Independent association between relative fat mass and total bone mineral density in U.S. adolescents: a cross-sectional study adjusting for Appendicular Lean Mass Index

decrease in total BMD (β = -0.005, P < 0.001). Furthermore, sex-specific non-linear relationships were identified. Threshold effect analysis revealed inflection points at an RFM of 21.28 for males and 39.44 for females. Beyond these respective thresholds, the decline in bone mineral accrual significantly accelerated (P < 0.001). After accounting for the mechanical loading effects of ALMI, higher RFM is independently associated with reduced total BMD in adolescents. These findings reveal observational sex-specific inflection points (21.28 for males, 39.44 for females) beyond which lower BMD was observed. These thresholds should be interpreted as exploratory rather than clinically actionable targets.