Body composition is a reflection of the metabolic state of the organism. However, because the time course of change in body composition is slower than that of metabolic processes, measurement of body composition offers a unique way of assessing the organism's physiologic status. The hormonal and immune mediators that control metabolism, and thus body composition, can be divided into three categories: day-to-day regulators (insulin and glucagon), life cycle-related hormones (estrogens and androgens, growth hormone, prolactin, thyroid hormones, catecholamines, corticosteroids) and immunologic mediators (the cytokines interleukin-1, tumor necrosis factor, and interleukin-6). Although the cytokines can clearly drive metabolism and thus body composition in various illnesses, it is not yet clear whether they also play a homeostatic role in the age-related changes in body composition that we now call sarcopenia.