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Body composition for paediatricians
Measuring body composition provides clinically useful information for many paediatric conditions such as cystic fibrosis, cerebral palsy, eating disorders and inflammatory bowel disease. Body composition measurements provide an insight into disease severity, nutritional status and long-term health risks, enabling paediatricians and allied health professionals to plan and deliver more holistic care. Waist circumference measurements and assessment of fat mass (FM) and percentage fat using dual-energy X-ray absorptiometry (DEXA) are two examples of methods in clinical use.
Component models of body composition
A variety of techniques exist to measure body composition, ranging in complexity and accuracy, and which have utility across both research and clinical settings (table 1). However, all rely on the same key principles of measuring different chemical constituents of the human body. Measurement of body composition at its simplest quantifies these key constituents into two components: FM and fat-free mass (FFM).
Body composition measurement techniques
Methods to measure body composition are therefore often described as component models (figure 1); the simplest being a two-component model which measures FFM from which FM can be derived. More detailed multicomponent methods of body composition measure individual constituents of FFM, such as bone, mineral or water content to form a more comprehensive model.
Components measured in body composition.
There are, however, many smaller constituents with different densities which cannot be accurately quantified, and the relative density of measurable components like fat differs between health states, populations and—crucially for paediatrics—by age. All component models therefore make assumptions based on the physical and chemical properties of the constituents of FM and FFM.1 The techniques used to measure the components that underpin composition models can be further classified as direct (eg, imaging or DEXA) or indirect (bioelectrical impedance analysis, skin fold thickness or waist circumference). An overview of the most used body composition measuring techniques is presented in table 1.
Methods of body composition assessment
Paediatricians wishing to measure body composition in clinical practice have several different options available (table 1). Choice of measurement technique will depend on equipment and expertise available together with what is most suitable for the population being measured.
Both MRI and CT can be used to directly quantify the volume of adipose tissue. A key advantage of CT and MRI is that they can be used to measure regional adipose tissue volumes and lean tissue volume. This is clinically and academically important because different adipose tissue depots have different properties: internal abdominal adipose tissue is associated with metabolic risk, whereas gluteal adipose tissue is associated with insulin sensitivity.2 Therefore, quantification of regional adipose tissue compartments may more accurately predict long-term metabolic risk.3 However, few hospitals will have access to imaging for the sole purpose of measuring body composition.
Air displacement plethysmography (ADP) measures body volume based on the principles of densitometry, from which body composition can be estimated. Owing to the specialist nature of the equipment required, its use is limited to research settings. The gold standard for determining body volume is hydrodensitometry or underwater weighing; this entails the subject being fully submersed in water following maximal expiration and is not suitable for paediatric patients. Hence, ADP has been developed, using BOD POD for adults or PEA POD (COSMED, Italy) for children, which measures the volume of air displaced. Using reference values for FM and FFM density, we found that the relative proportion of these can be estimated from the volume and weight of a child or baby. However, reference values vary widely, depending on ethnicity and gender. Body composition changes significantly through early infancy and childhood, so it is important to use reference values from a population like that being investigated.4
Dilution can be used to measure total body water (TBW), which can then indirectly estimate FFM. This technique usually involves administration of water labelled with deuterium (a non-radioactive, stable isotope tracer) followed by measurement of excreted deuterium in urine or saliva5 and therefore is most commonly performed in research laboratories. This methodology assumes that FM is anhydrous and that the water content of FFM is constant, and a key limitation in practice is that this varies between individuals and by age, in addition to the logistical challenges of collecting samples in young children. However, reference data for infants and children aged 6 weeks–5 years are now available.6 The addition of TBW with other two component methods such as DEXA and ADP provides more accurate data on FFM by separating FFM into water and dry FFM.
Clinical application of body composition measurements
Simple proxy measures of body composition, like waste circumference, have widespread clinical use in the assessment of childhood obesity. Body mass index (BMI) is relatively poor at identifying children at risk of obesity-related metabolic complications compared with simple measures of body composition such as waist circumference and waist:height ratio. Like adults, children with increased abdominal fat, independent of BMI, have a clustering of cardiovascular risk factors.7 Waist circumference is a marker of ectopic fat deposition,8 which is a strong predictor of insulin resistance.9 It is also a criterion for diagnosing the metabolic syndrome in adults10 and can be used to predict risk in adolescents11 as it correlates with abdominal fat volume on imaging.12 Waist:height ratio can be used to identify cardiometabolic risk factors in paediatric and adolescent populations.13 Both waist circumference and waist:height ratios are easy to perform, non-invasive and do not require specialist equipment.
In children14 and adolescents15 with cystic fibrosis, lean mass predicts pulmonary function. Given that undernutrition (and resultant loss of lean mass) is common in cystic fibrosis, the assessment of body composition is now routinely recommended as part of monitoring nutritional status16 using DEXA. Children and young people with eating disorders, such as anorexia nervosa, also have deficits in lean and FM. While the long-term deficits of lean mass in childhood are not known,17 it is hypothesised and biologically plausible that it predisposes to an increased risk of osteoporosis in later life.18 As such, yearly DEXA scans are recommended for children with persistently low weight (over 1 year).
Lower lean muscle mass with corresponding accumulation of FM, particularly around the trunk and abdomen, has been observed in children undergoing treatment for paediatric malignancies19 with differences in body composition persisting into young adulthood.20 21 A similar pattern is also observed in young adult survivors of preterm birth.22 This pattern of body composition is strongly associated with cardiovascular and metabolic risk factors in adults.23 24 Although longitudinal body composition data through childhood and beyond is lacking, early evidence suggests that abdominal fat tracks from childhood25 to adulthood.26 This suggests that body composition may be a biomarker suitable to predict, as well as potentially inform, interventions that target later metabolic risk. Given that over 85% of children and young people with malignancy survive,27 with similar survival following preterm birth,28 the potential impact on population health is significant. Assessment of body composition is therefore likely to become a key aspect of long-term follow-up for these groups.
Body composition in research settings
Whole body MRI has been used to analyse body composition in preterm and term infants in natural sleep and children as young as 5, without sedation.29 It is, however, expensive and time-consuming (with each scan taking up to an hour, including time for child to get used to the scanner). Due to the risks of ionising radiation, CT is used only to evaluate body composition through single abdominal slices in children; these have been demonstrated to correlate with abdominal and whole body adipose tissue in adults where this approach is more commonly used. DEXA and whole-body MRI have been used as an end point in clinical trials to assess the impact of different nutritional interventions on the body composition of preterm infants.30 31
In children with cerebral palsy, more severe impairments (Gross Motor Function Classification levels III–V) have been demonstrated to be associated with increased body fat and lower lean body mass. While the optimal body composition parameters for infants and children with specific disease states are unknown, targeting lean mass through dietary interventions has been demonstrated to improve short-term outcomes in other settings such as children dependent on long-term ventilation,32 and in adults, there is a strong inverse relationship between predicted lean body mass and mortality from respiratory disease.33
A negative correlation between disease severity and lean mass34 has been consistently observed in children with inflammatory bowel disease, suggesting utility in regular measurement of body composition. Body composition could therefore prove to be a useful end point in clinical trials testing nutritional interventions for children with chronic conditions and complex neurodisability. This would enable a more targeted approach to nutrition to improve overall health outcomes.17
Conclusion
There is no single measure of body composition which can be considered the overall ‘gold standard’, and the optimal choice of method will be determined by the body composition component or compartment of interest. All techniques rely on assumptions at some stage in the measurement or analysis. Consequently, paediatricians face specific challenges in measuring body composition owing to lack of age-specific, sex-specific and ethnicity-specific reference data, and due to the changes that occur throughout infancy and childhood as a result of chemical maturation.35 Measurement of body composition does, however, provide information for clinical and research purposes that exceed simple anthropometrics, particularly for predicting future disease risk.36 37 Users need to be aware of specific assumptions associated with different models and the sparsity of reference data for paediatric populations.
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References
Footnotes
Twitter @dremilyprior, @DrCGale
Contributors EP and CG planned the manuscript. EP wrote the manuscript, supervised by SNU and CG. All authors contributed to the final version of the manuscript.
Funding EP received support from a clinical research training fellowship from the Medical Research Council and Chelsea and Westminster Medical Research Charity. CG received support from the UK Medical Research Council through Clinician Scientist Fellowship and Transition Support awards and from Chiesi Pharmaceuticals to attend an educational conference; in the past 5 years, he has been investigator on received research grants from Medical Research Council, National Institute of Health Research, Canadian Institute of Health Research, Department of Health in England, Mason Medical Research Foundation and Westminster Medical School Research Trust and Chiesi Pharmaceuticals. SNU declares no funding nor conflicts of interest in relation to the work submitted.
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.