This study was conducted to assess the usefulness of fat-free mass index (FFMI) and fat mass index (FMI) as novel expression methods of body composition in children.

A total of 466 Second grade students-248 boys and 218 girls- from all elementary schools the Gwacheon City underwent anthropometric measures including bioelectrical impedance analysis (BIA) and biochemical tests. The correlation coefficients between obesity indices, including FMI and FFMI, and metabolic risk factors, were assessed. Metabolic risk factors of children with increased FMI were compared with those of children with normal FMI. We compared FMI and FFMI percentile distribution between this study's subjects and the subjects of the Fukuoka body composition study.

FMI was lower and FFMI was higher in this study's subjects compared to the subjects of the Fukuoka body composition study. FMI was correlated with other obesity indices and several metabolic risk factors. Metabolic risk was higher in children with increased FMI than in children with normal FMI.

FMI and FFMI were useful indicators in comparing difference of body composition among children that had different body size and growth. High FMI was related to increase of metabolic risk in children.