Comparison of the body mass index to other methods of body fat evaluation in ethnic Japanese and Caucasian adolescent girls

OBJECTIVE: The aim of this study was to assess the body mass index (BMI) and its relationship with other methods of body fat evaluation in pre- and post-menarcheal, Japanese and Caucasian female adolescents, using hive different cut-off points for obesity: 28% and 30%. DESIGN: A cross-sectional study with incomplete sampling, using the subject as the evaluation unit. SUBJECTS: A total of 436 Japanese and Caucasian female adolescents in two age groups: 10-11 (pre-menarcheal adolescent); and 16-17 (post-menarcheal adolescents). METHODS: For the BMI the cut-off point for thinness was set at the 5th percentile of the BMI distribution of the NCHS reference population and the cut-off point for overweight and obesity was set at the 85th percentile. Body composition was assessed using foot-to-foot bioelectrical impedance analysis (BIA), near-infrared interactance (NIR) and Slaughter skinfold equations (SKI). The statistical comparison of the methods was performed using the kappa agreement test and the McNemar disagreement test. RESULTS: In the 10- and 11-y-old girls, the BMI was considerably and significantly correlated with the other methods. The major agreements were: in Japanese adolescents BMI x NIR = 82.3% (cut-off point of 28%), BMI x BIA = 85.7% (cut-off point of 30%); in Caucasian adolescents BMI x NIR = 80.7% (cut-off point of 28%), BM I x BIA = 87.4% (cut-off point of 30%). The disagreement above the diagonal between BMI x NIR was higher within the two groups for both the cut-off points, revealing that the girls identified as obese by the BMI were considered eutrophic by NIR. In the 16- and 17-y-old adolescents, the BMI demonstrated low or no correlation with the other methods. Furthermore, it presented disagreements below the diagonal, revealing that the BMI identified fewer obese subjects than the other methods. CONCLUSION: Among the 10- and 11-y-olds, the BMI presented a good correlation with the other methods, independent of ethnicity. The BMI can therefore be used in place of these methods, although it may underestimate obesity. Among the 16- and 17-y-olds, the BMI presented low or no agreement with the other methods, suggesting that it is probably not a suitable index for this age-group in studies focusing on the identification of obesity. In such cases the choice of one of the other methods, depending on availability, cost or technical experience, may represent a better approach.