What does insulin actually do in your body?

Insulin is the hormone that tells your body what to do with sugar. After a meal, glucose from your food floods into your blood. Cells in your pancreas, called beta cells, sense the rise and release insulin. Insulin then promotes the absorption of glucose from the blood into liver, fat and muscle cells, where it is turned into glycogen (a storage form of sugar) or into fat. It also strongly holds back the liver from releasing more glucose of its own.

The amount at stake is tiny. A 70 kg person has only about four grams of dissolved glucose in the blood at any moment, and the body works to keep that level steady. The brain alone uses about 60% of the blood glucose of a resting, fasting person, so the supply cannot be allowed to run out or pile up.

Between meals the system runs the other way. When glucose is low, beta cells hold back insulin and neighbouring alpha cells release a second hormone, glucagon, which acts on the liver to put glucose back into the blood. Together, these two hormones are the primary way the body keeps blood glucose in balance.

When insulin activity is decreased or absent, the result is diabetes: persistently high blood sugar. That is the story of the rest of this path, and it begins with this one hormone and a surprisingly delicate balancing act.