Body Composition Assessment

There are a number of ways to assess body composition. These procedures differ in the time, expense, and expertise required and in accuracy of the results they provide. All of these procedures are subject to some error and can only provide an estimate of the actual percent body fat. Errors in body composition estimates can result from either the actual measurement procedures, from the equations selected to calculate percent body fat, or from both. Moreover, because most methods are based on the average population, they are less accurate with very lean or obese individuals. Because of individual deviations from the norms, accuracy of percent fat values also vary from person to person. For these reasons, most procedures currently used for body composition assessment have a standard error of 3 to 4% at best. This means that only approximately two-thirds of all measurements are within three percentage points of the actual percent fat. Thus, for someone who was measured at 18% body fat, the true percent fat probably ranges between 15% to 21%. Consequently, one should not look at a body composition as a precise measurement, but as an estimate within a close range.

Underwater (hydrostatic) weighing, which is based on Archimedes' Principle, is generally regarded as the "gold standard" for body composition assessment, although this claim is being made less of late because of newer and more sophisticated procedures. Because body fat is less dense than water, it increases one's buoyancy while the fat-free mass, which has a density greater than water, makes one sink. After correcting for residual volume–which increases buoyancy and decreases the underwater weight–percent fat can be calculated based on the underwater weight. The largest source of error in underwater weighing is thought to be the determination of residual volume (RV). When RV is estimated rather than measured, the precision of underwater weighing is no better than anthropometric determination.

The development of skinfold (anthropometric) measurements came as the result of investigations for simpler and less expensive method for determining body composition. Body circumferences and/or skinfold thickness are used in a regression equation, of which there are many available, for prediction of body composition. Among the most commonly employed are the generalized equations for use with skinfold measurements developed for adult males and females by Jackson and Pollock (1978) and Jackson, Pollock, and Ward (1980), respectively. They are termed generalized because they are most accurate in predicting body composition of people with relatively average amounts of body fat. Generalized equations tend to be less accurate with the very lean (e.g. athletes), obese, or other special populations. Other, more appropriate, equations have been developed for such populations which increase their accuracy for prediction of body composition.

The major source of error in anthropometry lies in the actual skinfold measurement. Making accurate skinfold measurements is more than simply pinching the skin somewhere around a particular area and measuring the thickness. There are precise sites on which the measurements are to be taken. A well-trained technician can obtain results that approach the precision of underwater weighing. Unfortunately, most people who take skinfold measurements are not well trained. Obtaining consistently accurate skinfold measurements requires training and experience.

Electrical impedance is a newer method used for body composition assessment. A major advantage of the bioelectrical impedance analysis (BIA) is its simplicity. The resistance is measured of an extremely low current, which is unable to be felt, passed through a subject's body. The underlying principle to this procedure is that a current passes more easily through lean tissue than through adipose tissue as lean tissue contains more water and electrolytes. Another advantage of this procedure is that no special training or skill is required. While the accuracy of BIA is reported to be similar to skinfold measurements, research is continuing to develop better equations for use with BIA in order to improve its accuracy. This procedure holds promise for making accurate measurements both easily and quickly.

Infrared interactance is another new method to estimate body composition. A probe is placed on the biceps and an energy beam emitted to a depth of 1 cm. Energy from the beam is scattered, absorbed, and reflected by the underlying tissue. A detector in the probe measures the energy reflected back. The analyzer to predict body composition interprets the reflected energy. A major assumption of this procedure is that measurement from a single adipose site can be used to predict body composition on different types of individuals which is also a potential source of error.