BMI and recommended levels of physical activity in school children

Background: Physical activity (PA) and its health benefits are a continuous point of discussion. Recommendations for children's daily PA vary between guidelines. To better define the amount of PA necessary to prevent overweight and obesity in children, further research is needed. The present study investigates children's compliance to physical activity guidelines (PAGs) and the association between objectively measured PA and body mass index (BMI). Methods: Participating children were 11 years old (n = 419) and part of the European CHOP trial, which was conducted in Germany, Belgium, Poland, Spain, Italy. At least 2 days of PA measurements were collected from each child using a SenseWear™ armband. BMI was calculated from children's height and weight. Thresholds of min·day-1 in PA needed to differentiate between normal and excess weight (overweight/obesity) were determined with Receiver Operator Characteristics (ROC) analysis. Additionally, adjusted linear and logistic regressions models were calculated for group differences and effects of a 5, 15 and 60 min·day-1 increases in PA on BMI. Results: Median time spent in total PA was 462 min·day-1 (25th percentile; 75th percentile: 389; 534) and 75 min·day-1 (41; 115) in moderate to vigorous PA (MVPA). Girls spent 36 min·day-1 less in MVPA than boys and overweight/obese children 24 min·day-1 less than normal weight children (linear regression, p < 0.001). 63.2% of the children met PAGs of 60 min·day-1 in MVPA. The optimal threshold for min·day-1 in MVPA determined with ROC analysis was 46 min·day-1. Comparing 5, 15 and 60 min·day-1 increases in PA revealed that an additional 15 min·day-1 of vigorous PA had the same effect as 60 min·day-1 of MVPA. Sedentary time and light PA showed contrary associations to one another, with light PA being negatively and sedentary time being positively associated with excessive weight. Conclusions: Current PAGs are met by 2/3 of children and seem appropriate to prevent excess weight in children. An official recommendation of daily 15-20 min of vigorous PA and further reduction of sedentary time could help to fight youth overweight and thus be of potential public health importance.