Muscle mass and body composition in Japanese children measured by bioelectrical impedance analysis.

Age- and sex-specific reference values for muscle mass and body composition are essential for evaluating childhood cancer survivors at risk of muscle loss and abnormal body composition. To establish standard values for Japanese children, we conducted a cross-sectional study of healthy children aged 6 to 17 years. From April to June 2023, height, weight, and body composition were measured via bioelectrical impedance analysis (Tanita MC 780 A-N) in 9,202 students at 16 schools in Fukuoka City and nearby areas in Japan. Boys had significantly higher lean mass index (LMI) than girls at all ages (p < 0.001). In boys, LMI and skeletal muscle mass index (SMI) showed strong positive correlations with age (γ = 0.748, γ = 0.806), whereas girls exhibited moderate correlations (γ = 0.482, γ = 0.622). Although no sex difference in SMI was observed at age 6, boys surpassed girls thereafter. Trunk LMI increased linearly with age in boys (γ = 0.904), while in girls it plateaued around age 13 (γ = 0.819). Upper limb LMI peaked in girls around 14 years (γ = 0.771), with strong growth seen in boys (γ = 0.824). Lower limb LMI correlated strongly with age in both sexes (boys γ = 0.909, girls γ = 0.868). Girls had consistently higher fat mass index (FMI) and body fat percentage (BFP) across all ages (p < 0.001). Boys showed little correlation between FMI/BFP and age (γ = 0.117, γ = 0.004), whereas girls exhibited weak positive correlations (γ = 0.544, γ = 0.595). In healthy Japanese children aged 6 to 17 years, boys show a progressive increase in lean body mass and muscle mass, whereas girls demonstrate an age-related increase in fat mass. These normative data would offer a useful reference for monitoring body composition in children, particularly in targeted interventions for childhood cancer survivors.