Associations between total and regional fat-to-muscle mass ratio and fracture risk in elderly population: a prospective cohort study in UK Biobank.

Background: Previous studies reported that the overall risk of fragility fractures increased with both obesity (high body mass index, BMI) and sarcopenia (low BMI) in elderly population, which highlights that BMI may not fully capture the crucial role of skeletal muscle and fat tissue distribution. The total and regional fat-to-muscle mass ratio (FMR), which consider the interaction of both muscle mass and fat tissue, might better reflect the body composition variations associated with fragility fractures. Methods: We included 211,881 participants aged ≥60 years from the UK Biobank who had complete bioelectrical impedance analysis-derived body composition data. FMR was calculated as fat mass divided by muscle mass. Incident osteoporotic fractures (OF) and major osteoporotic fractures (MOF) were recorded and multivariable Cox models assessed associations between total/regional FMR and fracture risk, followed by sex-stratified analyses. Non-linear relationships were examined using restricted cubic splines. Results: Among 12,274 participants with incident MOF, arms and trunk FMR showed the strongest positive associations with MOF risk after adjustment for BMI, bone mineral density, sex, and other confounders. Compared to the lowest quintile (First quintile, Q1), the highest quintile (Fifth quintile, Q5) of arms FMR was associated with a 24% increased risk of MOF, and trunk FMR with a 16% increase. Non-linear associations between arms and trunk FMR and fractures were observed ( Conclusions: Our findings indicate that elevated FMR in the arms and trunk is associated with an increased risk of MOF and OF, independent of sex, BMI, and BMD. While further studies are needed to formally compare its predictive performance with traditional metrics like BMI, region-specific FMRs may serve as a potential complementary tool for assessing fracture risk in the elderly.