Body Mass Index

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International Journal of Obesity (2006) 30, 590–594 & 2006 Nature Publishing Group All rights reserved 0307-0565/06 $30.00

Body mass index in children and adolescents: considerations for population-based applications A Must1,2 and SE Anderson2
1 2

Department of Public Health and Family Medicine, Tufts University School of Medicine, Boston, MA, USA and The Friedman School of Nutrition Science and Policy, Tufts University, Boston, MA, USA

International Journal of Obesity (2006) 30, 590–594. doi:10.1038/sj.ijo.0803300 Keywords: review; body mass index; pediatrics; BMI z-score; overweight classification

After several years of experience with body mass index (BMI)for-age reference standards in the US, the UK, and elsewhere, reflection on ‘how things are going’ seems timely. In this issue, Reilly1 offers a summary of the evidence base for the diagnostic accuracy of BMI in youth and his perspective on what is achieved by a definition of overweight and obesity based on high BMI. To complement this, in our short review, we describe the BMI measure itself, the utility of a BMI z-score (s.d. score), their utility in cross-sectional and longitudinal applications in public health/surveillance, clinical and population-based research settings.

centiles,5 and the inaccuracy of assessing overweight by observation or ‘eye-balling’ has been established.6,7 Therefore, for screening or for epidemiologic research, using a weight/height index to define obesity has advantages that outweigh its limitations. Despite the likelihood of misclassification of the small percentage of individuals whose high BMI is due to lean muscle mass (e.g. some professional athletes), the great majority of individuals with high BMI have excess body fat.

Use of body mass index in children and adolescents Body mass index defined Body mass index is a measure of weight adjusted for height. It is calculated as weight in kilograms divided by the square of height in meters. Although BMI is an imperfect tool – it does not distinguish overweight due to excess fat mass from overweight due to excess lean mass – it is the most commonly used measure for assessing obesity in adults. Other methods of determining adiposity are more accurate,2 but have limited applicability to screening or studying large populations. The BMI is well correlated with these more direct fatness measures,3,4 and weight and height are simple, inexpensive, non-invasive measurements that are recorded routinely in clinical and research settings. Others have discussed the limitations of clinical screening for high adiposity by comparing weight centiles to height Correspondence: Dr A Must, Department of Public Health and Family Medicine, Tufts University, 136 Harrison Ave, Boston, MA 02111, USA. E-mail: Received 20 December 2005; revised 24 January 2006; accepted 5 February 2006

The BMI is used to assess weight status in children and adolescents as well as adults, but whereas in adults the BMI cut points that define obesity and overweight are not linked to age and do not differ for males and females, in growing children BMI varies with age and sex. Thus, a 5-year-old boy with a BMI of 20 kg/m2 is likely to be overfat, but a 15-yearold boy with a BMI of 20 kg/m2 is likely to be lean. As a result, for BMI to be meaningful in children it must be compared to a reference-standard that accounts for child age and sex.

Choice of a reference standard
National and international BMI-for-age reference standards are available. The US BMI-for-age reference is based on nationally representative data from boys and girls ages 2–20 years collected between 1963 and 1980.8 National reference standards are also in use in the UK,9 and are under development elsewhere. An international BMI reference has been produced by the International Obesity Task Force

BMI in children and adolescents A Must and SE Anderson

(IOTF) with data from...
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