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There is increasing interest in body composition in paediatric research, as distinct from growth and nutritional status, as almost all diseases have adverse effects on either fatness or the fat-free mass. However, the approaches used to assess growth and nutritional status are not appropriate for separate evaluations of body fatness and lean mass. Traditional measurements such as body mass index and skinfold thickness do not measure fat in accurate quantitative terms. Various techniques have been used in recent years which divide body weight into fat mass and fat-free mass; however, the data tend not to be appropriately expressed. Body fatness is generally expressed as a percentage of weight, while fat-free mass typically remains unadjusted for size. A more appropriate approach is to normalise both body fatness and fat-free mass for height. This recommendation is relevant both to studies comparing patients with controls and to the expression of new reference data on body composition which are needed to allow informative comparisons. The same approach is appropriate for the classification of childhood obesity.
There is increasing interest in the relation between paediatric diseases and body composition,  which refers to the components of body weight. Various different models of body composition can be used, with the simplest differentiating the fat and fat-free components of weight. More sophisticated models can distinguish more specific components, such as water, mineral, or protein.  Historically, the effects of disease on children tended to be assessed in terms of growth (size) or nutritional status (a global index of health intended to identify overnutrition and undernutrition). Such indices are too crude, however, to distinguish changes in the composition of weight, and the last half century has seen rapidly growing interest in children's body composition. The current obesity epidemic has drawn attention to the development of body fatness, while growth of the fat-free mass is of increasing interest to clinicians, physiologists, and epidemiologists.
There is now a need to develop an approach whereby body composition can be assessed in cross sectional terms and can be followed over time in individuals, in the same way as is currently practised for growth. This approach requires that data be expressed as age and sex specific standard deviation scores relative to a reference.
Almost all diseases influence body composition in one way or another, and measurements are required both to characterise these effects and to assess the efficacy of treatment programmes. Until recently, such measurements were extremely difficult to make with accuracy in infants and children. Many sophisticated techniques (for example, magnetic resonance imaging, in vivo neutron activation analysis, whole body potassium counting) may not be applicable to younger subjects, while relatively more acceptable methods (hydrodensitometry; dual energy x ray absorptiometry; isotope dilution) rely on information concerning the relative chemical immaturity of the body. Only recently have these difficulties been overcome, through the development of multicomponent models which combine several measurement methods simultaneously and distinguish the components of the fat-free mass. [2-4] These models vary in complexity, but appropriate techniques are now available, at least for children; infants and toddlers present greater practical difficulties which are still being addressed. During the next decade we can expect, through use of such models, to see a marked improvement in our understanding of the effects of disease on paediatric body composition, especially fat mass and fat-free mass.
The renewed interest in this area, combined with awareness of the growing obesity epidemic, has resulted in calls for improved reference data for body composition. [4-6] Current United Kingdom growth reference data are presented as weight, height, and body mass index (BMI). [7 8] These variables evaluate size or nutritional status but not body composition, and are of limited value for independent comparisons of body fatness and lean mass. New reference data specifically describing body composition are therefore required.
There tends to be a tacit …