Physical activity at the government-recommended level and obesity-related health outcomes: a longitudinal study (Early Bird 37)

Arch. Dis. Child. 2008;93;772-777
B S Metcalf, L D Voss, J Hosking, A N Jeffery, T J Wilkin

Background: In the UK and USA, government guidelines for childhood physical activity have been set (>60 min/ day at >3 metabolic equivalents of thermogenesis (METs)), and body mass index (BMI) chosen as the outcome measure.
Aim: To determine the extent to which physical activity at the government-recommended intensity is associated with change in body mass/fat and metabolic health in prepubertal
Methods: Non-intervention longitudinal study of 113 boys and 99 girls (born 95/96) recruited from 54 schools. Physical activity (Actigraph accelerometers),changes in body mass (raw and age/gender-standardised BMI), fatness (skin-fold thickness and waist circumference)
and metabolic status (insulin resistance, triglycerides, cholesterol/HDL ratio and blood pressure separately and as a composite metabolic z score) were measured on four annual occasions (5, 6, 7 and 8 years).
Results: Mean physical activity did not change over time in either sex. Averaging the 7-day recordings from four time points rather than one increased the reliability of characterising a child’s activity from 71% to 90%. Some 42% of boys and 11% of girls met the guideline. There
were no associations between physical activity and changes in any measurement of body mass or fatness over time in either sex (eg, BMI standard deviation scores: r=20.02, p=0.76). However, there was a small to moderate inverse association between physical activity and change in composite metabolic score (r=20.19, p,0.01). Mixed effects modelling showed
that the improvement in metabolic score among the more active compared to the less active children was linear with time (20.08 z scores/year, p=0.001).
Conclusions: In children, physical activity above the government-recommended intensity of 3 METs is associated with a progressive improvement in metabolic health but not with a change in BMI or fatness. Girls habitually undertake less physical activity than boys, questioning whether girls in particular should be encouraged to do more, or the recommendations adjusted for girls.