Effect on prevalence rates of using three different definitions of obesity in 9-10 year old children

Introduction Body Mass Index (BMI) is frequently used to define body fatness. In children, curves extrapolated from adult BMI values and percentile curves are often used to plot overweight and obesity cut-offs throughout childhood, taking phases of growth into account. Three methods are commonly used to classify childhood BMI: UK 1990 Growth Foundation, Cole et al (2000) and Chinn and Rona (2002) cut-off values. There is little agreement about which of these methods is the most reliable. This study examined the prevalence of obesity in 9-10 year old children using the three cut-off measures. Methods BMI data on 9-10 year old school children collected by the Liverpool SportsLinx project were examined using the three classification methods to estimate the prevalence of overweight and obesity. This was conducted on six successive cohorts of children, from academic years 1998-99 to 2003-04. Results Substantial differences in the prevalence of obesity were apparent when using the three cut-offs. For example, obesity prevalence in girls in 2004: UK 1990 = 14.4%, Cole et al (2000) = 10.5%, Chinn and Rona (2002) = 10.3%. The UK 1990 cut-off points resulted in an increased prevalence of obesity and reduced prevalence of overweight compared to the other cut-offs (e.g. boys' obesity in 2004: UK 1990 = 16.79%, Chinn and Rona = 12.1%). Conclusion The Cole et al results gave a similar prevalence of obesity in girls when compared with the Chinn and Rona findings, but appeared to classify boys and girls differently. Since the Cole et al (2000) figures are based on a limited international data set, we suggest the wider use of the Chinn and Rona reference cut-offvalues.