Body Composition and Its Correlates in Children and Adolescents Living in Germany: A Cross-Sectional Study.

Body composition is an important health parameter during childhood and adolescence. In this study, we investigate the associations between body composition and age, physical activity, side jump, standing long jump, physical working capacity at 170 beats per minute pulse, screen time, and socioeconomic status in a nationwide German sample. A total of 2.869 children and adolescents (1.456 girls) aged 6-17 years from the Motorik-Modul study (2014-2017) were stratified by sex and three age groups (6-10, 11-13, 14-17 years). Physical activity was quantified by accelerometers, while physical fitness parameters were measured as part of the Motorik-Modul field-based fitness test battery. Body composition analysis (BIA 2000-S; Data Input, Frankfurt, Germany) included phase angle as well as height-adjusted indices for fat mass and fat-free mass. Potential correlates of body composition were examined by testing 18 preregistered hypotheses via multiple regression analyses. We found a general increase in fat mass index, fat-free mass index, and phase angle during childhood with sex-dependent changes in these trajectories occurring between the ages of 10 and 14 and persisting up to age 17. Besides age, the most important correlates were standing long jump and physical working capacity. Higher screen time and lower socioeconomic status accompany higher fat mass index but do not predict a lower fat-free mass index. Physical activity correlates negatively with fat mass index only among 6 to 13-year-olds. These findings emphasize the complex interplay between body composition, physical fitness parameters, lifestyle factors, and socioeconomic background in childhood and adolescence.