Approximately 14% of children and adolescents in Norway are overweight or obese. Overweight and obesity can affect physical and psychosocial health. Treatment may provide several health benefits.

We have assessed the effectiveness of interventions to reduce body mass index (BMI) and/or body mass index standard deviation score (BMI z-score) in children and adolescents with overweight or obesity and conclude that:

• Combined lifestyle interventions involving structured strategies for change in diet and physical activity may reduce BMI and BMI z-score compared with little or no treatment. We have moderate confidence in the documentation.
• Increased physical activity may lead to decrease in BMI compared with little or no treatment. We have moderate confidence in the documentation.
• We do not know if low glycemic index or load diet may reduce BMI z-score more than low-fat diet and diet with high glycemic index. We were unable to conclude on other dietary interventions.
• Metformin, in addition to lifestyle interventions, may reduce BMI z-scores more than placebo in obese adolescents with impaired glucose tolerance. We have low confidence in the documentation.

Surgical treatment may reduce BMI and BMI z-scores more than lifestyle interventions in adolescents with obesity. We have moderate confidence in the documentation.