Determining body composition in adults

INTRODUCTION — Methods for determining body composition continue to improve, greatly increasing the accuracy and ease of making these measurements [1-4]. Certain measurements, such as height, weight (to calculate body mass index [BMI]), and waist circumference are the minimal clinical criteria for evaluating patients suspected to have excess adiposity.

This topic will review body composition and critique most methods available for its measurement. Other considerations when evaluating patients with excess body weight are discussed elsewhere. (See "Obesity in adults: Prevalence, screening, and evaluation".)

WHY MEASURE BODY COMPOSITION? — When considering health and disease, body weight is not nearly as important as the composition of that weight. Body weight reflects the combined weight of all the body's tissues, while body composition measures the relative proportions of fat and lean mass in the body. Lean mass refers to bones, tissues, organs, and muscle.

Measurements of body composition are most commonly done for research purposes. However, body composition measurements may also be useful in a number of clinical settings to:

●Evaluate undernourished patients.

●Identify patients who have an increase in visceral, but not overall, body fat. This former circumstance is associated with a substantially increased risk of heart disease and diabetes.

●Assess body changes associated with growth and development (infant, childhood), aging (sarcopenia), and in certain disease states (eg, HIV, diabetes) [5-11].

●Evaluate patients in whom the body mass index (BMI) is suspected to be discordant with total fat mass (eg, body builders, in whom BMI may be high but adiposity low; older adult or frail patients with sarcopenic obesity, in whom muscle mass is reduced but adiposity is high (ie, normal BMI but excess body fat) [12]; and patients with edema in whom excess fluid increases body weight).

METHODS — Methods for determining body composition continue to improve. Each method has its own degree of operator-patient ease, cost, precision, and reproducibility [13,14]. The most widely utilized techniques in clinical practice and research are described below.

Clinical practice — Routinely, anthropometric measures are sufficient to estimate adiposity in a clinical setting. Nevertheless, other techniques to measure body composition (usually confined to research) may be appropriate for certain patients. For example, patients in whom body mass index (BMI) and adiposity are suspected as discordant, such as in sarcopenia (normal BMI but high body fat), body builders (high BMI but low body fat), and in persons where visceral fat deposition occurs at lower BMI, may be candidates for a more rigorous assessment of body composition.

Anthropometric measurements

●Height and weight are the most commonly measured and can be determined with great accuracy. They are important in making clinical decisions regarding treatment of obesity [15]. Weight can be related to height by several methods, but the most widely used is the BMI, which is weight (in kilograms) divided by the height (in meters) squared (calculator 1 and calculator 2). Height and weight should be measured by a provider in the office. "Self-reported" height and/or weight should never be used, as height is typically overestimated and weight is underestimated, resulting in an underestimate of BMI. (See "Obesity in adults: Prevalence, screening, and evaluation", section on 'Body mass index'.)

Yearly height measure measurements are particularly important in older individuals as loss of height can be an indicator of vertebral fracture related to osteoporosis. (See "Osteoporotic thoracolumbar vertebral compression fractures: Clinical manifestations and treatment", section on 'Height loss'.)

BMI is used routinely in the clinical setting to diagnose obesity. Although it correlates with body fat, it does not directly measure body fat. Percent body fat varies with age and sex. For normal-weight women, healthy percent body fat ranges from 20 to 35 percent and, for men, from 8 to 25 percent [16]. At any given BMI, women have approximately 12 percent more body fat than men, and percent body fat increases with age, even if total weight stays the same. (See "Obesity in adults: Prevalence, screening, and evaluation".)

●Waist circumference is another essential anthropometric measurement. It is measured with a flexible tape placed on a horizontal plane at the level of the iliac crest as seen from the anterior view (figure 1). Increasing central adiposity, as measured by waist circumference, is associated with an increased risk of morbidity and mortality. (See "Obesity in adults: Prevalence, screening, and evaluation", section on 'Waist circumference' and "Metabolic syndrome (insulin resistance syndrome or syndrome X)".)

●Multisite testing of skinfold thickness is considerably less accurate than measurements of height or weight, particularly in people with obesity. Thus, they have little practical clinical value and are primarily used in epidemiologic research studies.

Research methods

Dual-energy x-ray absorptiometry — Dual-energy x-ray absorptiometry (DEXA) is one of the more commonly used methods for determining body composition. This method is based on the attenuation of signals from two energy sources to provide a three-compartment model of body composition and is generally the preferred method in a clinical setting when a more rigorous determination of body composition is needed. It has been validated against other techniques [14], uses a very low dose of radiation, and is relatively simple for the operator and patient to perform.

In a study comparing DEXA-derived percentage body fat estimates with a four-compartment model of body composition measured by underwater weighing and isotope dilution, estimates of mean percent body fat were similar between the two methods [14]. However, there was considerable intraindividual variability, ranging from -3.0 to +4.0 percent, with DEXA.

Impedance measurement — Bioelectric impedance analysis measurement is simple and widely used but has limitations. Impedance is measured by applying electrodes to one arm and one leg or by standing on the foot plates of a special scale. Impedance is proportional to the length of the conductor and inversely related to the cross-sectional area of the conductor. Accuracy in placement of electrodes is essential because variations can cause relatively large errors in the measurement of impedance and corresponding errors in the estimate of body water.

A variety of formulas have been developed to convert impedance, which measures body water, into an estimate of fat [17,18]. Most formulas for estimating fat from bioelectric impedance analysis underestimate body fat [13]. As an example, in a study comparing two bioelectric impedance devices with DEXA for the measurement of body fat, percent body fat measured with both bioelectric impedance devices was 2 to 6 percent lower in persons with normal BMI [17]. Among people considered overweight, the values were lower in women but similar in men. When using bioelectric impedance devices, it is important to use validated formulas to estimate body fat [17]. Recent developments in bioelectric impedance technologies involve systems that incorporate multiple frequencies and multiple body segments.

Imaging techniques — Patterns of body fat distribution can be reliably determined by either computed tomography (CT) or magnetic resonance imaging (MRI) [1,19,20]. The most common technique is to obtain a single cross-sectional image at the interspace between the fourth (L4) and fifth lumbar (L5) vertebrae. The image is used to quantitate subcutaneous and visceral fat. Images obtained between the second and third lumbar vertebrae provide similar information [21].

●CT uses radiograph and computer analysis to determine the structure of internal organs. It is possible to obtain an accuracy of less than 1 percent margin of error for body fat using a series of scans. To minimize radiation dose, however, a single cut at the L4-5 position is used clinically.

●MRI requires the use of a powerful magnet that surrounds the subject. It has no risks but takes considerably longer to perform than CT. Movement will blur the images. Newer techniques using quantitative MR have demonstrated better sensitivity to measure change in fat mass during weight loss or weight gain and may be used longitudinally [22,23].

●Nuclear magnetic resonance (NMR) spectroscopy utilizes similar technology to MRI but is able to discern fat and glycogen within tissues (ie, in the liver) as well as fat and lean mass overall.

The estimates of subcutaneous and visceral fat obtained by these three methods may differ in absolute terms, but the relative ranking of subjects is similar.

Isotopic measurement of body water — Isotopic dilution with a tracer that measures body water (D₂0; ³H₂0, H₂¹⁸O) also provides an accurate assessment of body fat. In a study of children, three methods (DEXA, isotopic measurement of body water, and densitometry) provided a similar estimate of body fat [1,13]. Nevertheless, the need for mass spectroscopy to analyze samples has limited the use of isotopic measurement of body composition to research pursuits.

Water or air displacement-based measures — Partitioning fat and fat-free components of the body can be done with hydrodensitometry (underwater weighing) or whole-body plethysmography. The first technique weighs the individual on dry land and then after complete submersion, preferably with a simultaneous measurement of lung volume.

By contrast, whole-body plethysmography uses air displacement rather than water displacement to measure body fat. It is similar in principle to hydrodensitometry or underwater weighing but does not require submersion. In a study that compared air displacement versus water displacement (underwater weighing), the two methods were highly correlated (r = 0.94) with a standard error of the estimate of 0.0073 [24]. Underwater weighing was long considered to be the gold standard for determining body composition but has largely been replaced by contemporary techniques.

SUMMARY

●Clinical measurement – Careful measurement of waist circumference, height, weight, and calculation of body mass index (BMI) are the measurements needed to evaluate patients' body mass and determine if they are overweight or obese (calculator 1 and calculator 2). (See 'Methods' above and "Obesity in adults: Prevalence, screening, and evaluation".)

●Other methods of measurement - Other techniques to measure body composition (usually confined to research) may be appropriate for certain patients. For example, patients in whom BMI and adiposity are suspected as discordant, such as in sarcopenia with normal BMI, body builders, and people in whom visceral fat deposition occurs at lower BMI, may be candidates for a more rigorous assessment of body composition. Visceral fat is associated with a substantially increased risk of heart disease and diabetes. (See 'Introduction' above and "Overweight and obesity in adults: Health consequences", section on 'Diabetes mellitus'.)

●Dual-energy x-ray absorptiometry (DEXA) is considered the preferred method to determine body composition when BMI and adiposity are considered discordant.

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges George Bray, MD, who contributed to an earlier version of this topic review.