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The present study examined how body mass index (BMI) is associated with body composition in children who are in around puberty. Fat mass relative to body mass (%FM) was estimated using a bioelectrical impedance analyzer in 416 Japanese children aged 9 to 14 yr. Fat mass relative to body height squared (FM/ht²) and fat-free mass relative to body height squared (FFM/ht²) were calculated from body height, body mass and %FM. On the basis of the heights at the ages of peak height velocity (PHV) of Japanese boys (153.4 cm) and girls (141.3 cm) reported in literature, the participants were allocated to prepubertal or pubertal group. Regardless of sex, BMI was significantly higher in pubertal group than in prepubertal group. %FM and FM/ht² for girls were significantly higher in pubertal group than in prepubertal group, whereas those for boys were similar between both groups. In both sexes, BMI percentile was significantly correlated to %FM and FM/ht², regardless of group. In the corresponding relationships, the slope of the regression line was similar between both groups, regardless of sex. BMI percentile was significantly correlated to FFM/ht², regardless of group and sex. In the girls, there were no significant differences between both groups in the slope of regression line for the corresponding relationship, whereas, in the boys, the slope differed between both groups. In Japanese boys and girls, while BMI percentile can be a convenient measure for assessing %FM, FM/ht² and FFM/ht², adolescents have higher fat and fat-free mass than children across BMI percentile.

Key words: bioelectrical impedance analysis, peak height velocity, prepuberty, puberty, maturity-related difference