Insulin

Insulin is a small protein hormone made by the beta cells of the pancreatic islets, the clusters of endocrine cells scattered through the pancreas. It is first made as a single chain, proinsulin, which is cut into two shorter chains, A and B, held together by disulfide bonds. Insulin is the body's main signal that fuel is plentiful, and the beta cells release it when blood glucose rises after a meal.

Insulin lowers blood glucose in several ways. In skeletal muscle and fat tissue it makes cells move glucose transporter proteins to their surface, so glucose can flow in from the blood. It tells the liver and muscles to store glucose as glycogen, promotes the building of fat and protein, and blocks both the breakdown of glycogen and the making of new glucose. Its effects oppose those of glucagon, and together the two hormones hold blood glucose in a narrow range through negative feedback.

Problems with insulin cause diabetes mellitus. In type 1 diabetes the immune system destroys the beta cells, so the body makes little or no insulin and needs it supplied from outside. In type 2 diabetes, by far the more common form, cells respond poorly to insulin and the beta cells eventually struggle to keep up.