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Recently there has been renewed interest in body composition from both the lay public and the scientific community. The ability to measure lean body mass and body fat mass in living human beings has permitted a better understanding of human energy metabolism. For example, much of the difference in resting metabolic rate between individuals can be explained by variations in total lean body mass (1). A relative excess of body fat is associated with an increased risk of hypertension, hyperlipidemia, and non insulin-dependent diabetes mellitus conditions that result in premature cardiovascular disease. Finally, loss of more than 30% to 50% of lean body mass secondary to undernutrition may portend a fatal outcome of disease processes (2,3). Thus, the ability to measure body composition precisely and accurately in human beings has the potential to advance our understanding of various disease processes and ultimately to aid in the design of more rational treatments.
Although indispensable for research, measurement of body composition in patients has yet to prove itself as a valuable clinical tool. A patient's energy expenditure can be measured if necessary (4) or estimated using validated equations; lean body mass measurements are not needed. Similarly, when evaluating obese patients, knowing the exact amount of body fat is less helpful than knowing body fat distribution. Upper body fat accumulation is more predictive of the metabolic consequences of obesity than total body fat (5-7). More importantly, the potential adverse health effects of increased adipose tissue mass are as easily or more easily measured (eg, via blood pressure, blood lipids, plasma glucose concentrations) than is body composition. Finally, although loss of lean tissue is a risk factor for adverse disease outcome, the need for nutrition support in hospitalized patients can be predicted by information such as recent weight loss, recent decrease in food intake, physical examination, and routine laboratory studies (8) without resorting to body composition analysis.
For those of us involved in nutrition, however, an understanding of the various measures of body composition is critical if we are to interpret the literature intelligently. Many studies are published in which body composition data are used to determine the effect of fat on health or to assess the effect of nutrition intervention on the amount of lean tissue. The various techniques measure different aspects of body composition, and each method has its unique strengths and weaknesses. Thus, some measures may be appropriate to address some questions but not others. The purpose of this review is to familiarize you with the general principles and assumptions of the major techniques for measurement of body composition so that you will be able to assess whether a particular method is being used appropriately. More extensive reviews, focused primarily on the theory and details of the various techniques, are recommended to the interested reader (9-11).
TERMS AND TECHNIQUES
The terms used to describe body composition are summarized in Table 1. Body compartments are often divided into fat mass and fat-free mass. Fat mass is defined as pure fat, whereas adipose tissue includes the fat and its supporting cellular and extracellular structures. Adipose tissue is composed of approximately 83% fat, 2% protein, and 15% water. Lean body mass is the part of the body free of adipose tissue, and fat-free mass consists of lean body mass plus the nonfat components of adipose tissue. The terms lean body mass and fat-free mass are sometimes used interchangeably. This usage results in few problems in lean individuals in whom adipose tissue contributes very little to the fat-free mass; however, in very obese individuals, the excess adipose tissue can increase fatfree mass without increasing lean body mass. Lean body mass consists of body cell mass, extracellular water, and extracellular solids. Body cell mass lacks the extracellular components of lean body mass and therefore reflects the cellular components of the body, which are involved in energy transfer and chemical work. Extracellular solids consist of total body bone mineral (ie, the skeleton, which accounts for approximately 85% of extracellular solids), fascia, cartilage, and so forth.
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The various body composition measurement techniques may preferentially measure certain compartments or subcompartments of the body. Thus, in addition to the technical limitations of various measurement methods, there may also be conceptual problems depending on which compartment the investigator is primarily interested in measuring. These limitations tend to be specific to the technique, and, if appreciated, it is possible to obtain reliable body composition information. In addition, some body composition techniques are "direct" measures of body compartments, whereas others are indirect and require extensive cross-validation with other methods. Although the direct methods may require numerous assumptions, some of which may not always be valid, they are more intellectually satisfying than indirect techniques, which not only have their own problems, but inherit those from which they have been cross-validated. Table 2 summarizes the compartments measured by each body composition measurement method and indicates whether the technique is direct or indirect.
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This article summarizes the available body composition measurement methods. The techniques range from low cost and rapid to expensive and time consuming. The principles and theory of the following techniques will be described: anthropometry; body water spaces; densitometry; body potassium counting; total body electroconductivity; bioelectrical impedance analysis; dual photon absorptiometry/dual energy X-ray absorptiometry; computed tomography, magnetic resonance imaging, …