Nonlinear association between body roundness index and depressive symptoms in women of reproductive age: The mediating role of systemic inflammation

Background: Previous studies reported a linear association between Body Roundness Index (BRI) and depressive symptoms in general adults, but the relationship in women of reproductive age remains unclear. This study aimed to investigate the association between Body Roundness Index (BRI) and depressive symptoms and to examine the statistical mediating role of inflammatory markers in this association. Methods: This cross-sectional study analyzed nationally representative data from 7451 women aged 18−45 years in the National Health and Nutrition Examination Survey (NHANES) 2005−2018. BRI was calculated from height and waist circumference. Depressive symptoms were assessed using the Patient Health Questionnaire-9 (PHQ-9). Inflammatory markers included white blood cell count (WBC), Systemic Immune-Inflammation Index (SII), Systemic Inflammation Response Index (SIRI), platelet-to-lymphocyte ratio (PLR), neutrophil-to-lymphocyte ratio (NLR), and monocyte-to-lymphocyte ratio (MLR). Weighted linear regression, threshold effect analysis, and mediation analysis were performed. Results: BRI was nonlinearly associated with PHQ-9 score, with an exploratory turning point at approximately BRI = 3.85 ( P for likelihood ratio = 0.020). Above the threshold, BRI was significantly associated with depressive symptoms (β = 0.20, 95% CI: 0.15, 0.25; P  < 0.0001). The statistical association between BRI and PHQ-9 was partially accounted for by WBC (indirect association proportion: 9.3%, 95% CI: 1.1%, 18.6%; P  = 0.024); the other five markers showed no significant mediation. Conclusions: Among U.S. women of reproductive age, BRI was nonlinearly associated with depressive symptoms, with a significant positive association only above a turning point. The statistical pattern through WBC is consistent with a hypothesized immune-to-brain pathway linking visceral adiposity to mood; however, given the cross-sectional design, these findings do not establish causality.