the atherogenic index of plasma (AIP) with MetS and evaluated AIP’s mediating role across populations. Methods: We analyzed data from the National Health and Nutrition Examination Survey (NHANES) (2013–2020; n = 7971) and the China Health and Retirement Longitudinal Study (CHARLS) (2011; n = 9240). DII and AIP were calculated; their associations with MetS were assessed using multivariable logistic regression. Mediation analysis quantified AIP’s mediating effect, with subgroup analyses by body mass index (BMI), age, and sex. In CHARLS, a DII-Proxy was derived from standardized food expenditure and C-reactive protein data. Results: MetS prevalence in NHANES was 22.3%. In both cohorts, higher AIP was strongly associated with MetS. In NHANES, each 1-standard deviation increase in AIP corresponded to markedly higher MetS odds (odds ratio = 10.9, P < 0.001). After adjustment, the direct DII–MetS association weakened, but mediation analyses showed a significant indirect effect via AIP. In CHARLS, AIP mediated 62.9% of the association between DII-Proxy and MetS. Dose–response analyses revealed a nonlinear relationship between DII and MetS and an S-shaped association between AIP and MetS. Stronger mediation effects were observed in females and individuals with normal BMI. Conclusions: AIP is a key mediator linking proinflammatory diets to MetS, supporting the diet–inflammation–lipid axis and identifying targets for precision prevention.