Residual Appendicular Lean Mass as a Marker of Metabolic Capacity: Association with Insulin Resistance in Korean Adults.

BACKGROUND: The skeletal muscle plays a central role in glucose metabolism, but absolute muscle mass is strongly influenced by both body size and adiposity. Herein, we investigated whether residual appendicular lean mass (ALM), a body-size-adjusted measure of the skeletal muscle reserve, was associated with insulin resistance and whether this association differed according to the obesity status of Korean adults. METHODS: This cross-sectional study included 14,647 adults extracted from the Korea National Health and Nutrition Examination Survey (KNHANES) 2008-2010. Residual ALM was derived as the standardized (z-score) residual from a linear regression model including the age, sex, and body mass index (BMI). Associations between residual ALM and log-transformed homeostasis model assessment of insulin resistance (HOMA-IR) were examined using complex survey-weighted linear regression models. Metabolic syndrome was evaluated as a secondary clinical outcome. RESULTS: In the unadjusted analyses, residual ALM was positively associated with HOMA-IR (β = 0.041, 95% confidence interval [CI] 0.030-0.052; P < 0.001). After adjusting for age, sex, and BMI, higher residual ALM was associated with lower HOMA-IR (β = -0.060, 95% CI -0.070 to -0.049; P < 0.001); this association remained significant after further adjustment for hypertension, dyslipidemia, diabetes mellitus, chronic kidney disease, smoking status, and hepatic steatosis index (β = -0.043, 95% CI -0.053 to -0.033; P < 0.001). The magnitude of the inverse association differed according to obesity status (P for interaction = 0.007), and was more pronounced among individuals without obesity. Higher residual ALM was further associated with lower odds of metabolic syndrome after adjusting for BMI and hepatic steatosis index (odds ratio 0.86, 95% CI 0.81-0.92; P < 0.001). CONCLUSIONS: Higher residual ALM was independently associated with lower insulin resistance and lower odds of metabolic syndrome in Korean adults. Higher body-size-adjusted skeletal muscle reserve was associated with a more favorable metabolic profile, particularly among non-obese adults.