Body composition measurements: Clinical and technical considerations

Standard weight-for-age and height-for-age charts have been used for many years to assess growth and body compositions of children, including curves specific to the Prader-Willi syndrome (PWS). However, a number of direct measures of body compositions have been developed in recent years. Dural-energy x-ray absorptiometry (DXA) provides a measure of bone mineral content and bone mineral density for the whole body and at specific bone regions (spine, hip, forearm). This technology now has been extended to include assessment of the body's lean tissue mass and body fat mass. An alternate technique, called bioelectrical impedance analysis (BIA), has gained wide use in recent years as an indirect measure of the fat-free mass. The older, underwater weighing method is giving way to the air-displacement plethysmographic technique for assessing body fatness. Total body potassium still remains the method of choice for a direct as- sessment of body cell mass, and body nitrogen is recognized as the best assay for body protein mass. Imaging techniques, such as a computer tomography and magnetic resonance imaging, continue to receive increasing attention for body composition assays, especially for the subcutaneous vs. visceral distribution of body fat. Although total body water and extracellular water can be measured by dilution techniques, these methods are slowly being replaced by the BIA assay. When these various techniques are used in combination, the patient's nutritional status can be obtained based on a comprehensive 5-level multicompartment model of the body composition. Each of these methods will be reviewed, and their specific strengths and limitations presented, relative to their application for assessing changes in body composition of the PWS patient.