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

Objective To determine cut offs to define thinness in children and adolescents, based on body mass index at age 18 years. Design International survey of six large nationally representative cross sectional studies on growth. Setting Brazil, Great Britain, Hong Kong, the Netherlands, Singapore, and the United States. Subjects 97876 males and 94851 females from birth to 25 years. Main outcome measure Body mass index (BMI, weight/ height). Results The World Health Organization defines grade 2 thinness in adults as BMI <17. This same cut off, applied to the six datasets at age 18 years, gave mean BMI close to a z score of −2 and 80% of the median. Thus it matches existing criteria forwasting in childrenbasedonweight for height. For each dataset, centile curves were drawn to pass through the cut off of BMI 17 at 18 years. The resulting curves were averaged to provide age and sex specific cut-off points from 2-18 years. Similar cut offs were derived based on BMI 16 and 18.5 at 18 years, together providing definitions of thinness grades 1, 2, and 3 in children and adolescents consistent with the WHO adult definitions. Conclusions The proposed cut-off points should help to provide internationally comparable prevalence rates of thinness in children and adolescents. INTRODUCTION Much has been written about the epidemic of child obesity but malnutrition—meaning undernutrition— in infants, children, and adolescents poses a considerably larger public health problem internationally, and in the developed world anorexia nervosa is the third most common chronic condition of adolescence. Obesity and malnutrition represent opposite extremes on the spectrum of adiposity, and both are routinely quantified in terms of weight and height relative to the child’s age. Yet the classification of malnutrition in later childhood and adolescence is currently unsatisfactory because of the lack of suitable cut offs for international use. Fifty years ago Gomez introduced his malnutrition classification of weight below a specified percentage of median weight for the child’s age. This included three components: a measurement, a reference for age adjustment, and a set of cut offs. Later Seoane and Latham proposed splitting weight for age into weight for height and height for age, allowing underweight to be defined as wasting or stunting, or both. Subsequently Waterlow et al recommended the use of z scores for the definitions of underweight, wasting, and stunting, with the cut offs defined in terms of standard deviations (SDs) below the median rather than as percentages of themedian. This ensures that the false positive screening rate is constant across age as applied to the reference population. In 1983 theWorldHealthOrganization (WHO) formally recognised the US National Center for Health Statistics (NCHS) classification as the international reference and has used it since to classify children as underweight, wasted, or stunted, each based on a cut off of −2 z scores. Wasting in particular is assessed with the NCHS/WHO weight for height reference, which compares the child’s weight to the average weight of children of the same height. This ignores the child’s age, which allows nutritional status to be assessed when age is not known. It also assumes that, on average, children of a given height weigh the same whatever their age; in infancy and adolescence, however, the weight-height relation depends on age. 19 This can be seen by considering the index weight/ height, where the height power p is allowed to vary with age. The index is adjusted for age and sex by dividing it by the same ratio based on median weight and height for the child’s age and sex. For a weight for height index such asNCHS, the value of p is the ratio of the percentage growth rates in weight and height at each age, so it is largest when weight is growing fastest relative to height—that is, in infancy and adolescence when p is 3 or more as against 1.5 in mid-childhood. In later adolescence, as weight growth continues after height growth has stopped, p increases to infinity and height adjustment becomes impossible. This is an important general limitation of weight for height references in that they cannot be used in adolescence. 20 For this reason the NCHS weight for height reference was truncated at age 10 in girls and 11.5 in boys. The weight/height index can alternatively be adjusted for height for age, where p is chosen to make the index uncorrelated with height among children at Centre for Paediatric Epidemiology and Biostatistics, UCL Institute of Child Health, London WC1N 1EH National Center for Health Statistics, Centers for Disease Control and Prevention, Hyattsville MD 20782, USA Department of Child and Adolescent Mental Health, Great Ormond Street Hospital for Children, London Institute of Human Nutrition, University of Southampton, Southampton Correspondence to: T J Cole tim. cole@ich.ucl. ac.uk doi:10.1136/bmj.39238. 399444.55 BMJ | ONLINE FIRST | bmj.com page 1 of 8 on 26 June 2007 bmj.com Downloaded from