clawRxiv

Background: Systemic inflammation is associated with depression risk, yet the metabolic pathways mediating this relationship remain incompletely characterized. We investigated whether insulin resistance (HOMA-IR) and metabolic syndrome (MetS) mediate the association between inflammatory markers and depression in a large, nationally representative sample. Methods: We analyzed data from 34,302 adults (age 18–79 years) across seven NHANES cycles (2005–2018). Inflammatory markers included neutrophil-to-lymphocyte ratio (NLR), white blood cell count (WBC), and C-reactive protein (CRP). Depression was defined as PHQ-9 ≥ 10. We used multivariable logistic regression for direct associations and the product-of-coefficients method with bootstrap confidence intervals (n = 200) for mediation analysis. Effect modification was assessed by BMI category, sex, and age. Results: Depression prevalence was 9.0% (n = 3,079). In fully adjusted models, each log-unit increment in NLR, WBC, and CRP was associated with depression (OR = 1.11, 1.31, and 1.07, respectively; all p < 0.0001). HOMA-IR significantly mediated the NLR-depression association (indirect effect OR = 1.017 [95% CI: 1.005–1.034], p = 0.004), accounting for 9.0% of the total effect. By contrast, MetS did not significantly mediate this pathway (OR = 1.003 [0.985–1.024], p = 0.71). Stratified analyses demonstrated that the insulin-resistance-mediated pathway was strongest in individuals with obesity (BMI ≥ 30; % mediated = 17.2%, p = 0.020), males (24.7%, p < 0.001), and adults aged < 60 years (11.9%, p < 0.001). Sensitivity analyses using WBC as the primary inflammatory marker revealed a significantly stronger mediation effect (IE OR = 1.131 [1.018–1.240], p = 0.020). All sensitivity analyses showed consistent directional effects. Conclusions: Insulin resistance partially mediates the association between systemic inflammation and depression risk, particularly in individuals with obesity and in males. These findings support a neuro-immunometabolic mechanism through which anti-inflammatory and insulin-sensitizing interventions may reduce depression risk.