Assessment of Body Composition, Body Fat Distribution, Physical Activity, and Food Intake
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Measurement Issues Related to Studies of Childhood Obesity: Assessment of Body Composition, Body Fat Distribution, Physical Activity, and Food Intake Michael I. Goran, PhD ABSTRACT. This article reviews the current status of imal studies only). The lack of direct methods has led various methodologies used in obesity and nutrition re- to development of various models and indirect meth- search in children, with particular emphasis on identify- ods for estimation of fat and fat-free mass, all of ing priorities for research needs. The focus of the article which are imperfect and require a number of as- is 1) to review methodologic aspects involved with mea- sumptions, many of which require age-specific con- surement of body composition, body-fat distribution, en- siderations, because the usual assumptions in multi- ergy expenditure and substrate use, physical activity, and food intake in children; and 2) to present an inventory of compartmental models (eg, hydration of fat-free research priorities. Pediatrics 1998;101:505–518; obesity, mass, density of fat-free mass) are known to be in- 2,3 body composition, energy expenditure, physical activity, fluenced by age and state of maturation. In addi- diet, methodology. tion, practical aspects limit the availability of tech- niques for use in younger prepubescent children to specialized research-based techniques such as total ABBREVIATIONS. DXA, dual energy x-ray absorptiometry; SEE, 4 standard error of the estimate; MRI, magnetic resonance imaging; body water, dual energy x-ray absorptiometry 5 6 CT, computed tomography; RQ, respiratory quotient; METs, met- (DXA), total body electrical conductivity, total body abolic equivalents. potassium,7,8 and other more convenient and widely available techniques such as bioelectrical resistance,4 BODY COMPOSITION skinfolds,2 and other clinical anthropometric evalua- ccurate assessment of body composition is tions (eg, weight for height, ideal body weight, body important in many areas of obesity and nu- mass index). Some of the more frequently used body Atrition-related research. In addition to pro- composition techniques in children are discussed viding fundamental whole-body descriptive charac- briefly below. teristics, accurate measures of body composition Densitometry often are required as scaling factors to normalize Densitometry is based on estimating body compo- physiologic variables (eg, metabolic rate, physical sition from measurement of total body density. The activity, physical fitness, etc). As described by Wang most widely used approach is to measure body vol- 1 the composition of the human body can be et al, ume by underwater weight and determine density thought of in terms of an atomic model (ie, oxygen, by dividing body mass by body volume. The tech- carbon, hydrogen, etc), a molecular model (ie, water, nique is a two-compartment model and is based on lipid, protein, minerals, and glycogen), a cellular the different tissue densities of the fat and fat-free model (ie, cell mass, extracellular fluid, etc), or a compartments of the body. If total body density and tissue model (ie, skeletal muscle, adipose tissue, the specific densities of fat and fat-free mass are bone, etc). In terms of obesity research, the most known, an equation can be generated for converting useful model is probably the molecular model, in total body density to percentage of body fat based on which the body’s composition is broken down into the Archimedes principle.9 Generally, at least in its main molecular components: lipids (includes es- adults, the densities of fat and fat-free mass are as- sential and nonessential lipids), water, protein, min- sumed to be 0.9 g/mL and 1.1 g/mL; however, the erals, and glycogen. Fat and fat-free mass are terms density of fat-free mass is known to be influenced by used frequently that refer to the classic two-compo- factors such as age, gender, and ethnicity,3 and little nent body composition model in which body mass is information exists in children. broken down into fat and nonfat tissue masses. Current limitations for applying densitometry to Measurement of the masses of the individual com- the pediatric population include practical problems partments of body mass is extremely challenging, and theoretic considerations. The technique requires because no direct method exists other than in vivo climbing into a large tank of water, emptying the neutron activation analysis (very limited availability) lungs by maximal exhalation, and sitting still under- and chemical analysis of the cadaver (useful for an- water for several seconds. Thus, from a practical standpoint, testing adherence is extremely difficult From the Division of Physiology and Metabolism, Department of Nutrition for young children and impossible for infants. Recent Sciences, University of Alabama at Birmingham, Birmingham, Alabama. Received for publication Oct 24, 1997; accepted Nov 6, 1997. developments using air rather than water displace- 10 PEDIATRICS (ISSN 0031 4005). Copyright © by the American Academy of ment for measurement of volume may be more Pediatrics. practical for pediatric populations. Such a device, Downloaded from www.aappublications.org/news by guest on September 29, 2021 SUPPLEMENT 505 called the BodPod,10 has been developed, and it pro- showed that carcass lean and fat content were highly duces an alternative method for measuring body correlated with DXA measurements (Pearson r val- volume that is simpler, quicker, and more practical ues .0.98). The regression between carcass and DXA than hydrostatic weighing. data deviate by a small but significant amount, sug- From a theoretic standpoint, the limitations of suc- gesting that specific correction factors may need to cessfully applying densitometry requires additional be used to improve the measurement accuracy of knowledge of the specific densities of fat and fat-free total body composition by DXA. This approach ef- mass in children of different states of maturity, gen- fectively calibrates the DXA technique to the labora- der, and ethnicity. One previous study suggests that tory standard of carcass analysis in a pig model. The between birth and 22 years of age, the density of the Hologic QDR-1000/W DXA instrument with pediat- fat-free mass increases from 1.063 to 1.102 g/mL in ric whole-body software failed to measure fat mass boys and from 1.064 to 1.096 g/mL in girls. The accurately (as derived by chemical analysis) in small density of fat mass probably does not change, be- (;1.57 kg) and large (;6 kg) piglets.19 In larger pigs cause it is more or less fixed by the biophysical (5 to 35 kg), the Hologic QDR-2000 using adult scan properties of fat in vivo at a density of 0.9 g/mL.11 analysis also failed to measure fat mass accurately.18 Thus, as described by Lohman,3 application of den- Collectively, the validation studies of DXA suggest sitometry and development of accurate age-, gen- that the relationship between actual chemical content der-, and ethnic-specific equations requires knowl- of the carcass and DXA estimates may be affected by edge of the density of fat-free mass for each factors such as the size of the animal, the equipment subgroup being investigated. used, and the operation mode. These validation stud- ies and generation of new calibration equations are DXA important steps in development of standardized Recent advances in techniques to measure body techniques to measure body composition in chil- composition have provided DXA for assessment of dren.20 whole-body as well as regional measurements of bone mass, lean mass, and fat mass.12 DXA is based on the exponential attenuation resulting from ab- Skinfolds and Anthropometry sorption by body tissues of photons emitted at two Estimation of fat mass from anthropometry in- energy levels to resolve body weight into bone min- volves development of prediction models in which eral and lean and fat soft tissue masses; the theoretic anthropometric measures (eg, sum of skinfolds) are considerations are reviewed elsewhere.5 The advan- related to body-fat mass. Thus, the use of an accurate tages of DXA are the relatively quick scan time (#20 criterion method is important for development of minutes with newer machines equipped with a fan these equations. Several prediction equations have beam) and minimal radiation dose (,1 mSv or been developed for children based on use of a mul- ,1/100th of the equivalent radiation exposure of a ticompartmental model2 or DXA21 as criterion chest x-ray). In addition, because the technique is method. based on a whole-body scan, additional information Slaughter and colleagues2 developed body compo- is collected about aspects of regional body composi- sition prediction equations from data on 310 subjects tion. Thus, the technique is of potential use to the (8 to 29 years of age), including 66 prepubescent pediatric population. children (50 boys and 16 girls). A multicompartmen- Since the introduction of DXA, numerous studies tal model of body composition was used as a crite- have compared this technique with other research- rion method by combining measures of total body based methods.13–16 These studies have proved valu- density (from underwater weight), total body water able in cross-validating and comparing the individ- (from deuterium dilution), and bone mineral density ual techniques. A general limitation of many widely (from photon absorptiometry) on the right and left used body composition techniques is the lack of true radius and ulna. This study led to development of validation studies that have performed comparisons gender-, race-, and maturation-specific equations for with known chemical composition. One advantage estimating body fat based on measurement of either of DXA is that there are several studies that have triceps plus calf (two gender-specific equations) or demonstrated validity by comparison with chemical triceps plus subscapular skinfolds (nine equations analysis of the carcass by using a pig model.17–20 recommended depending on gender, race, matura- Chemical analysis of the carcass is an ideal labo- tion state, and sum of skinfold thickness).