[Relationship between body mass index and fat mass percentage in children aged 3-17 years in China].

<b>Objective:</b> To analyze the relationship between body mass index (BMI) and fat mass percentage (FMP) in children aged 3-17 years in China. <b>Methods:</b> The BMI and FMP data of children aged 3-17 years from the National Nutrition and Health Systematic Survey in 0-18 years old children in China was analyzed. BMI-<i>Z</i> score/BMI and FMP were used to classify the subjects, respectively. Spearman correlation was used to analyze the correlation between BMI and FMP. The consistency between BMI-<i>Z</i> score/BMI and FMP in classifying the subjects was measured using Kappa coefficient. <b>Results:</b> The FMP of malnutrition, normal and overweight/obesity in boys was higher in age group 10-13-year than in other age groups (all <i>P</i><0.001). The FMP of all nutritional status in girls increased with age (all <i>P</i><0.05). The BMI of boys in all the FMP levels increased with age (all <i>P</i><0.05). When the FMP of girls was 25%- or ≥30%, BMI increased with age (all <i>P</i><0.001). The relationship between BMI and FMP was strong (<i>r</i>=0.705, <i>P</i><0.001), with <i>r</i> of 0.618 in boys and 0.884 in girls. The consistency between BMI-<i>Z</i> score/BMI and FMP in classifying the subjects was found to be moderate (Kappa=0.574, <i>P</i><0.001). <b>Conclusions:</b> There was a strong relationship between BMI and FMP. The consistency between BMI-<i>Z</i> score/BMI and FMP in classifying the subjects was moderate.