Prevention of hypoglycemia is essential for the preservation of brain metabolism and survival of the whole body. Normally, glucose is the only substrate used by the brain to meet its metabolic requirements. Therefore, a continuous supply of circulatory glucose is a necessary prerequisite for normal cerebral metabolism. When plasma glucose concentration decreases (e.g., during prolonged fasting or after administration of glucose-lowering drugs) several physiologic responses are activated to prevent further decreases in blood glucose. The first response is known as counterregulation, a system that prevents and corrects hypoglycemia through the release of counterregulatory hormones.