Body weight is the most important index of nutritional status that is readily available to the physician; however, the ease and simplicity of the weighing procedure have tended to obscure the complexity of the physiologic processes that underlie weight change. Weight loss reflects a deficit of one or more body substances sufficient to produce a net decrease in body mass. For practical purposes, the constituents that can contribute appreciably to weight loss over the short term are water, fat, protein and glycogen. In much longer-term situations, deficits of minerals (from both bone and soft tissues) also make a small contribution . . .