Objective: To examine the relationship of body fat mass measured by dual energy X-ray absorptiometry (DEXA) to BMI in young girls, according to age and normative BMI percentile groupings.
Design: Cross-sectional observation study.
Setting: Dunedin, an urban town in the South Island of New Zealand.
Subjects: 196 healthy Caucasian girls aged 4-16 y.
Measurements: Body weight, height and BMI, total fat mass and % body fat (by DEXA).
Results: Our BMI percentile standard groupings were < 5 centile (n = 7); 5-50 centile (n = 71); 51-75 centile (n = 50); 76-90 centile (n = 42); 91-95 centile (n = 12); > 95 centile (n = 14). In this whole population sample DEXA-derived fat mass correlates well with BMI (n = 0.934) indicating that BMI accounts for 87.2% of the variance in body fat mass. However, at the extremes of BMI the association is weaker. A nomogram for predicting DEXA fat mass from BMI in girls is presented.
Conclusion: Because DEXA-derived fat mass correlates well with BMI throughout the 'normal' range of BMI (5-95th centiles) our study supports the usefulness of BMI as a simple measure of fatness in girls. Children with BMI values outside the normal BMI range may benefit from more exact body composition assessment using DEXA.