The ability to assure continuous availability of energy despite highly variable energy supplies in the environment is a major determinant of the survival of all species. For higher organisms including mammals, a solution to this problem involved development of the capacity to efficiently store excess energy as triglycerides in adipose cells, from which stored energy could be rapidly released for use at other sites. In addition, physiological mechanisms arose to enhance survival by stabilizing adipose energy stores during prolonged periods of deficient or excess nutritional availability. The outlines of such physiological systems are simple: with prolonged nutritional deprivation, falling energy stores are sensed, leading to increased food-seeking behavior and decreased energy expenditure; with prolonged nutritional abundance, food intake is reduced and energy expenditure is increased to avoid excessive energy storage, ie obesity. The systems involved in such adaptations are referred to as systems for regulated energy balance.

A physiological system for regulation of energy balance and avoidance of obesity would be expected to have several requirements. The first is a mechanism by which information on the level of energy stores in adipose tissue can be sensed, and the resulting information relayed to regulatory sites elsewhere in the body. Although a number of candidate mechanisms have been described for a feedback signaling system reflecting the amount of adipose energy stores (