Because energy balance is important for survival, a system is required to monitor energy status and to make appropriate adjustments in energy intake and energy expenditure. In higher animals, a centrally located system has evolved to accomplish this task. When caloric intake exceeds expenditure, the surplus is channeled into energy storage pathways, primarily the synthesis of fatty acids, which are converted into fat and stored in adipose tissue. Thus, metabolic flux through the pathway of fatty acid synthesis, located in the lipogenic tissues, reflects the "energy status" of the animal. The enzymatic machinery of this pathway is also expressed in the brain, notably the hypothalamus. In the hypothalamus, intermediates in this pathway appear to serve as energy sensors that signal higher brain centers to produce appropriate responses, e.g., altered food intake and energy expenditure.
