It is widely known that the BMI has a number of limitations when used to assess overweight and obesity in children. This position is in part a result of its inability to differentiate between fat and fat-free masses, resulting in low sensitivity(Reference McCarthy1). Recently, references for body fatness using bioelectrical impedance analysis (BIA) have been developed for the UK childhood population(Reference McCarthy, Cole and Fry2). The prevalence of overweight and obesity and the prevalence of overfat and obesity were compared in a sample of children from low-income schools within London.
A total of 1088 Caucasian children from schools predominantly in east London boroughs aged between 5 and 13 years were selected for analysis. Height and weight were measured and BMI calculated. Percentage body fat (%BF) was predicted using BIA (Tanita BC418; Tanita UK Ltd, Yiewsley, Middlesex, UK). Both measurements were converted to a standard deviation score based on the current UK reference data(Reference McCarthy, Cole and Fry2,Reference Cole, Freeman and Preece3). The percentage of children exceeding the International Obesity Task Force (IOTF) cut-off for overweight and the proportion exceeding the BIA cut-off for overfat was calculated(Reference McCarthy, Cole and Fry2,Reference Cole, Bellizzi and Flegal4). The percentage of the original sample of children misclassified as either overweight and obese or normal weight by BMI was subsequently determined using %BF as the criterion.
In this sample 23% (n 254) of children were classified as overweight and obese based on the IOTF BMI cut-off, which contrasted with 30% (n 330) of the children classified as overfat and obese based on BIA. Further analysis of the data indicated that within the 23% of children, twenty-seven did not have excess body fat. However, within the group of children classified as normal BMI (n 834) 103 children were identified as having excess body fat. On a whole-group basis these data equated to 2.5% being misclassified as overweight and obese based on IOTF BMI and 9.5% being misclassified as normal BMI.
This study is the first to quantify the misclassification of a sample of UK children by BMI using the UK %BF references. These results indicate that whilst a relatively small number are wrongly classified as overweight and obese, a substantial number of children with high body-fat levels are missed using BMI. These findings are in general agreement with an earlier study(Reference Reilly, Dorosty and Emmett5). Whilst this group cannot be considered representative of UK children in general, the findings, if reproduced on a larger scale, would suggest current national data may seriously under-represent the true prevalence of children at risk of morbidity related to excess body fat. This widely-acknowledged but generally-ignored limitation of BMI should be considered when prevalence rates based on BMI are communicated and interpreted.
