Relationship of BMI z score to fat percent and fat mass in multiethnic prepubertal children

Objective The objective of the study is to examine the validity of body mass index z score (zBMI) as a measure of percent body fat in prepubertal children. Methods One hundred eleven multiethnic, healthy, Tanner 1 children aged 6-12 years had fat percent and fat mass measured by the four-compartment method as part of the Paediatric Rosetta Body Composition Cohort. Multiple regression models were developed with fat percent as the dependent variable and zBMI, age, sex and ethnicity as independent variables. Results Body mass index z score predicted fat percent, adjusted for age in both girls (P < 0.001, RMSE 5.67 and R-2 0.54) and boys (P < 0.001, RMSE 4.71, R-2 0.69). The average model percent error was 20.3% in girls and 21.6% in boys. zBMI(2) predicted fat mass when adjusted for age and zBMI in both girls (P < 0.001, RMSE 2.27 and R-2 0.82) and boys (P < 0.001, RMSE 2.08 and R-2 0.81). The average percent error was 7.2% in girls and 8.7% in boys. Age was associated with percentage body fat (P < 0.01), while ethnicity was not (P > 0.05). Conclusions Given the relatively large error in the models, zBMI are not a useful indicator of fat mass in healthy, Tanner 1 children. zBMI(2) scores are associated with significantly lower absolute percent errors in girls and boys.