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

Brad S Metcalf ^1*, Linda D Voss ¹, Joanne Hosking ¹, Alison N Jeffery ¹ and Terence J Wilkin ¹

¹ Peninsula Medical School, Plymouth, United Kingdom

^* To whom correspondence should be addressed. E-mail: brad.metcalf{at}phnt.swest.nhs.uk.

Accepted 28 March 2008

Abstract

Background: In the UK and US, Government guidelines for childhood physical activity have been set (60mins/day at 3 METs), and 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 pre-pubertal children.

Methods: Non-intervention longitudinal study of 113 boys and 99 girls (born 1995/96) recruited from 54 schools. Physical activity (MTI accelerometers), changes in body mass (raw and age/gender-standardized BMI), fatness (skin-fold thickness and waist circumference) and metabolic status (insulin resistance, triglycerides, cholesterol/HDL ratio and blood pressure - separately and combined as a composite metabolic score) were measured on four annual occasions (5, 6, 7 and 8y).

Results: Mean physical activity did not change over time in either sex. Averaging the seven-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 measure of body mass or fatness over time in either sex (e.g. BMI-SDS: r=-0.02 p=0.76). However, there was a small-to-moderate inverse association between physical activity and change in composite metabolic score (r=-0.19, p<0.01). Mixed effects modeling showed that the improvement in metabolic score among the more active compared to the less active children was linear with time (-0.08 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 (though direction of causality is not certain). 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.