The primary role of adipocytes is to store triacylglycerol during periods of nutritional caloric excess and to mobilize this reserve when caloric expenditure exceeds intake. Mature adipocytes are uniquely equipped to perform these functions. They possess the full complement of enzymes and regulatory proteins needed to carry out both de novo lipogenesis and lipolysis. The regulation of these processes in adipocytes is exquisitely responsive to hormones that regulate energy metabolism. For example, insulin promotes lipogenesis and inhibits lipolysis, whereas counterregulatory hormones [eg glucagon, epinephrine, and