Body Composition Changes During GLP-1 Receptor Agonist Therapy in Pediatric Obesity: A Pilot Study

Background and Objectives: GLP-1 receptor agonists (GLP-1 RAs) are effective weight-loss therapies, but data on body composition changes in pediatric obesity remain scarce. The primary objective was to evaluate the effects of GLP-1 RAs on body composition in children with obesity. Materials and Methods: We conducted a retrospective study of children with obesity evaluated at the National Institute for Mother and Child Health “Alessandrescu-Rusescu”, Bucharest, Romania, who initiated weekly injectable GLP-1 RA therapy (semaglutide) between January and December 2025. Patients were assessed at baseline and after a median follow-up of 5 months. Eight of ten participants with complete paired data were included in the final analysis; two were excluded because one was a non-responder with weight gain and suspected non-compliance, while one responder could not maintain the standing position for bioimpedance measurement. Bioimpedance analysis and anthropometry were performed at both visits. Paired data were analyzed using Wilcoxon signed-rank tests. Results: Eight children (4 boys, 4 girls; mean age 14.9 ± 1.8 years) completed the study. Significant Body Mass Index (BMI) Z-score improvements were observed (CDC: −0.14, p = 0.012; WHO: −0.37, p = 0.012), with a median weight reduction of 4.75 kg (p = 0.036). While absolute muscle mass showed non-significant change (−1.3 kg, p = 0.362), predicted muscle mass percentage increased significantly (+1.9%, p = 0.012), suggesting selective fat loss. Fat-free mass percentage increased (+2.0%, p = 0.012) with reciprocal fat mass reduction (absolute: −3.85 kg, p = 0.017; percentage: −2.0%, p = 0.012). Fat-free mass index remained stable (−0.67 kg/m2, p = 0.161). No serious adverse events occurred. Sensitivity analysis (n = 10) confirmed the robustness of the results, with improvements in BMI Z-scores remaining significant. Conclusions: In this preliminary pilot study, GLP-1 RA therapy in children with obesity was associated with significant improvements in BMI Z-scores and favorable shifts in body composition, consistent with selective fat loss and relative preservation of lean mass. These exploratory findings are hypothesis-generating and support the conduct of larger prospective controlled studies with body composition as a primary endpoint.