Appendicular-to-trunk lean mass ratio resolves the height-based normalization paradox in low muscle mass and metabolic syndrome: A population-based cross-sectional study

Whether height-based normalization indices for appendicular lean mass (ALM) are metabolically neutral remains uncertain, because such indices may be influenced by adiposity. We evaluated ALM normalized to trunk lean mass (ALM/TLM) - both with bone mineral content excluded - as a normalization strategy potentially less susceptible to adiposity-related distortion. Using the Korea National Health and Nutrition Examination Survey 2008-2010, we analyzed 3,176 adults aged ≥65 years (1,373 men; 1,803 women) for body composition by dual-energy X-ray absorptiometry. Sex-specific Class I and Class II cut-offs were derived from a healthy young reference group aged 20-39 years (n = 3,727) at 1 SD and 2 SD below the young-adult mean. Metabolic syndrome (MetS) was defined by NCEP-ATP III criteria with population-specific waist cut-offs. Survey-weighted regression models were adjusted for age, smoking, alcohol, and physical activity. The appendicular skeletal muscle mass index (ASMI; ALM/height2) correlated positively with fat mass index (FMI) (men r=+0.288; women r=+0.358; both p < 0.001), whereas ALM/TLM correlated negatively with FMI (men r=-0.199; women r=-0.243; both p < 0.001). ASMI Class I low muscle mass showed an inverse association with MetS (men odds ratio [OR] 0.496, 95% confidence interval [CI] 0.388-0.636; women OR 0.475, 95% CI 0.328-0.686). In contrast, ALM/TLM Class II showed positive associations with MetS (men OR 2.779, 95% CI 2.074-3.722; women OR 2.701, 95% CI 1.680-4.340; both p < 0.001). The inverse ALM/TLM-HOMA-IR association persisted after FMI adjustment (men β=-0.131, p < 0.001; women β=-0.065, p = 0.007). ASMI correlated positively with adiposity, while ASMI-defined low muscle mass was inversely associated with MetS, suggesting that ASMI may partly reflect body size/adiposity rather than relative muscle depletion alone; this inverse association should not be interpreted as protective. By contrast, ALM/TLM-defined low muscle mass showed consistently positive associations with MetS across adiposity strata, supporting further evaluation of ALM/TLM, although its clinical validity remains unestablished.