Diagnostic accuracy of the BMI for age in paediatrics

Abstract

Must and Anderson,1 in this issue of the journal, have described the basis of body mass index (BMI) for age for the assessment of weight status in children and adolescents. The aims of this short article are to summarise the evidence base on the diagnostic accuracy of BMI, and to expand on what we achieve when defining paediatric obesity on the basis of a high BMI for age. A high BMI for age is not 'obesity' (a high body fat content associated with increased morbidity), per se but a diagnostic test for obesity. Similarly, in adults a BMI>30 kg/m2 is not obesity, but a positive diagnostic test for obesity. It is also widely stated that 'BMI does not predict body fat content accurately'. This is true,2, 3, 4, 5 but should not be used to damn the diagnostic ability of BMI because it is not directly relevant to the issue of diagnosis of obesity. When diagnosing or defining obesity, we do not need to estimate body fat content precisely; rather we simply need to establish with confidence that any child defined as obese has a high body fat content relative to his/her peers. There are two main components to the issue of diagnostic accuracy: how well does a high BMI for age diagnose the fattest children in the population? Are children at high BMI for age at greater risk of morbidity than their peers at lower BMI for age

Age- and sex-standardised lean and fat indices derived from bioelectrical impedance analysis for ages 7–11 years: functional associations with cardio-respiratory fitness and grip strength

“…A major barrier to nutritional assessment in children is the widespread perception that the assessment methods and definitions of under-and over-nutrition which are currently available lack functional significance and are just 'arbitrary' (1) . We have previously described the development of the lean index and shown that it can discriminate between groups of children with chronic conditions expected to cause differing body compositions (6) .…”

“…There has been a recent resurgence of interest in nutritional assessment of children, driven mainly by the advent of the obesity epidemic (1) but there is also an increasing awareness of its importance in the assessment of underweight, with recognition that a low BMI can reflect a low lean mass rather than low fat stores, which has important implications for nutritional management (2,3) . Assessing body composition robustly is labour intensive and expensive, and is particularly difficult in a field or clinical setting.…”

Epidemiologic and Physiologic Approaches to Understanding the Etiology of Pediatric Obesity: Finding the Needle in the Haystack

“…Systematic reviews have shown that high BMI Z score is a good, perhaps surprisingly good, measure of high fat mass (1), but at lower points in the BMI distribution a very wide range of fat masses exist at the same BMI SD score (Z score) (1,44,45). The use of mean BMI Z score is a poor measure of overweight and obesity in a sample.…”

“…O besity continues to increase rapidly across much of the world (1)(2)(3)(4)(5). In England, obesity prevalence defined as body mass index (BMI) Ն95th percentile relative to UK 1990 reference data (4) in 11-15 y olds was 25% in 2004 (5) compared with 17% in 1996 (6) and 5% by definition in 1990 (4).…”

“…The specific aims of the present review are therefore to (1) highlight the paradoxical complexity of the etiology of obesity; (2) identify emerging problems in etiologic research; (3) suggest ways in which energy balance studies (which attempt to attribute etiology to energy intake and/or energy expenditure) and epidemiologic studies (which try to identify exposure variables for obesity) might be more productive in future; (4) suggest approaches to integrating the diverse body of evidence of etiology now available.…”