Childhood obesity is a highly prevalent, complex, and heterogenous condition putting children and adolescents with overweight and obesity at increased risk for several physical and psychological complications. Childhood obesity is impacted by the interaction of numerous factors, including nutrition, physical activity, psychological well-being, socioeconomic status as well as genetic predisposition. This evidence highlights the need to develop and enhance both preventive and treatment strategies for children and adolescents with overweight and obesity.

The aim of this PhD project was to examine the effect of adding 3 months of supervised high-intensity interval training (HIIT) to a 12 months validated multidisciplinary lifestyle intervention on BMI z-score, waist circumference, blood pressure, health-related quality of life, physical activity behavior and mental health symptoms in children and adolescents with overweight and obesity.

In total 173 children and adolescents with overweight and obesity were included and randomized in the project. Ninety children and adolescents participated in the HIIT intervention.

The three studies presented in this dissertation collectively indicates that while HIIT did not significantly augment reductions in anthropometric measurements, blood pressure or symptoms of mental health disorders, it did enhance adherence to lifestyle interventions, particularly in participants with severe obesity. Additionally, the HIIT intervention resulted in a significant improvement in health-related quality of life and moderate-to-vigorous physical activity after three months of intervention.

The high prevalence of childhood overweight and obesity highlights the need for integrated multidisciplinary care. This dissertation shows that close collaboration between primary and tertiary care sectors can create an effective and enjoyable physical activity intervention to improve both physical and psychological health parameters in children and adolescents with overweight and obesity.