Central regulation of energy homeostasis - The key role of insulin

Insulin has two important functions that relate to overall metabolic homeostasis. The phylogenetically oldest is the maintenance of sufficient energy stores to allow for development, growth, and reproduction. The newer is as a feedback regulator of plasma glucose. The key role of the central nervous system in both functions is reviewed from a personal perspective, and the development of the concept that both body weight (adiposity) and plasma glucose are critically regulated by the same hormone is described. The recent suggestion that diabetes and obesity are linked by their common reliance on this central nervous system insulin signaling system is reviewed. Recent efforts to understand the hypothalamic mechanisms involved are described, and the common use of insulin receptor substrate 2 and the phosphatidylinositol 3-kinase signaling mechanism is emphasized. Potential consequences of defects in the secretion of insulin or the action of insulin in the central nervous system are given, and linkage between obesity and diabetes is illustrated with a potential clinical representative.