Bioelectrical impedance analysis for measuring body composition and predicting low muscle mass in apparently healthy pediatric outpatients: a retrospective observational study.

Background: The bioelectrical impedance analysis-derived phase angle is a proposed indicator of sarcopenia in adults. This study assessed the body composition of pediatric outpatients without underlying medical conditions to evaluate the predictive value of the phase angle in identifying low muscle mass, a risk factor for pediatric sarcopenia.

Methods: Analyses were performed separately for each sex among 480 pediatric outpatients aged 5-18 years. Body composition variables were compared between low and normal body mass index-for-age z-score (BMIz) groups, including correlation analysis between the phase angle and other variables. The receiver operating characteristic curves of the phase angle, body mass index, and fat-free mass index (FFMI) were compared to predict a severely low appendicular skeletal muscle mass index (ASMI), defined as an ASMI below - 1 or - 2 standard deviations based on sex- and ethnicity-specific reference curves derived from dual-energy X-ray absorptiometry.

Results: The low BMIz group showed a greater prevalence of a low fat-mass percentage and severely low ASMI, accompanied by notable changes in fat mass, muscle mass, height-squared adjusted indices, body water, protein, visceral fat area, and the phase angle (P < 0.05) compared with the normal BMIz group. The phase angle exhibited moderate correlations (P < 0.001) with the FFMI and ASMI (positive) and the visceral fat area and the extracellular water/total body water (ECW/TBW) ratio (negative) but no or negligible correlation with fat mass, fat-mass percentage, the fat mass index, or minerals. The phase angle' area under the curve for predicting a severely low ASMI was 0.743-0.785 (sensitivity: 62.3-80.4%; specificity: 67.0-75.0%). The area under the curve of the FFMI was 0.853-0.931 (sensitivity: 78.4-92.9%; specificity: 79.6-87.1%).

Conclusions: Body composition can identify fat and muscle wasting in children with a normal BMIz. The phase angle moderately correlated with the FFMI, ASMI, visceral fat area, and ECW/TBW ratio. The phase angle is a reasonable, although not a surrogate, indicator of the sarcopenia risk in pediatric outpatients.