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) an d its relationship with other methods of body fat evaluation in pre- and po st-menarcheal, Japanese and Caucasian female adolescents, using hive differ ent 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 a ge groups: 10-11 (pre-menarcheal adolescent); and 16-17 (post-menarcheal ad olescents). METHODS: For the BMI the cut-off point for thinness was set at the 5th perc entile 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 analysi s (BIA), near-infrared interactance (NIR) and Slaughter skinfold equations (SKI). The statistical comparison of the methods was performed using the ka ppa agreement test and the McNemar disagreement test. RESULTS: In the 10- and 11-y-old girls, the BMI was considerably and signif icantly correlated with the other methods. The major agreements were: in Ja panese 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 disagree ment 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 b y the BMI were considered eutrophic by NIR. In the 16- and 17-y-old adolesc ents, the BMI demonstrated low or no correlation with the other methods. Fu rthermore, it presented disagreements below the diagonal, revealing that th e BMI identified fewer obese subjects than the other methods. CONCLUSION: Among the 10- and 11-y-olds, the BMI presented a good correlati on with the other methods, independent of ethnicity. The BMI can therefore be used in place of these methods, although it may underestimate obesity. A mong 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 ca ses the choice of one of the other methods, depending on availability, cost or technical experience, may represent a better approach.