body fat

INTRODUCTION — Methods for determining body composition have improved over the past 20 years, greatly increasing the accuracy and ease of making these measurements [1-4]. Body composition measurements may be useful in undernourished patients, or for identifying patients who do not have an increase in overall body fat, but who have an increase in visceral fat. This latter circumstance is associated with a substantially increased risk of heart disease and diabetes. Measurement of body composition is also instructive for assessing body changes associated with growth and development, aging (sarcopenia), and in certain disease states (eg, HIV, diabetes) [5-11].

Certain measurements, such as height, weight (to calculate body mass index), and waist circumference are the minimal clinical criteria for evaluating the overweight patient. This topic will review body composition and critique most methods available for its measurement. Other considerations when evaluating the overweight patient are discussed elsewhere. (See "Screening for and clinical evaluation of obesity in adults".)

MODELS OF BODY COMPOSITION — Body composition can be viewed from five perspectives (figure 1) [1,12]:

Atomic
Molecular
Cellular
Tissues
Whole body

Atomic composition — A reference human weighing 70 kg contains 61 percent oxygen, 23 percent carbon, 10 percent hydrogen, 2.6 percent nitrogen, 1.4 percent calcium, and less than 1 percent all other atoms (phosphorous, sulphur, potassium, sodium, chlorine, magnesium, and various trace elements). Thus, six elements - oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorous - account for more than 98 percent of body mass, while less than 2 percent is contributed by the other 44 elements that are present [1].

Molecular composition — The molecular model is the most common form in which body composition is expressed. More than 100,000 chemical compounds can be identified in the human body, ranging from simple to very complex; water, lipid, and protein are the major components (figure 1) [1].

Water comprises 60 percent or more of the reference male (and 50 percent of the reference female), of which approximately 26 percent is extracellular and 34 percent intracellular.
Body fat ranges from under 10 percent in well-trained athletes to slightly over 50 percent in obese patients. Two to three percent of lipids in fat are essential structural lipids; the remainder are fat stores.
Protein comprises 15 percent of normal body composition.
Mineral comprises 5 percent of normal body composition.

Thus, water, lipid, protein, and minerals account for 99.4 percent of the molecular constituents of the body.

Cellular composition — The body is composed of cell mass, extracellular fluid, and extracellular solids. The 10(18) cells in the body of an adult can be divided into four major categories: connective tissue cells, epithelial cells, neural cells, and muscle cells. Fat cells, osteoclasts, osteoblasts, and the cellular elements of blood are components of connective tissue. Muscle cells include skeletal muscle, smooth muscle, and cardiac muscle. Epithelial cells include the cells lining a hollow viscus. The body content of potassium, measured as the naturally occurring isotope (40K) or as exchangeable potassium (42K), is the most widely used indicator of body cell mass because potassium is the principal intracellular cation [1].

Extracellular fluid is approximately 94 percent water. It is distributed into two main compartments, the plasma in the intravascular space and the interstitial fluid in the space outside of the vascular compartment. Plasma and interstitial fluid account for approximately 5 and 20 percent, respectively, of body weight in the reference human.

Tissue and organ components — Body weight represents the sum of the muscle tissue, connective tissue, epithelial tissue, and nervous tissue. Bone, adipose tissue, and muscle make up 75 percent of body weight (figure 1). The majority of body fat (80 percent in men, 90 percent in women) is subcutaneous. However, fat also accumulates around the abdominal organs (visceral fat). Visceral adipose tissue is more difficult to measure [13]. Visceral adiposity is associated with a greater risk of metabolic and cardiovascular disorders including insulin resistance, type 2 diabetes mellitus, hypertension, and coronary heart disease. (See "Health hazards associated with obesity in adults".)

Whole body — The whole body represents the final perspective on body composition. There are at least 10 different components of the whole body that may be measured, including stature, segment lengths, circumferences, skinfold thicknesses, body surface area, body volume, body weight, body mass index, and body density.

METHODS OF MEASURING BODY COMPOSITION — There are several methods for determining body composition. Each provides a different view of the body's components. The various methods include two compartment models (fat and non-fat, aqueous and non-aqueous), three compartment models (fat, lean, bone mineral), or four compartment models (body density, weight, water, and bone mineral mass) [14,15]. They differ in ease of determination, cost, accuracy, use of radiation, and utility for assessing regional body fat (table 1). The most useful techniques are described below.

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 [16]. Weight can be related to height by several methods, but the most widely used is the body mass index (BMI), which is weight (in kilograms) divided by the height (in meters) squared (table 2A-B). (See "Screening for and clinical evaluation of obesity in adults".)
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 2). Increasing central adiposity, as measured by waist circumference, is associated with an increased risk of morbidity and mortality. (See "Screening for and clinical evaluation of obesity in adults", section on 'Waist circumference' and "The metabolic syndrome (insulin resistance syndrome or syndrome X)".)
Measurements of skinfold thickness are considerably less accurate than measurements of height or weight, particularly in obese subjects. Thus, they have little practical clinical value and are primarily used in epidemiological studies.

Dual-energy X-ray absorptiometry — Dual-energy X-ray absorptiometry (DXA) 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.

In a study comparing DXA with a four compartment model of body composition, estimates of mean percent body fat were similar between the two methods [15]. However, there was considerable intraindividual variability, ranging from -3.0 to +4.0 percent, with DXA. Thus, DXA is good for cross-sectional measurements, but is less reliable for individual measures.

Isotopic measurement of body water — Isotopic dilution with a tracer that measures body water (D20; 3H20) also provides an accurate assessment of body fat. In a study of children, three methods (DXA, isotopic measurement of body water, and densitometry) provided a similar estimate of body fat relative to a more complicated four-compartment model of body composition [1,14].

Body density and whole body plethysmography — Partitioning fat and fat-free components of the body can be done with hydrodensitometry (underwater weighing) or whole body plethysmography (Bod Pod). The first technique weighs the individual on dry land and then after complete submersion, preferably with a simultaneous measurement of lung volume. In 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 (Bod Pod) versus water displacement (underwater weighing), the two methods were highly correlated (r = 0.94) with a standard error of the estimate of 0.0073 [17]. Air displacement appears to be an important new instrument for measurement of body fat.

Impedance measurement — Impedance measurement is widely used but has significant 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 even small 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 [18,19]. Most formulas for estimating fat from bioelectric impedance analysis underestimate body fat [14]. As an example, in a study comparing two bioelectric impedance devices with DXA for the measurement of body fat, percent body fat measured with both bioelectric impedance devices was 2 to 6 percent lower in men and women with normal BMI [18]. Among the overweight individuals, 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 [18].

Imaging techniques — Patterns of body fat distribution can be reliably determined by either computed tomography (CT) or magnetic resonance imaging (MRI) [1,20,21]. The most common technique is to obtain a single cross sectional image at the interspace between the fourth and fifth lumbar vertebrae. The image is used to quantitate subcutaneous and visceral fat. Images obtained between the second and third lumbar vertebrae provide similar information [22].

CT uses X-radiation 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.

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

SUMMARY — Careful measurement of height, weight, and waist circumference are the minimal measurements needed to begin evaluation of overweight patients. (See "Screening for and clinical evaluation of obesity in adults".) If there is concern about whether fat is increased, particularly visceral fat, DXA may be beneficial. Although impedance measurements are used in many clinical settings, they do not contribute more than the methods outlined a