Insulin is the hormone that controls the entrance of blood sugar, the most fundamental source of nutrition and energy, into most cells of the body. Without insulin, blood sugar (glucose) remains in the blood, where its concentration keeps increasing. This concentration damages blood-vessel walls and leads to other kinds of serious and permanent damage, for example to nerves. Meanwhile, the body loses the source of energy that cells need to function.

Insulin is normally made by specialized cells in the pancreas called beta cells. In type 1 diabetes, the body's immune defenses turn against, and destroy, beta cells. In type 2 diabetes, insulin is present but for a number of reasons can't do its job effectively.