Municipality-Based Lifestyle Intervention of Childhood Overweight and Obesity: 4-Years Follow-Up on BMI Trajectories.

To examine 4-year changes in Body Mass Index z-score (BMIz) among children and adolescents with overweight or obesity who participated in municipality-based interventions, and to assess whether effects varied by age, sex or baseline weight category. This matched cohort study included children aged 5-15 years. The intervention group received a municipality-based early treatment intervention. Controls (up to five) were matched on age, sex, baseline weight category and calendar year, and received school-based surveillance consisting of mandatory school health examinations with repeated anthropometric assessments. BMIz trajectories were analysed using mixed effects models with linear splines, adjusting for socioeconomic factors derived from national registries. Among 1274 children and adolescents (216 intervention, 1058 control), 56.9%-58.0% in both groups were living with overweight at baseline. No significant differences in BMIz trajectories were observed over the 4-year period. Both groups showed an initial decrease in BMIz during the first 6 months (intervention: -0.18; control: -0.28), followed by an increase from 6 to 12 months (intervention: 0.12; control: 0.21) and a plateau from 12 to 48 months (intervention: -0.01; control: -0.03). Municipality-based interventions did not result in greater long-term BMIz reductions than matched controls. Both groups had modest short-term improvements, which may reflect beneficial effects of both the structured interventions and the active school-based surveillance received by controls. The similar BMIz trajectories suggest system-level equivalence within the Danish school health system. These findings underscore the need for understanding the mechanisms behind these early changes and how to support long-term maintenance.