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BODY COMPOSITION ANALYSIS TECHNIQUES IN ADULT AND PEDIATRIC PATIENTS: HOW RELIABLE ARE THEY? HOW USEFUL ARE THEY CLINICALLY?

Renal Unit, Leeds General Infirmary, Leeds, U.K.

Correspondence to: G. Woodrow, Renal Unit, Leeds General Infirmary, Great George Street, Leeds LS1 3EX U.K. graham.woodrow{at}leedsth.nhs.uk

	ABSTRACT

TOP ABSTRACT THE PROBLEM OF UNDERLYING... REFERENCE BODY COMPOSITION... BODY COMPOSITION TECHNIQUES IN... INACCURACY IN CLINICAL METHODS USEFULNESS IN PEDIATRIC CARE CONCLUSIONS REFERENCES

 

Complex abnormalities of body composition occur in peritoneal dialysis (PD). These abnormalities reflect changes in hydration, nutrition, and body fat, and they are of major clinical significance. Clinical assessment of these body compartments is insensitive and inaccurate. Frequently, simultaneous changes of hydration, wasting, and body fat content can occur, confounding clinical assessment of each component. Body composition can be described by models of varying complexity that use one or more measurement techniques. "Gold standard" methods provide accurate and precise data, but are not practical for routine clinical use. Dual energy X-ray absorptiometry allows for measurement of regional as well as whole-body composition, which can provide further information of clinical relevance. Simpler techniques such as anthropometry and bioelectrical impedance analysis are suited to routine use in clinic or at the bedside, but may be less accurate. Body composition methodology sometimes makes assumptions regarding relationships between components, particularly in regard to hydration, which may be invalid in pathologic states. Uncritical application of these methods to the PD patient may result in erroneous interpretation of results. Understanding the foundations and limitations of body composition techniques allows for optimal application in clinical practice.

KEY WORDS: Body composition; hydration; nutrition; dual energy X-ray absorptiometry; bioelectrical impedance analysis.

Malnutrition and fluid overload are important complications in patients on peritoneal dialysis (PD). They manifest as complex abnormalities of body composition and have major impacts on patient outcome. Protein–energy malnutrition is common in PD and is associated with increased mortality (1). Diagnosis is essential if reversible elements are to be improved through optimal management such as enhancing dialysis clearances and providing nutritional supplementation, but clinical assessment of nutrition is imprecise and insensitive. Assessment of nutrient intake does not give the whole picture—especially when a catabolic wasting state is the primary problem.

Management of fluid balance is a primary objective of PD, with important effects on patient outcome (2). Occult fluid overload commonly leads to hypertension and cardiac dysfunction in PD (3). Fluid retention may conceal the presence of malnutrition by preserving stable body weight and body contours. Other important abnormalities of body composition include fat gain associated with carbohydrate absorption from dialysate, fat loss from malnutrition, and bone mineral depletion.

Body composition techniques measure components of the body on the basis of their differing physical characteristics (4), and body composition can be described by models of varying complexity that use one or more measurement techniques. Body composition analysis has been used to study physiologic processes such growth, development, aging, and exercise physiology and is increasingly being applied to the study and clinical management of pathologic conditions.

In renal disease, body composition analysis is of particular interest in providing information about nutrition and hydration, and enabling simultaneous and often confounding body composition changes to be distinguished. The available techniques produce objective values for different aspects of body composition; they reflect nutrition and hydration, with the aim of diagnosing abnormal states or detecting longitudinal changes; and they allow assessment of the effects of therapeutic interventions and are important in a number of areas of PD research.

	THE PROBLEM OF UNDERLYING ASSUMPTIONS

TOP ABSTRACT THE PROBLEM OF UNDERLYING... REFERENCE BODY COMPOSITION... BODY COMPOSITION TECHNIQUES IN... INACCURACY IN CLINICAL METHODS USEFULNESS IN PEDIATRIC CARE CONCLUSIONS REFERENCES

 
A number of the assumptions that underlie body composition methodology may be invalid in pathologic states. Uncritical application of certain methods to the PD patient may lead to misleading interpretations of results. It is crucial to understand exactly what is being measured and how estimates of body composition are derived. As in clinical assessment, variable fluid status is an important factor that affects the validity of the assessments produced by some body composition analysis techniques (5).

A number of body composition models of varying complexity have been described (6). The basic model is the two-compartment model, comprising fat and fat-free mass (FFM). This model is involved in a number of methods of body composition analysis, but it has important limitations.

Fat-free tissue is heterogeneous, with components including total body water (TBW), protein, and bone mineral. Variations in the proportions of these components, which the model assumes to be constant (both within and between individuals), confounds interpretation and validity. In renal disease, that variability—especially of hydration (with TBW being assumed by the model to be a constant 73% of FFM)—may occur to a greater degree.

More sophisticated models divide the body into three, four, or more components. Body cell mass (BCM), a physiologically and chemically more homogeneous compartment than FFM, reflects components involved in energy transfer, meaning chemical work (7), and is a more important marker of nutrition and wasting than FFM is. The main constituent of FFM, TBW, can be divided into extracellular water (ECW), which reflects hydration, and intracellular water (ICW), which closely reflects BCM and thus nutrition rather than hydration.

There is a great need for simple, portable body composition techniques that can be used for routine assessment and monitoring of nutrition and hydration in the clinical setting. Currently, at one end of the spectrum of body composition techniques are the "gold standards," including densitometry, dilution techniques, neutron activation analysis, and measurement of total body potassium. These methods provide accurate and precise data, and they are the standards against which other methods are assessed and calibrated. However, they are unsuited to routine clinical practice, being costly, time-consuming, and hard on frail patients. In addition, they sometimes involve radiation and may require complex measurement facilities that are not widely available. Simpler, clinically applicable techniques such as anthropometry and bioelectrical impedance analysis (BIA) are more suited to routine use in clinic or at the bedside, but their limitations may include lesser accuracy and a greater effect of disease on validity.

	REFERENCE BODY COMPOSITION TECHNIQUES

TOP ABSTRACT THE PROBLEM OF UNDERLYING... REFERENCE BODY COMPOSITION... BODY COMPOSITION TECHNIQUES IN... INACCURACY IN CLINICAL METHODS USEFULNESS IN PEDIATRIC CARE CONCLUSIONS REFERENCES

 
Densitometry: Densitometry was central to the development of the discipline of body composition analysis (8). Body density is determined by measuring weight after whole-body immersion. This method is clearly impractical for most dialysis patients. Also, it uses the two-compartment model, which assumes that FFM has a constant density, an assumption that is unlikely to be true with the variable hydration seen in PD. A more patient-friendly equivalent is air displacement plethysmography, but that method is still based on the two-compartment model (9).

Neutron Activation Analysis: Neutron activation analysis involves irradiating the study subject with a neutron source. The resulting interactions with atomic nuclei in the body produce element-specific gamma-ray emissions that can be measured, determining the body content of that element. All major elements in the body can be measured. Total body nitrogen is of particular interest in renal disease, providing a good measure of body protein (body nitrogen = 0.16 x body protein). This information can be of immense value in PD and has been associated with patient outcomes (10), but unfortunately, few measurement facilities are available worldwide.

Total Body Potassium: Total body potassium is measured by detecting emissions of the 1.46 MeV gamma rays produced by ⁴⁰K, a naturally occurring isotope accounting for 0.012% of the potassium in the body (no exogenous irradiation is involved). Because 98% of body potassium is intracellular, the assumption of a constant intracellular potassium concentration provides a measurement of BCM. This technique has been validated in PD patients. It has high accuracy and precision, but it requires specialized measurement facilities (11,12).

Imaging Techniques: Magnetic resonance imaging and computed tomography scanning are also used to measure body composition. They can determine anatomically defined compartments—for example, limb muscle volume and intra-abdominal versus subcutaneous fat.

Dilution Techniques: Dilution techniques involve administration of a tracer substance that is distributed within the volume of interest (13). These techniques include measurement of TBW after administration of water containing isotopes of hydrogen [²H₂O (stable), ³H₂O (radioactive)] and oxygen (H ¹⁸₂O), and ECW by bromide dilution, with ICW being determined by the difference between TBW and ECW. The FFM can then be estimated by assuming fixed hydration of 73% (unlikely to be constant in PD), and BCM can be determined from ICW by assumption of fixed water content (ICW/0.72).

	BODY COMPOSITION TECHNIQUES IN THE CLINICAL SETTING

TOP ABSTRACT THE PROBLEM OF UNDERLYING... REFERENCE BODY COMPOSITION... BODY COMPOSITION TECHNIQUES IN... INACCURACY IN CLINICAL METHODS USEFULNESS IN PEDIATRIC CARE CONCLUSIONS REFERENCES

 
Dual-energy X-Ray Absorptiometry: Dual-energy X-ray absorptiometry (DEXA) was developed for diagnosis of osteoporosis, but additionally, it provides useful information about body composition (14). It is a clinically applicable, if not "bedside," method that provides detailed and precise information. It involves measurement of the relative attenuation of two different energy X-rays by the body and produces a three-component model of body composition comprising fat, bone mineral, and lean tissue. Scans are relatively quick and involve minimal radiation exposure. The technique also allows for regional analysis, for example, limb lean tissue (which approximates to limb muscle), and body fat distribution.

With DEXA, body weight can be estimated to a high degree of accuracy from radiologic data alone (which encourages belief in its accuracy), and measurement precision is high. However, the results produced by different scanners show some variation (15). Lean tissue by DEXA contains body water as its dominant component, and changes in hydration will be reflected as change in lean tissue, confounding nutritional interpretation.

In patients with severe renal failure (pre-dialysis, PD, and hemodialysis patients), we found that lean tissue wasting was greater in limb lean tissue (reflecting muscle) than in trunk lean tissue (including vital organs), with the limb/trunk lean tissue ratio being more sensitive to wasting than total lean tissue is (16). The implications are that measurement of regional DEXA with estimation of limb lean tissue expressed as a ratio with trunk lean tissue may be a more sensitive, less hydration-dependent measure of lean tissue depletion. In hemodialysis, reduced limb/trunk lean tissue ratio is associated with reduced survival (17).

Anthropometry: Anthropometry is long established in nutritional assessment. The technique includes measurement of skinfold thicknesses with calipers, which provides an index of body fat content, and measurement of limb circumference, which reflects limb muscle and thus the state of protein nutrition.

Skinfold measurements and limb circumference (usually mid-arm circumference) can both be compared with standard population data to assess "normality," usually in terms of centiles of the population. However, in an individual, a significant decline in nutrition can occur before anthropometric measurements fall clearly outside the population normal range. Also, care is needed with the selection of "normal" ranges for comparison, because these vary across time and between countries and ethnic groups, and depend on the type of subjects from which they are derived. The sum of skinfold measurements (usually at four sites) can be used to estimate body density and thus body fat (18).

Anthropometry is simple and easily applicable. Some operators have shown good precision with the method, but significant inter-observer variability can occur, leading to reduced sensitivity for detecting change. Body compartment values may show significant discrepancies when compared with values obtained by reference methods (19). Skinfold measurements are difficult to obtain in very obese subjects, and edema can exaggerate measurements.

Bioelectric Impedance Analysis: Bioelectric impedance analysis has been available for a long time, and its use has greatly increased in recent years with a variety commercially produced systems coming onto the market. Several different technologies and methods of interpreting data and deriving results for body composition measurements are available (20), and validity and interpretation of results produced by a BIA system therefore vary with the technology and analysis technique used. Users of BIA must understand what exactly is being measured by the particular system, what meaning can be drawn from the results produced, and what the limitations of the system and the analysis are. In renal disease, BIA has great potential to probe the complex combinations of body composition abnormalities, but the abnormalities may themselves reduce the validity of some of the derived measurements.

In BIA, impedance to the passage of a small AC electrical current through the body is measured. Electrical current is conducted by body water, and impedance is inversely related to water volume. The BIA technique assumes that the body behaves as a uniform electrical conductor, which obviously isn't true: the limbs provide a disproportionate contribution to impedance because of a smaller cross-sectional area than that of the trunk, to which BIA is relatively insensitive (21). The original design of BIA systems (still widely used) employs a single-frequency 50 kHz AC current.

The BIA technique estimates TBW from equations derived by regression from comparison with reference methods such as dilution. Many different equations are in use. One criticism is that the equations include anthropometric data such as height and weight that may account for a significant part of the estimate. Derivation of other body compartments includes FFM, which is based on the assumption that FFM contains 73% water (FFM = TBW / 0.73), and is thus prone to misinterpretation, with variable hydration being interpreted as nutritional change. Fat is estimated simply from body weight minus FFM (fat being an electrical resistor and so not actually reflected by the electrical data).

In BIA, conduction of current through the body is complex. The current passes freely through the ECW space, but penetration of the ICW space is reduced by a capacitance effect of cell membranes. The impedance comprises a resistance component (R) and a smaller reactance component (Xc) attributable to the cell-membrane capacitance effect. The relationship between R and Xc is called the phase angle and reflects the ratio of ECW to ICW. Phase angle is reduced in PD, with lower values being associated with malnutrition and reduced survival (22). One approach uses R, Xc, and phase angle to plot the results of R and Xc on a graph, where areas of normality can be defined for healthy controls, with changes in vector length and phase angle helping to distinguish between changes in ECW and ICW or BCM (23).

At very low frequencies, the AC current passes through ECW almost exclusively; ICW is increasingly penetrated at higher frequencies. Separating ECW and ICW (and thus hydration and nutrition) is fundamental to a useful and meaningful assessment of body composition in renal failure patients. This frequency-dependent differential penetration of current through body water compartments is exploited by techniques that perform BIA measurements at varying frequencies to determine ECW and ICW volumes.

One widely used multiple-frequency BIA technique is bioimpedance spectroscopy (BIS) (24). The BIS technique involves measurement of R and Xc at a large number of frequencies. Mathematical modeling (Cole–Cole plot) produces estimates of theoretical resistance at zero frequency (purely reflecting ECW) and at infinite frequency, reflecting free passage through the TBW space, including ECW and ICW. The results provide estimates of ECW, ICW, and TBW, and can independently determine changes in hydration and BCM. Calculation of FFM still depends on assumption of 73% hydration (and is a less useful nutritional measure than BCM is), and fat is again indirectly determined by subtracting estimated FFM from body weight.

A careful and methodical approach to performing BIA is important, because the technique is affected by other factors. Position and posture of the subject are particularly important, and measurements should be performed after a standard period of recumbence. With care, the precision of the technique is high, making it particularly suited to longitudinal monitoring, but using BIA to diagnose abnormal states on a single reading is more problematic. Significant errors can potentially arise in BIA measurements as compared with reference standards. Thus, although the technique is potentially sensitive to change, it exhibits only moderate accuracy in individual measurements of actual values.

	INACCURACY IN CLINICAL METHODS

TOP ABSTRACT THE PROBLEM OF UNDERLYING... REFERENCE BODY COMPOSITION... BODY COMPOSITION TECHNIQUES IN... INACCURACY IN CLINICAL METHODS USEFULNESS IN PEDIATRIC CARE CONCLUSIONS REFERENCES

 
The degree of inaccuracy for BIA and anthropometry as compared with "gold standard" measures for body composition may be greater in renal failure patients (including those on PD) than in healthy subjects (19). And even if BIA (or another technique) is accurate in determination of absolute values, problems arise with determinations of "normality."

Healthy individuals can vary very significantly in body composition. Thus, a significant change in nutrition or hydration in an individual may not be sufficient to move the reading out of the necessarily wide normal range for a healthy population. In addition, body composition changes with normal physiologic processes such as aging and growth and development, and variations occur between racial groups.

The BIA ratios of ECW/ICW and ECW/TBW are often expressed as measures of hydration. But the denominators in these ratios reflect BCM, and thus nutrition status, and so the ratio also inversely reflects nutrition status or BCM loss. As a measure of hydration, ECW may be better normalized to another index of individual patient size, such as body height (25).

	USEFULNESS IN PEDIATRIC CARE

TOP ABSTRACT THE PROBLEM OF UNDERLYING... REFERENCE BODY COMPOSITION... BODY COMPOSITION TECHNIQUES IN... INACCURACY IN CLINICAL METHODS USEFULNESS IN PEDIATRIC CARE CONCLUSIONS REFERENCES

 
Body composition techniques provide valuable information in children on PD (26). Much of the theory and methodology discussed so far is also applicable to children, although measurement in pediatric practice raises certain specific issues. A marked change in size and body composition occurs between birth and adulthood, with proportions of the different components varying in healthy individuals during this time. Unlike the situation in adults, body composition change is expected in children as part of normal growth, making longitudinal assessment particularly important but more complicated to interpret. Change reflects growth and development as well as the effects of disease or therapy.

Growth retardation is common in children on PD. Malnutrition is an important factor slowing growth, but many other metabolic factors are also important, and so growth cannot be used as a marker of nutrition status. Because of lesser stature, comparison of measures of body composition with age-matched normal ranges can lead to an underestimation of nutrition status. It may be more appropriate to compare with "height–age" equivalents [the age at which a child's height equates to the 50th height centile (26)]. Also, comparisons should be made with children at the same pubertal stage. Some specific equations and software have been developed for BIA and DEXA in children.

	CONCLUSIONS

TOP ABSTRACT THE PROBLEM OF UNDERLYING... REFERENCE BODY COMPOSITION... BODY COMPOSITION TECHNIQUES IN... INACCURACY IN CLINICAL METHODS USEFULNESS IN PEDIATRIC CARE CONCLUSIONS REFERENCES

 
Body composition analysis provides vital clinical information in the PD patient. Several techniques, including DEXA, anthropometry, and BIA are readily applicable to clinical use. A central issue is the effect of hydration on nutritional and body composition assessment and the importance of distinguishing changes in fluid and nutrition status. The complexity of body composition in renal disease and the effects of abnormalities on analysis techniques mean that a clear understanding of the methodology and of the limitations of these measurements is essential to ensure maximal use of the information obtained and avoidance of misinterpretation.

	REFERENCES

TOP ABSTRACT THE PROBLEM OF UNDERLYING... REFERENCE BODY COMPOSITION... BODY COMPOSITION TECHNIQUES IN... INACCURACY IN CLINICAL METHODS USEFULNESS IN PEDIATRIC CARE CONCLUSIONS REFERENCES

 

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This Article Abstract Full Text (PDF) Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Woodrow, G. Search for Related Content PubMed Articles by Woodrow, G.