OBJECTIVE: The beginning of the post-infancy rise in the body mass index (BMI, kg/m^2) has been termed the BMI-rebound (or adiposity rebound), and the fat-free mass index (FFMI=fat-free mass/stature^2, kg/m^2) and fat mass index (FMI=fat mass/stature^2, kg/m^2) are concepts comparable to that of BMI. We examined whether the relation of stature to FFMI and FMI, respectively, is independent of childhood BMI-rebound. DESIGN: Cross-sectional study. SUBJECTS: A total of 752 apparently healthy children in Japan (337 boys and 415 girls) , varying in age from 3 to 8 yrs. MEASUREMENTS AND METHODS: Stature and body mass were measured, and body composition was assessed by bioelectrical impedance analysis (BIA). Fat-free mass (FFM) and fat mass (FM) were both adjusted for stature to give FFMI and FMI. RESULTS: The relationship between BMI and %FM was linear for boys (r=0.610; p<0.0001) and girls (r=0.596; p<0.001) at ages 3-8 yrs. In both sexes, levels of BMI tended to dip slightly with increasing age from 3 to 5 yrs, at which it reached a nadir, then rose steadily with age, by 1.4kg/m^2 from 5 to 8 yrs. There were no significant correlations between stature and FFMI and FMI in the 3-5 yrs age group, but there were significant correlations in the 6-8 yrs age group (0.347 for boys and 0.276 for girls, and 0.377 for boys and 0.406 for girls, respectively). CONCLUSION: The convenience with which BMI can be measured has understandably made it popular with both pediatric clinicians and epidemiologists, despite awareness of its shortcomings. However, the present study has demonstrated that the relationships between stature and FFMI and FMI, respectively, are reasonably sound after the age of 5. This implies that the BMI offers a reasonable measure of fatness in early childhood (～5yrs), but the BMI-rebound is not necessarily equivalent to an increase in fatness. Caution should therefore be used in generalizing from the findings in this study, and further investigation of the issue are required.
