Independent Associations of Appendicular Skeletal Muscle Mass Index and Waist Circumference with Site-Specific Bone Mineral Density in Postmenopausal Women: A Real-World Cross-Sectional Study

Background/Objectives: Postmenopausal aging is accompanied by progressive loss of skeletal muscle mass and increased abdominal adiposity, both of which may influence bone health through distinct mechanical and metabolic pathways. However, their independent associations with bone mineral density (BMD) at different skeletal sites remain incompletely characterized. This study investigated the independent associations of appendicular skeletal muscle mass index (ASMI) and waist circumference (WC) with site-specific BMD in postmenopausal women. Methods: This cross-sectional study included 345 postmenopausal women who attended a university hospital menopause clinic in Thailand. Body composition was assessed using multi-frequency bioelectrical impedance analysis, and ASMI was calculated according to the Asian Working Group for Sarcopenia (AWGS) 2019 consensus criteria. WC was measured using standardized anthropometric procedures. BMD at the lumbar spine (L1–L4), total hip, and femoral neck was determined by dual-energy X-ray absorptiometry. Multivariable linear regression analyses were performed to evaluate the independent associations of ASMI and WC with site-specific BMD after adjustment for age, years since menopause, hormone replacement therapy use, osteoporosis medication use, calcium supplementation, vitamin D supplementation, and fracture history. Robust standard errors were used in the final models. Results: Higher ASMI was independently associated with higher BMD at the lumbar spine (standardized β = 0.362, p < 0.001), total hip (standardized β = 0.371, p < 0.001), and femoral neck (standardized β = 0.353, p < 0.001). WC was positively associated with total hip BMD (standardized β = 0.158, p = 0.006), whereas no statistically significant associations were observed at the lumbar spine (standardized β = 0.009, p = 0.899) or femoral neck (standardized β = 0.105, p = 0.092). Conclusions: ASMI was consistently associated with BMD across all measured skeletal sites, whereas the association between WC and BMD varied according to skeletal region. These findings support the complementary assessment of skeletal muscle mass and central adiposity for characterizing body composition and skeletal health in postmenopausal women.