[Validity of body mass index and fat mass index as indicators of overweight status in Spanish adolescents: Esccola Study].

Background And Objectives: To determine the accuracy of body mass index (BMI), fat mass index (FMI) and fat-free mass index (FFMI) to find cut-off points related to overweight in adolescents aged 12-18 years.

Subjects And Method: 150 adolescents were assessed (75 boys aged 14.81+/-1.86yr, height 168.8+/-8.8cm, weight 65.84+/-16kg and BMI 22.89+/-4.4kg x m(2), and 75 girls aged 14.88+/-1.94yr, height 159.4+/-7.6cm, weight 54.77+/-9.54kg and BMI 21.48+/-3.1kg x m(2)). Body composition was measured by using the deuterium dilution technique with an isotope ratio mass spectrometer and fat free mass hydration constants were applied. Taylor's age-related overweight classification was applied and receiver operating characteristic (ROC) curves analyses were performed. Areas under curve were calculated for every index (BMI, FMI and FFMI). Sensitivity and specificity values of BMI and FMI to diagnose overweight were evaluated.

Results: Among boys, predictive positive values for BMI were 78.1% and 89.2% for FMI; predictive positive values in girls was 34.8% for BMI and 81.4% for FMI. Predictive negative values were 81.4% and 100% in boys and 98.1% and 100% in girls for BMI and FMI. Areas under curve of FMI for boys (0.988, 95%CI: 0.930-0.998) and girls (0.996, 95%CI: 0.943-1.000) showed high accuracy for overweight diagnose. Cutoff points of 4.58kg x m(2) in boys and 7.76kg x m(2) in girls for FMI and 23.07kg x m(2) in boys and 23.18kg x m(2) in girls for BMI had high sensitivity and specificity as indicators of overweight classification.

Conclusions: FMI showed high accuracy for overweight screening. Thus, our cut-off points may be useful as a tool to surveillance children's overweight on clinical and epidemiologic settings.