insulin
Insulin is the body's “store it away” signal. Released after a meal, it tells cells throughout the body to pull glucose out of the blood and either burn it or stash it, lowering blood sugar in the process. It is the only hormone capable of lowering blood glucose, which makes it irreplaceable and explains why its absence is lethal without treatment.
Chemically, insulin is a small peptide hormone of two amino-acid chains held together by disulfide bonds. It binds the insulin receptor, a receptor tyrosine kinase, on muscle, fat, and liver cells. This triggers an intracellular signaling cascade that, among other things, moves GLUT4 transporters to the cell surface so glucose can enter, and switches the liver from making glucose to storing it as glycogen.
Insulin is far more than a glucose hormone: it is broadly anabolic, promoting fat storage, protein synthesis, and growth. When cells stop responding well to it — insulin resistance — the pancreas compensates by making more, until beta cells can no longer keep up and type 2 diabetes emerges. Discovered in 1921, purified insulin transformed a fatal disease into a manageable one.
After a person eats a bowl of rice, blood glucose climbs, beta cells release insulin, muscle and fat cells take up the sugar, and within a couple of hours blood glucose returns near baseline.
Insulin orchestrates the post-meal disposal of glucose.
Insulin is secreted in two phases: a rapid first-phase burst from pre-formed granules within minutes of a glucose rise, then a slower sustained second phase. Loss of the first-phase response is one of the earliest detectable defects in type 2 diabetes.