Body Mass Index and Bone Mineral Density in Postmenopausal Women: Nonlinear Associations and Discriminatory Performance for Osteoporosis at Three Skeletal Sites

Abstract Purpose Body mass index (BMI) is positively associated with bone mineral density (BMD), but its shape, usefulness, and variation across skeletal sites remain uncertain. Methods In this cross-sectional study, 1,707 postmenopausal women undergoing dual-energy X-ray absorptiometry in Isfahan, Iran (April 2022–March 2023) were analyzed after excluding secondary causes of bone loss and bone-active medications. BMD was measured at the lumbar spine, femoral neck, and total hip. Logistic and ordinal logistic regression, adjusted for age, current smoking, exercise, number of pregnancies, years of menstruation, and fragility fracture history, compared BMI categories (< 25, 25–<30, 30–<35, ≥ 35 kg/m²). RCS assessed nonlinearity; receiver operating characteristic analysis with the Youden index evaluated BMI for identifying absence of osteoporosis. Results Mean age and BMI were 61.1 ± 8.0 years and 29.7 ± 4.7 kg/m²; 606 women (35.5%) had osteoporosis at one or more sites. BMD increased across BMI categories at all sites. Compared with BMI < 25 kg/m², BMI ≥ 35 kg/m² was associated with lower adjusted odds of osteoporosis overall (OR 0.21; 95% CI 0.13–0.32) and at the total hip (OR 0.04; 95% CI 0.01–0.19). Splines showed significant nonlinearity overall and at each site (all P ≤ 0.007). AUCs for absence of osteoporosis were 0.64 (lumbar spine), 0.65 (femoral neck), and 0.76 (total hip), with Youden cut-offs of 28.0–29.1 kg/m². Conclusions BMI was nonlinearly and positively associated with BMD across skeletal sites. A BMI of approximately 28–29 kg/m² provided the best discrimination for absence of osteoporosis, particularly at the total hip, warranting prospective validation.