Dietary Behaviours Mediate Intervention Effects on Body Composition Outcomes in Children: Evidence from a Cluster-Randomised Controlled Trial

Background/Objectives: Few paediatric obesity trials have examined behavioural mediators of intervention effects on detailed body composition outcomes beyond body mass index (BMI). Given the complexity of multiple correlated body composition indicators, this study aimed to employ principal component analysis (PCA) to derive a composite adiposity factor and to apply mediation analysis to identify whether changes in dietary behaviours and physical activity mediated the effects of the DECIDE-Children intervention on body composition. Methods: This study included 1180 children from a 9-month cluster-randomised controlled trial that delivered a multifaceted, digitally assisted intervention targeting children (health education, physical activity reinforcement, and app-based BMI monitoring/feedback) and their environments (school food policies and family engagement). Outcomes were changes in body composition (whole-body and trunk fat mass, fat-free mass, and muscle mass indices measured by bioelectrical impedance analysis). Potential mediators included changes in dietary behaviour, screen time, and moderate-to-vigorous physical activity (MVPA), assessed via validated questionnaires. PCA was used to reduce the dimensionality of body composition variables and to derive a composite adiposity factor (PCA_1), with higher values indicating greater adiposity and lower muscle mass. Mediation was tested using linear mixed-effects models adjusted for age, sex, and region. Results: The intervention significantly reduced adiposity indicators (e.g., whole-body and trunk fat mass percentage decreased by 1.003%, and PCA_1 decreased by 0.942 SD) and concurrently increased fat-free mass percentage and muscle mass percentage at both whole-body and trunk levels. Dietary behaviour significantly mediated the intervention effects on all body composition outcomes (e.g., mediating 16.1% of the effect on PCA_1), whereas physical activity (both screen time and MVPA) showed no mediation. Sensitivity analyses supported the primary findings. Conclusions: The DECIDE-Children intervention improved body composition by reducing adiposity and increasing fat-free and muscle mass percentage, effects primarily mediated by dietary behaviour. These findings highlight dietary behaviour as a key component in paediatric obesity intervention.