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BMJ. 2007 July 28; 335(7612): 166–167.
PMCID: PMC1934439

Body mass index cut offs to define thinness in children and adolescents

Noël Cameron, professor of human biology

A new chart will be most useful in countries in social, economic, and nutritional transition where both undernutrition and overnutrition are prevalent

Assessment of risk for overweight by monitoring body mass index is now recommended in developed countries and more recently in urban areas of less developed countries. Body mass index is known to track significantly from childhood, to adolescence, and then to adulthood.1 Body mass index should therefore be assessed and monitored during childhood and adolescence to allow for early, and perhaps more effective, intervention strategies. Tim Cole and colleagues' validated thresholds or cut offs for body mass index, recommended by the International Obesity Task Force to define and identify overweight and obesity in childhood,2 are internationally recognised.

Rather less attention has been paid, however, to the importance of assessing body mass index to monitor undernutrition. In this week's BMJ, Cole and colleagues extend this work to provide cut offs for body mass index to define “thinness” in children and adolescents.3

Malnutrition, or more specifically undernutrition, in children has long been defined in terms of height and weight in relation to age in relation to various cut offs, which are usually based on representative samples of European or American children. The choice of cut off is fundamentally important to identify correctly those children at risk and, ideally, should be related to known outcomes for morbidity and mortality. Yet, while adult body mass index values of 25 (overweight) and 30 (obesity) are related to morbidity, evidence on morbidity related to cut offs for thinness, particularly in children, is less clear. Current cut offs for thinness in children are related to either the third or fifth centile of reference charts for body mass index and cut offs for malnutrition (undernutrition) to weight for height z scores (standard deviation scores).

In France, for instance, the third centile of the French reference chart for body mass index is recommended for defining thinness in adolescents,4 while the World Health Organization (WHO) expert committee on anthropometry5 recommends the fifth centile of the American National Health and Nutrition Examination Survey (NHANES) reference database to define thinness in adolescence.6 Cole and colleagues argue that the current WHO recommendations for defining thinness are inappropriate because the NHANES dataset dates from the early 1970s and is of “uncertain validity.” In addition, the latest WHO growth standards7 are truncated at age 5, leaving no current reference that effectively covers the age range of childhood to adolescence.

Cole and colleagues' response to this void is to use the same technique on a sample of almost 200 000 subjects from six countries (Brazil, United Kingdom, Hong Kong, the Netherlands, Singapore, and the United States), the source of the data for the International Obesity Task Force reference standards for overweight and obesity, to generate cut offs throughout childhood and adolescence that identify the child at risk because of thinness. The chosen cut off is a body mass index of 17 kg/m2 at age 18 coinciding with the WHO grade 2 cut off for thinness in adults, and to a value of −2 z scores for body mass index in Cole's combined dataset.

In addition, cut offs of 18.5 kg/m2 and 16 kg/m2 are also included to coincide with WHO grade 1 and grade 3 thinness in adults and allow the distinction between different grades of undernutrition and thus different levels of risk in children. This is important, given that the prevalence of child mortality is directly related to the degree of malnutrition.8 Furthermore, a value of −2 z scores has the added advantage of being about 80% of the median body mass index and is equivalent to the WHO definition of wasting (low weight for height).

These new cut offs are most suitable for use with samples of children in comparative studies of the prevalence of thinness, rather than as references or standards for current or recommended body mass index by age and sex. Limitations are that they use the same cut offs for males and females and do not adjust for pubertal development or the tempo of adolescent growth and maturation. Both sex and pubertal development are associated with dramatic changes in body composition and detailed statistical control of both variables is usually needed during analyses. Moreover, body mass index is not a direct measure of total body fat or total body lean mass, even though it correlates surprisingly well with fat and lean tissue.9

The new cut offs proposed by Cole and colleagues need to be tested in studies of the association between thinness and morbidity in children and adolescents. They are potentially most useful in countries that are experiencing social, economic, and nutritional transition such as South Africa, Brazil, China, and Russia,10 and in which both overnutrition and undernutrition are prevalent. Having a single chart that is consistent at both ends and is constructed from international data is helpful in both epidemiological and clinical settings.


Competing interests: None declared.

Provenance and peer review: Commissioned; not externally peer reviewed.


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