Body roundness index and self-reported oral health among US adults: Nonlinear patterns and an exploratory indirect association through the systemic immune-inflammation index

Evidence linking body roundness index (BRI) to self-reported oral health is limited. We examined this association, nonlinearity, subgroup differences, and an indirect association through the systemic immune-inflammation index (SII). This cross-sectional study included 14,656 adults from the 2009 to 2014 National Health and Nutrition Examination Survey. Self-reported oral health was analyzed as a 5-level ordered outcome. Survey-weighted ordinal logistic regression, restricted cubic spline, fixed-folding point, subgroup, and threshold-specific sensitivity analyses were performed. Exploratory mediation analysis used 1000 bootstrap simulations. In the fully adjusted model, each 1-unit increase in BRI was associated with higher cumulative odds of poorer self-reported oral health (odds ratio [OR], 1.076; 95% confidence interval [CI], 1.055-1.097). Compared with the lowest BRI tertile, the OR for the highest tertile was 1.448 (95% CI, 1.272-1.647). The spline showed modest nonlinearity (P = .048). Below the fixed-folding-point at BRI = 10, the OR was 1.092 (95% CI, 1.063-1.120); above 10, the association was attenuated (OR, 0.978; 95% CI, 0.913-1.048). Although the proportional-odds assumption was not fully supported (P = .011), threshold-specific ORs remained positive (1.065-1.099). Interactions were significant for age and education level. SII accounted for 3.06% (95% CI, 0.99%-5.82%) of the association. Higher BRI was associated with poorer self-reported oral health among US adults. The association varied modestly across thresholds and attenuated at higher BRI levels, while SII explained only a small proportion. Prospective studies with clinical dental assessments are needed.