Physical activity to prevent obesity in young children: [1]

Reilly et al entitle their article, "Physical activity to prevent obesity â[euro]¦" and conclude, "Alternative interventions [to physical activity] to prevent obesity in young children are required." 1 The emphasis on a lack of impact of physical activity on obesity is unfortunate for four reasons.

Firstly, the independent variable was not successfully manipulated: the intervention failed to generate differences in physical activity or sedentary behaviour between groups. Why would one expect a "physical activity intervention" which has no impact on physical activity to alter energy balance or "obesity"?

Secondly, the principal outcome measure, body mass index (BMI), is an inappropriate measure of obesity for two reasons.

Studies of obesity should focus on body composition, rather than measures of body weight (BMI). We, like Reilly et al, observed no differences in weight or BMI in supervised exercise studies in obese children. 2 However, dual energy x ray absorptiometry (DEXA) showed significant decreases in central fat mass. Weight and BMI did not concomitantly change because lower limb muscle mass increased. We also observed improvements in euglycaemic hyperinsulinaemic clamps and vascular function with training (A M Thompson et al, 14th annual scientific meeting, Australasian Society for the Study of Obesity, Adelaide, October 2005). 2 A review of DEXA studies in obese children showed similar countervailing impacts of exercise on fat and lean body mass. 3 A conclusion of no impact on obesity based on lack of change in BMI is misleading.

BMI corrects for individuals who are heavy because they are tall, a valid concept in cross sectional population comparisons. However, in within subject longitudinal experiments, change in BMI reverts to a measure of change in weight because interventions do not alter height. Using weight as an index of obesity is flawed (above). In children, change in BMI is not even a valid measure of weight change because height also changes. Reilly et al did not include data on changes in height or weight or on waist girth.

Thirdly, no amount of statistical manipulation alters the fundamental fact that BMI is not a measure of obesity (adiposity) and change in BMI within subjects is not a proxy for change in obesity.

Finally, BMI may be practical, precise, and easy to collect, but these characteristics do not make it a valid measure of obesity in this context.

Research studies fundamentally need to manipulate an independent variable and measure the impact on a legitimate dependent variable. The study of Reilly et al failed on both counts. The paper generated widespread and unfortunate, but understandable, international media attention. 4 Exercise remains a key interventional strategy for the management of obesity in young people.

Footnote

Competing interests: None declared.

References

1 Reilly JJ, Kelly L, Montgomery C, Williamson A, Fisher A, McColl JH, Physical activity to prevent obesity in young children: cluster randomised controlled trial. BMJ 2006; 333: 1041-3. (18 November.)

2 Watts K, Beye P, Siafarikas A, Davis EA, Jones TW, O'Driscoll G, Exercise training normalizes vascular dysfunction and improves central adiposity in obese adolescents. J Am Coll Cardiol 2004; 43: 1823-7.

3 Watts K, Davis E, Jones T, Green DJ. Effect of exercise training in obese children and adolescents. Sports Med 2005; 35: 1-18.

4 Boseley S. More exercise does nothing to stop obesity in youngsters. Guardian 2006 Oct 6.