[Characteristics of fat-free mass distribution in children aged 3-17 years in China].

<b>Objective:</b> To describe the distribution of fat-free mass (FFM) and fat-free mass index (FFMI) in children aged 3-17 years in China. <b>Methods:</b> Data were collected from National Nutrition and Health Systematic Survey in 0-18 years old children in China. By using multi-stage stratified randomized cluster sampling method, the project was conducted in 28 survey points in urban and rural areas in 14 provinces (autonomous regions and municipalities) in 7 regions in China from 2019 to 2021. FFM was measured using bioelectrical impedance meter. Finally, the body composition data of 70 853 children were included in the analysis. <i>M</i> (<i>Q</i>₁, <i>Q</i>₃) was used to describe the gender and age specific FFM and FFMI of the children in different regions. Kruskal-Wallis <i>H</i> rank sum test was used to compare FFM and FFMI of boys and girls in same age group, boys in different age groups, girls in different age groups, as well as boys in same age group and girls in same age group in different regions. DSCF method was used for pairwise comparisons. <b>Results:</b> After the age of 11 years, the difference of FFMI between boys and girls increased year by year. The FFMI was 14.2 kg/m² in boys and 13.8 kg/m² in girls at 11 years old, the difference was significant (<i>χ</i>²=135.86, <i>P</i><0.001). The difference of FFMI between boys and girls exceed 1.0 kg/m² from 12 years old, and FFMI was 15.3 kg/m² in boys and 14.2 kg/m² in girls at 12 year old, the difference was significant (<i>χ</i>²=597.27, <i>P</i><0.001). The FFMI was 17.5 kg/m² in boys and 14.7 kg/m² in girls at 16 years old, the difference was significant (<i>χ</i>²=2 543.60, <i>P</i><0.001). The FFMI was higher in boys in northeast China, while the FFMI was lower in both boys and girls in northwest China. <b>Conclusions:</b> Gender specific difference was observed in the increase of FFMI with age. The FFMI was significantly higher in boys than in girls after 11 years old. It is necessary to pay attention to the problem of FFM in children in northeastern and northwestern China.