Body roundness index and its association with QUS-defined osteoporosis and bone mineral density in middle-aged and older adults: a cross-sectional analysis.

OBJECTIVES: This study evaluated the association between Body Roundness Index (BRI) and QUS-defined osteoporosis, its discriminative ability, empirical cutoff, and subgroup heterogeneity in middle-aged and older adults. METHODS: We analyzed cross-sectional data from 1,444 community-dwelling adults aged 35-75 years. Osteoporosis was defined using tibial quantitative ultrasound-derived T-scores. Receiver operating characteristic curves assessed discrimination, restricted cubic splines examined dose-response relationships, logistic and robust regression analyses quantified associations, and subgroup analyses evaluated effect modification. RESULTS: The Youden index identified an optimal BRI cutoff of 4.5. High BRI (≥ 4.5) was associated with higher odds of osteoporosis in the unadjusted model (OR = 1.606, 95% CI: 1.292-1.996; P < 0.001), the age- and sex-adjusted model (OR = 1.432, 95% CI: 1.141-1.797; P = 0.002), and the fully adjusted model excluding BMI (OR = 1.404, 95% CI: 1.105-1.783; P = 0.005). The association was approximately linear (P for non-linearity = 0.539; P-overall < 0.001). BRI alone showed modest discrimination (AUC = 0.562, 95% CI: 0.529-0.594), which increased after adjustment for age and sex (AUC = 0.680, 95% CI: 0.652-0.710) and after full covariate adjustment (AUC = 0.695, 95% CI: 0.674-0.731). The association was stronger among participants with metabolic syndrome (OR = 2.62, 95% CI: 1.641-4.179) than among those without it (OR = 1.39, 95% CI: 1.040-1.860; P for interaction = 0.022). CONCLUSIONS: Higher BRI was associated with QUS-defined osteoporosis, but BRI alone showed limited discrimination, and the fully adjusted model remained only moderately discriminative. BRI should therefore be considered an adjunctive anthropometric indicator within a broader assessment framework, rather than a stand-alone predictor or screening tool. CLINICAL TRIAL REGISTRATION: https://www.chictr.org.cn/showproj.html?proj=187762, identifier ChiCTR2300071492.