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Background Overweight and obesity in childhood is associated with physical, psychological and social consequences. Physical inactivity is regarded as one of the main factors that have contributed to the increase in childhood obesity through out the world. Overweight and obesity as well as physical activity level are shown to track from childhood to adolescence and adulthood, thereby influencing not only the current health status but also long-term health. The general purpose of this thesis was to evaluate the effect of a 2-year family-based lifestyle intervention on physical activity and metabolic health among children with overweight and obesity.

Methods Children with overweight or obesity living in northern Sweden were recruited to the study. In total 105 children, mean age 10.5 years (SD±1.09), were randomized into either an intervention or a control group. The intervention group was offered as a 2-year family-based lifestyle intervention; the 1^st year consisted of 14 group sessions and during the 2^nd year the intervention was web-based. The control group did not participate in any intervention sessions, but performed all measurements. Physical activity was measured in all children using SenseWear Pro₂ Armband (SWA) during 4 consecutive days before, in the middle and after the intervention, data regarding anthropometrics and blood values were collected in the same periods. Twenty-two of the children wore SWA during 14 days before the intervention in order to validate energy expenditure (EE) estimated by SWA against EE measured with double labelled water.

Results The SWA, together with software version 5.1, proved to be a valid device to accurately estimate EE at group level of overweight and obese children. There were no statistically significant differences between the groups neither before nor after the intervention regarding physical activity and screen-time. All children significantly decreased their time being active ≥3 METs during the study period. After the study period, significantly fewer in the control group achieved the national physical activity recommendation, and they had significantly increased their screen-time. However, these changes were not seen within the intervention group. The intervention group had a significantly lower apolipoprotein B/A1 compared to the control group at 1-year measurement; no other significant differences were found regarding metabolic markers. No statistical difference was found between the groups regarding body mass index after the 2-year study period.

Conclusion Even though a comprehensive program, the 2-year family-based lifestyle intervention had limited effects on physical activity and metabolic health in overweight and obese children. SWA is a device that can be used in future studies to measure energy expenditure among free-living overweight or obese children.