Osteosarcopenia: More Than the Sum of Its Parts? A Community-Based Study Across Sarcopenia Definitions

Osteosarcopenia, the co-existence of osteoporosis and sarcopenia, is described as a geriatric syndrome arising from shared determinants of muscle and bone. We asked how far the prevalence of sarcopenia and osteosarcopenia depends on the definition of low muscle mass, whether osteoporosis and sarcopenia co-occur more often than expected under statistical independence, and whether the muscle-bone relationship is expressed in areal bone mineral density (BMD) or in femoral geometry. Cross-sectional study of 649 community-dwelling adults aged 65 years and over (72% women; mean age 71.5 years; mean body mass index (BMI) 29.6 kg/m²) assessed with dual-energy X-ray absorptiometry (GE Lunar iDXA), grip dynamometry (Jamar Hydraulic Hand Dynamometer) and the Short Physical Performance Battery. Osteoporosis followed World Health Organization criteria. Sarcopenia was classified according to EWGSOP2, and low muscle mass was defined three ways (appendicular lean mass (ALM)/height2; absolute ALM; and the Foundation for the National Institutes of Health (FNIH) ALM/BMI index). Observed co-occurrence was compared with the value expected under independence (Fisher exact test, odds ratios (OR) with 95% confidence intervals (CI)); for the best-powered analysis the two one-sided tests (TOST) equivalence procedure using pragmatic equivalence bounds of OR 0.5 to 2.0. Hip structural analysis provided section modulus and cross-sectional area. Low muscle mass ranged from 3.7 to 15.7%, and confirmed sarcopenia from 1.4 to 4.2%, according to the metric. Osteoporosis prevalence was 17.6% (95% CI 14.9 to 20.8). Under none of the sarcopenia definitions was there statistically significant evidence of excess co-occurrence (Fisher p = 0.50 to 1.00), although observed overlap under the absolute-ALM definition was slightly above the independence expectation. For probable sarcopenia (low grip strength), the best-powered comparison, the OR was 0.89 (95% CI 0.47 to 1.67) and equivalence within broad OR bounds of 0.5 to 2.0 was supported (90% CI 0.52 to 1.51; TOST p = 0.037); equivalence was not supported under narrower bounds. Appendicular lean mass was not associated with areal BMD after adjustment for body size (femoral neck standardised β = 0.03, 95% CI - 0.18 to 0.24) but was associated with femoral section modulus substantially more strongly in men than women (globally standardised interaction Δβ = 0.30, 95% CI 0.15 to 0.44, p < 0.001). In this community sample, there was no statistically significant evidence that osteoporosis and sarcopenia co-occurred more often than expected under statistical independence, across several sarcopenia definitions, although estimates for confirmed sarcopenia were imprecise. The muscle-bone association was more apparent for femoral geometry than areal BMD and was seen predominantly in men. These findings do not support assuming that low bone mass and low muscle mass identify the same individuals, but they concern statistical co-occurrence and do not establish biological independence.