Reply to G Rodríguez et al

¹ Department of Paediatrics and Child Health, University of Sydney, Westmead, NSW 2006, Australia, E-mail: louiseb3{at}chw.edu.au
² James Fairfax Institute of Paediatric Nutrition, Royal Alexandra Hospital for Children, Westmead, NSW 2145, Australia
³ Children’s Nutrition Research Centre, Department of Paediatrics & Child Health, University of Queensland, Royal Children’s Hospital, Brisbane, QLD 4029, Australia

Dear Sir:

We are surprised that Rodríguez et al find the results of our doubly labeled water study in children "categorical and surprising" because our results are entirely consistent with the results of a meta-analysis of doubly labeled water studies in adults (1). Like us, Westerterp and Goran (1) found that in males, but not in females, a lower physical activity level was associated with a higher percentage of body fat.

Rodríguez et al cast doubt on our use of the Schofield equations in estimating resting energy expenditure (REE) in children. However, it has been shown in our laboratory (2) as well as in other laboratories (3, 4) that there is good agreement between REE measured with the use of indirect calorimetry and REE estimated with the use of the Schofield equations. We agree that it would have been preferable to have measured REE by indirect calorimetry. Unfortunately, this was not logistically possible with school-aged children who lived some distance from the study center. Moreover, the constraints placed on the subject for a true estimate of REE could be said to be too great for young children. For these reasons, the practical alternative of using the Schofield equations was chosen.

Rodríguez et al refer to some of their own work that was recently published (5). We note that there was a relatively small sample size of 20 for each sex and that a high proportion (40%) of their subjects were obese. In addition, percentage of body fat was determined from skinfold thicknesses, a relatively imprecise method, by using an equation derived from only 23 subjects aged 1–11 y (6). This could explain some of their findings, but without seeing their data, it is difficult to comment.

The argument put forward by Rodríguez et al also does not explain the distinct sex differences found in our study. If there is an inherent error in estimating REE (and hence physical activity level and activity energy expenditure) with the use of the Schofield equations, then this should be the case for males and females alike. However, we found an inverse relation between percentage of body fat and both physical activity level and activity energy expenditure in boys, but not in girls. Rodríguez et al do not comment on this apparent discrepancy.

The relation between physical activity and body composition clearly is not straightforward, and further information is required. Nevertheless, we believe that our data make a useful contribution to the debate.

REFERENCES

1. Westerterp KR, Goran MI. Relationship between physical activity related energy expenditure and body composition: a gender difference. Int J Obes Relat Metab Disord 1997;21:184–8.
2. Allen JR, McCauley JC, Waters DL, O’Connor J, Roberts DCK, Gaskin KJ. Resting energy expenditure in children with phenylketonuria. Am J Clin Nutr 1995;62:797–801.
3. Firouzbakhsh S, Mathis RK, Dorchester WL, et al. Measured resting energy expenditure in children. J Pediatr Gastroenterol Nutr 1993;16:136–42.
4. Schofield WN. Predicting basal metabolic rate, new standards and review of previous work. Hum Nutr Clin Nutr 1985;39(suppl):5–41.
5. Rodríguez G, Moreno LA, Sarría A, Fleta J, Bueno M. Resting energy expenditure in children and adolescents: agreement between colorimetry and prediction equations. Clin Nutr 2002;21:255–60.
6. Brook CG. Determination of body composition of children from skinfold measurements. Arch Dis Child 1971;46:182–4.