Evid Based Med 18:147-148 doi:10.1136/eb-2012-101092
  • Therapeutics
  • Randomised controlled trial

Randomised trial of exercise dose in children reduces diabetes risk (as measured by insulin resistance) in both 20-min and 40-min doses

  1. Aaron L Carrel
  1. Department of Pediatrics, American Family Children's Hospital, University of Wisconsin, Wisconsin, USA
  1. Correspondence to : Dr Aaron L Carrel
    Department of Pediatrics, American Family Children's Hospital, University of Wisconsin, 600 Highland Avenue H4-436, Madison, WI 53792, USA; alcarrel{at}

Commentary on [CrossRef][Medline][Web of Science]Google Scholar


An increasing number of children are classified as obese and sedentary.1 Poor physical fitness and obesity contribute risk towards type 2 diabetes mellitus (T2DM).2 Unfortunately, the simple ‘cure’ for childhood obesity, ‘move more, eat less’, remains elusive. A constant stream of studies addressing this global health ‘epidemic’ have not led to significant progress, in part because add-on physical activity programmes do not compensate for the degree to which the environment, and our choices within it, has reduced physical movement in our daily routine. This problem is highlighted in schools, where physical education and active recess have been whittled away by economic constraints.


Davis and colleagues present an impressive randomised controlled trial studying the effect of exercise ‘dosage’ on insulin resistance (IR), visceral fat, and fitness in overweight children. The authors randomly assigned 222 overweight children to low dose (20 min/day), high dose (40 min/day) or control (usual physical activity) for 13 weeks after school. IR was assessed by oral glucose tolerance test with insulin area under the curve calculated via the trapezoidal rule. Body fat was measured by dual-energy x-ray absorptiometry and visceral fat by magnetic resonance. Cardiovascular fitness was measured by multistage treadmill.


An intention-to-treat analysis of the data demonstrated that after 13 weeks, 20 or 40 min of daily aerobic training improved fitness and demonstrated dose–response benefits for IR and body composition in overweight children compared to controls. The results of the study were notable, with findings that 20-min and 40-min interventions resulted in significant health improvements, but did not significantly differ from each other. The different ‘dosages’ of exercise resulted in nearly identical effects on fitness, and no discernable difference on improved IR. Careful evaluation of the results reveals some results that are more statistically significant than clinically significant. On the positive side, these results suggest that measurable health benefits can be accomplished with a small increase in school-based vigorous physical activity.


Schools are logical sites for public health interventions for children. Supporting this, the Institute of Medicine asserts that the obesity epidemic has been driven by environmental change to a greater extent than poor individual decision-making.3 Establishing policy to promote daily vigorous activity in school provides an evidence-based environmental shift to address obesity. Many parents romanticise the notion that kids left to their own devices will run around the playground; this study shows that they do not.

This study also adds an important finding to the current literature: the role that fitness plays in children's risk of T2DM and IR. And IR has been shown to be a strong marker for risk of T2DM in children.4 Children in this study, regardless of the study group, remained obese after 13 weeks of intervention. Thus, as previously demonstrated, detectable fitness-induced benefits occur without significant reductions in weight.5 How does this new study fit into the urgent need for evidence-based treatments for childhood obesity? This study proved that adding just 20 min of exercise, achievable for most schools, was sufficient at improving health.

The most interesting aspect of this study is not its demonstration of the obvious (that increased activity improves fitness and IR), but rather the lack of dose–response, which the authors felt was explained by sample size. This study takes what seems to be an obvious question and provides a surprising outcome. It is interesting that more exercise is not better, but some is essential. An ‘add-on’ programme of 20 min does not change the environment, and will not reach all children, but provides evidence to build upon. The real question this study does not address is how sustainable is this intervention? The long-term answer remains unknown, but this study adds a piece of evidence towards this question.

An important message for school policy-makers, which is supported by this study, is that adding just 20 min/day of vigorous activity will improve the overall health of children. If these findings can be proven to be sustainable, then we may say: ‘Focus on fitness, not solely weight and diet: just give us 20 mins!’


  • Funding None.

  • Competing interests None.


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