Physical activity in young children

Abstract

John Reilly and colleagues1Reilly JJ Jackson D Montgomery C et al.Total energy expenditure and physical activity in young Scottish children: mixed longitudinal study.Lancet. 2004; 362: 211-212Summary Full Text Full Text PDF Scopus (308) Google Scholar report interesting data on total energy expenditure and physical activity levels in 3-year-old to 5-year-old children and put forward the proposal that modern British children establish a sedentary lifestyle at early age. Although we agree that this is possible, we do not feel that the data in the paper allow such a conclusion to be drawn. First, the total energy expenditure and physical activity levels (ie, total energy expenditure divided by resting energy expenditure) do not indicate a sedentary lifestyle by comparison with values established in other populations. Although physical activity levels of 1·56 and 1·61 would be low for adults, they are indicative of at least average activity levels in 3-year-old to 5-year-old children (normative data from Institute of Medicine).2Institute of Medicine Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, and amino acids (macronutrients): a report of the panel on macronutrients, subcommittees on upper levels of nutrients and interpretation and uses of dietary reference intakes, and the standing committee on the scientific evaluation of dietary reference intakes. National Academies Press, Washington, DC2002Google Scholar Second, the accelerometer data reported need to be interpreted with caution. Reilly and colleagues applied threshold values for light, moderate, and vigorous intensity of physical activity on the basis of a study by Puyau and colleagues.3Puyau MR Adolph AL Vohra FA Butte NF Validation and calibration of physical activity monitors in children.Obes Res. 2002; 10: 150-157Crossref PubMed Scopus (788) Google Scholar These thresholds of activity were derived from the laboratory-based relation between accelerometer counts and energy expenditure in children with an average age of about 11 years. Laboratory-established correlations between activity counts and energy expenditure, and the derived intensity thresholds from these equations, are dependent on the calibration activities used to establish them. Laboratory-based thresholds are therefore unlikely to be valid for free-living purposes. To test this hypothesis, we applied the prediction equation for physical activity energy expenditure from Puyau and colleagues3Puyau MR Adolph AL Vohra FA Butte NF Validation and calibration of physical activity monitors in children.Obes Res. 2002; 10: 150-157Crossref PubMed Scopus (788) Google Scholar to free-living physical activity energy expenditure, simultaneously assessed by the doubly labelled water method and accelerometry in 9-year-old to 10-year-old children.4Ekelund U Sjöström M Yngve A et al.Physical activity assessed by activity monitor and doubly labeled water in children.Med Sci Sports Exerc. 2001; 33: 275-281Crossref PubMed Scopus (334) Google Scholar As the figure shows, in a paediatric population, the equation applied by Reilly and colleagues overestimated physical activity energy expenditure by a mean of about 11·4 kcal per kg per day (47%) and a significant inverse relation (r=–0·90, p<0·0001) between the difference of the methods and the mean of the methods was seen, indicating that the applied equation is invalid for prediction of free-living physical activity energy expenditure and thus for generating intensity thresholds. Therefore, time spent in intensity categories defined on the basis of laboratory-derived thresholds that are disproportionately high when applied to free-living will result in a biased estimate of the time spent at different intensity levels and of individuals classified as sedentary. Third, in Reilly and colleagues' study, the total volume of physical activity (counts per min per day—a measure previously validated in children4Ekelund U Sjöström M Yngve A et al.Physical activity assessed by activity monitor and doubly labeled water in children.Med Sci Sports Exerc. 2001; 33: 275-281Crossref PubMed Scopus (334) Google Scholar) was similar to, or even higher, than that reported from a large (n=2185) randomly selected population-based sample of European children and adolescents.5Riddoch CJ Bo Andersen L Wedderkopp N et al.Physical activity levels and patterns of 9- and 15-yr-old European children.Med Sci Sports Exerc. 2004; 36: 86-92Crossref PubMed Scopus (696) Google Scholar In this study, 97% of children were judged to have achieved the current health-enhancing physical activity guidelines of 60 min of moderate-intensity physical activity per day.5Riddoch CJ Bo Andersen L Wedderkopp N et al.Physical activity levels and patterns of 9- and 15-yr-old European children.Med Sci Sports Exerc. 2004; 36: 86-92Crossref PubMed Scopus (696) Google Scholar Since accelerometry is widely used when assessing physical activity in children, it is of great concern that data from this method could be interpreted so differently depending on the intensity thresholds applied. We agree with Reilly and colleagues that public-health interventions are urgently needed to curb the obesity epidemic in young people, but such strategies need to be based on sound epidemiology of physical activity in children and its relation with obesity and metabolic risk. Physical activity in young childrenAuthors' reply Full-Text PDF