Insulin is a hormone secreted by the islets of the pancreas. The pancreas is both an endocrine gland and an exocrine gland, and the cells with endocrine function form the islets in the pancreas, among which the beta cells of the pancreas have the ability to secrete insulin. Insulin is the only hypoglycemic hormone in the body, and the role of insulin is: first, to promote glycogen synthesis and inhibit glycogen decomposition so as to lower blood sugar; second, to promote protein synthesis and inhibit protein decomposition, and inhibit the process of gluconeogenesis; third, to promote fat synthesis and inhibit fat decomposition. Therefore, insulin is a pro-synthetic hormone that can promote the synthesis of protein, fat and glycogen, and is the only hypoglycemic hormone in the body, playing an important role in maintaining the dynamic balance of blood glucose.