Named After Islands Inside You
The name is a small piece of anatomy trivia hiding in plain sight. Insulin comes from the Latin *insula*, meaning "island" — because the hormone is made in scattered clusters of cells that look like islands within the pancreas, the pancreatic islets.
More precisely, it is produced by the beta cells of those islets, and in mammals it is produced there exclusively. It was also the first peptide hormone ever discovered, which makes it something of a founding member of modern endocrinology.
What it actually does is act as the body's main anabolic hormone — the build-things-up signal. It tells the liver, fat tissue and skeletal muscle to pull glucose out of the blood and store it, either as glycogen or converted into fats. It also nudges tissues throughout the body to build proteins. In short, insulin is the message that says: supplies have arrived, start stockpiling.
A Thermostat Made of Cells
Blood sugar is not managed by a single switch. It is managed by two hormones arguing in opposite directions, which is a far more stable arrangement.
Beta cells are directly sensitive to blood sugar. High glucose makes them secrete insulin; low glucose shuts secretion down. Insulin then drives glucose into cells and blood sugar falls.
Their neighbours, the alpha cells, take their cue from the beta cells and do exactly the reverse with glucagon — releasing more when blood glucose is low, less when it is high. Glucagon raises blood sugar by prompting the liver to break down its glycogen stores and manufacture fresh glucose. This push-and-pull between insulin and glucagon is the primary mechanism of glucose homeostasis. Take insulin away and the effect flips: the body swings into widespread breakdown, burning through reserve body fat.
Two Ways the Signal Fails
When insulin activity drops or disappears, the result is diabetes — persistently high blood sugar, or hyperglycaemia. It arrives by two quite different routes.
In type 1 diabetes, the beta cells are destroyed by an autoimmune reaction. The factory is gone, so insulin can no longer be made or released at all.
Type 2 diabetes is messier and less well understood. Beta cell destruction is far less pronounced and is not autoimmune. Instead, amyloid accumulates in the pancreatic islets, likely disrupting their structure and function. Three things appear to combine: fewer surviving beta cells, weaker secretion from the ones that remain, and insulin resistance in the body's tissues. Type 2 also involves increased glucagon secretion that stubbornly ignores blood glucose levels. Insulin is still released in response to sugar — it simply cannot keep up, and glucose piles up in the blood.
The First Protein We Ever Read
Insulin has spent a century being the molecule scientists test their newest tools on, and it keeps saying yes.
Frederick Banting and Charles Best, working in John Macleod's laboratory at the University of Toronto, first isolated it from dog pancreas in 1921. Three decades later Frederick Sanger sequenced its amino acid structure in 1951, making insulin the first protein ever fully sequenced. Dorothy Hodgkin determined its crystal structure in the solid state in 1969.
It was also the first protein to be chemically synthesised, and the first produced by recombinant DNA technology. That last step mattered enormously in practice. Before it, type 1 diabetics were treated largely with porcine insulin — pig insulin, which is especially close to the human version but not identical, and slightly different in effect. The human protein itself is small: 51 amino acids, a molecular mass of 5808 Da, built as an A-chain and a B-chain held together by disulfide bonds.
