Physical activity in 6-10 year old children : variations over time, associations with metabolic risk factors and role in obesity prevention

Abstract

Background and Aims: Childhood overweight and obesity are emerging health problems in all western countries and also in urban areas in developing countries. In Sweden, the prevalence of overweight and obesity has been rapidly increasing. The aim of this thesis was to assess the efficacy of a school-based intervention programme in reducing the prevalence of overweight in 6-10 year old children. Another aim was to examine the stability of physical activity (PA), and differences in PA patterns in relation to age and gender. Also, to study differences in metabolic risk between groups of normal weight and moderately overweight children and associations with PA and cardio-respiratory fitness (CRF).

Material and Methods: 3135 boys and girls aged 6 to 10 years were included in the study. Ten schools were selected in Stockholm county area and randomised to intervention (n=5) and control (n=5) schools. Low fat dairy products and whole grain bread were promoted and all sweets and sweetened drinks were eliminated in intervention schools. PA was aimed to increase by 30 minutes per day during school time and sedentary behaviour restricted during after school care time. Eating habits at home were assessed by parental report. Eating attitudes were evaluated by self-report. PA was objectively measured during 7 days by accelerometry in 1293 children. Two sub-groups of children provided repeated measurements of physical activity. 68 children were selected based on their BMI and categorised into normal weight and overweight/obese. CRF was measured with a maximal ergometer cycle test and body composition with dual-energy x-ray absorptiometry (DXA). Oral glucose tolerance test was performed and triglycerides (TG) and HDL-cholesterol were measured from fasting blood samples. A metabolic risk score was computed and calculated as the mean of the standardised values of the outcome scores (i.e. insulin, glucose, TG, HDL and blood pressure).

Results: The prevalence of overweight and obesity decreased by 3.2% (from 20.3 to 17.1) in intervention schools compared to an increase of 2.8% (from 16.1 to 18.9) in control schools (P<0.05). The results showed no difference between intervention and controls, after cluster adjustment, in the longitudinal analysis of BMI SDS changes. However, a larger proportion of the children who were initially overweight reached normal weight in the intervention group (14%) compared to the control group (7.5%) (P=0.017). PA did not differ between intervention and control schools after cluster adjustment. Eating habits at home were found to be healthier among families with children in intervention schools at the end of the intervention. There was no difference between children in intervention and control schools in selfreported eating attitudes. Mean daily PA differed significantly across age groups (P<0.001) and was significantly lower during weekends compared to weekdays in all age groups (P<0.001). This decline was similar across low, medium and highly active children. The difference in mean PA between boys and girls was highest during school time (P<0.001) and after school care time (P<0.001). Baseline physical activity was significantly correlated with physical activity at follow-up (r=0.59). There was a significant difference between normal weight and overweight children in clustered metabolic risk (P=0.03), insulin (AUC) (P=0.01), fasting insulin (P=0.003) and systolic blood pressure (P=0.02). PA and CRF did not differ significantly between groups. PA and CRF were negatively associated with insulin levels.

Conclusions: A school-based intervention can reduce the prevalence of overweight and obesity in 6-10 year old children and may affect eating habits at home. The effect of the intervention was possibly due to its effect on healthy eating habits at school and at home rather than on increased levels of PA. The decline in PA in children may start already at the age of 6 years. The difference in PA levels between girls and boys is most pronounced during school time. PA levels are disproportionally low during weekends. Repeated measurements show that PA levels are fairly stable over time. Metabolic risk factors are elevated in moderately overweight pre-pubertal children compared with normal weight controls. This is not due to lower PA or CRF in the overweight group although PA and CRF were associated with lower insulin levels. This suggests a detrimental effect of a moderate excess of adiposity already at an early age.