Reference values of skeletal muscle mass, fat mass and fat-to-muscle ratio for rural middle age and older adults in western China

Skeletal muscle mass (SMM), fat mass (FM) and fat-to-muscle ratio (FMR) are significant indicators in epidemiology studies and clinical settings. The aim of this study was to establish age-related and sex-specific reference values for skeletal muscle mass index (SMMI), fat mass index (FMI) and FMR by bioelectrical impedance analysis (BIA) for healthy rural adults in western China. This study is a cross-sectional study from Ningxia cohort study, included 13,790 individuals aged 35 to 74 years. Bioelectrical impedance analysis (BIA) was performed to measure body composition. Lambda-mu-sigma method was used to establish age-related and sex-specific percentile curves for SMMI, FMI and FMR. Overall, men had higher SMMI, but lower FMI and FMR than women for all ages. The SMMI decreased rapidly with age for men and women after 55 years and 45 years, respectively. FMI in men remain stable until 70 years; women's FMI showed a rapidly increasing after 50 years. The FMR increased consistently after 35 years for both men and women. These age-related and sex-specific reference values were established with the mean ± SD as the normal reference range. These reference values could be used as simple tools to identify age-specific low SMMI or high FMI and facilitate earlier identification sarcopenia or sarcopenic obesity in rural Chinese adults.