A POLYPEPTIDE hormone (see HORMONES) produced in the PANCREAS by the beta cells of the ISLETS OF LANGERHANS. It plays a key role in the body's regulation of CARBOHYDRATE, FAT, and PROTEIN, and its deficiency leads to DIABETES MELLITUS.

Insulin is made biosynthetically by recombinant DNA technology using Escherichia coli, or semisynthetically by enzymatic modification of porcine insulin to produce human insulin. It acts by enabling muscles and other tissues requiring sugar for their activity to take up this substance from the blood. It is generally available in a standard strength of 100 units/millilitre (U100). Numerous different insulin preparations are licensed for use; these differ in their speed of onset and duration of action.

Insulin is inactivated by gastrointestinal enzymes and is therefore generally given by subcutaneous injection, usually into the upper arms, thighs, buttocks, or abdomen. Some insulins are also available in cartridge form, which may be administered by injection devices (‘pens’). The absorption may vary from different sites and with strenuous activity. Insulin may also be given by continuous subcutaneous infusion with an infusion pump. for patients who are well motivated and able to monitor their own blood glucose, with access to expert advice. About 25 per cent of diabetics require insulin treatment: most children from the onset, and all patients presenting with KETOSIS. Insulin is also often needed by those with a rapid onset of symptoms such as weight loss, weakness, and sometimes vomiting, often associated with ketones in the urine (ketonuria).

The aim of treatment is to maintain good control of blood glucose concentration, while avoiding severe HYPOGLYCAEMIA; this is usually achieved by regular injections of short-acting insulin before meals (often with an extra injection at bedtime of long-acting insulin).

Hypoglycaemia is a potential hazard for many patients converting from porcine to human insulin, because human insulin may result in them being unaware of classical hypoglycaemic warning symptoms. Drivers must be particularly careful, and individuals may be forbidden to drive if they have frequent or severe hypoglycaemic attacks.