Insulin, hormone, produced in the islets of Langerhans of the pancreas, that regulates the metabolism of carbohydrates, fats, and starches in the body. Like other proteins, insulin is partially digested if administered orally and hence must be injected into a muscle when used clinically. In the treatment of diabetes mellitus, which is caused by a deficiency of insulin production or by inhibition of its action on cells, insulin is often combined with protamine, which prolongs the period of absorption of the hormone. Insulin crystallized from the pancreas contains zinc, which also lengthens absorption. A preparation called protamine zinc insulin extends the hormone's action still further.
Insulin is used in the treatment of patients with diabetes of all types. The need for insulin depends upon the balance between insulin secretion and insulin resistance. All patients with type 1 diabetes need insulin treatment permanently, unless they receive an islet or whole organ pancreas transplant; many patients with type 2 diabetes will require insulin as their beta cell function declines over time.
Insulin was first extracted from the pancreatic tissue of dogs in 1921 by the Canadian physiologists Sir Frederick Grant Banting and Charles Herbert Best and the British physiologist John James Rickard Macleod. The Canadian biochemist James Bertram Collip then produced it in sufficiently pure form to be injected into humans. The molecular structure of insulin was determined in 1955 by the British biochemist Frederick Sanger; it was the first protein to be deciphered. Human insulin, the first human protein to be synthesized, was made in 1965. In 1981 insulin made in bacteria by genetic engineering became the first human hormone obtained in this way to be used to treat human disease. For the biochemistry of insulin, see Sugar Metabolism.
Insulin should be given to all patients with type 1 diabetes, which is due to autoimmune islet-cell injury that eventually leads to virtually complete insulin deficiency. The peak incidence of type 1 diabetes is around the time of puberty, but about 25 percent of cases present after 35 years of age . Insulin is also indicated for patients with secondary diabetes due to pancreatic insufficiency, as well as in many patients with type 2 diabetes to manage hyperglycemia.
A more difficult question is when to use insulin in patients with type 2 diabetes. This disorder is characterized by both insulin resistance and relative insulin deficiency. Therapy should begin with diet, weight reduction, and exercise, which can frequently induce normoglycemia if compliance is optimal. Patients with persistent hyperglycemia are often started on one or more oral hypoglycemic drugs. Metformin is a good early choice, based on its safety profile, neutral effect with regard to weight gain, and ability to lower glycemia. Insulin is added if goal glycemic control is not attained.
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