In this review, we attempt to deduce teleologically the physiological mission of leptin. Because overnutrition and diet-induced obesity are the only known causes of hyperleptinemia, we contrast the differences in overnutrition in normally leptinized rodents, in which the added lipids are confined to adipocytes, with those of unleptinized rodents, in which the added lipids are distributed in liver, pancreatic islets, and heart and skeletal muscle, causing organ dysfunction and cell death with a disease cluster resembling metabolic syndrome. We focus here on lipid-induced cardiac dysfunction and the remarkable ability of hyperleptinemia to prevent it. We conclude that the hyperleptinemia of overnutrition prevents the ectopic lipid deposition by: (1) acting on hypothalamic appetite centers to limit the caloric surplus to fit the available adipocyte storage capacity and, (2) upregulating of fatty acid oxidation and downregulating lipogenesis in peripheral tissues to minimize ectopic lipid deposition. The causes of failure of this system and its clinical consequences are discussed.