Insulin is made by specialized cells in your pancreas called beta cells. These cells sit in tiny clusters of hormone-producing tissue known as the islets of Langerhans, scattered throughout the pancreas. A healthy adult pancreas contains roughly one million of these islets, and beta cells make up 50% to 80% of the cells within each one. When it comes to the insulin used as medication, the story has shifted dramatically over the past century, from animal organs to genetically engineered bacteria.
How Your Body Makes Insulin
The pancreas is a small organ tucked behind your stomach, and most of it is devoted to producing digestive enzymes. But peppered throughout that tissue are the islets of Langerhans, tiny clusters of endocrine cells that release hormones directly into the bloodstream. Each islet contains between 1,000 and 3,000 cells. About 60% of them are beta cells, which are the only cells in the body that produce insulin. The remaining cells produce other hormones: alpha cells release glucagon (which raises blood sugar), while smaller populations of cells produce hormones involved in appetite and digestion.
Insulin production inside a beta cell begins as a larger precursor molecule called preproinsulin. This early form gets threaded into a compartment of the cell where its signal tag is snipped off, leaving proinsulin. Proinsulin then folds into its proper shape and moves through the cell’s packaging system, where a connecting segment (called C-peptide) is cut away. What remains are the two chains of mature insulin, locked together by chemical bonds, stored in tiny granules and ready to be released when needed.
What Triggers Insulin Release
Beta cells are constantly receiving signals from your body, but rising blood sugar is the primary trigger. When glucose levels climb after a meal, sugar enters the beta cell through a transporter on its surface. The cell breaks that glucose down to generate energy molecules, which cause specific channels in the cell membrane to close. This shifts the electrical charge across the membrane, opening calcium channels. Calcium rushes in, and that influx is the direct signal that causes insulin-packed granules to fuse with the cell surface and dump their contents into the bloodstream.
Glucose isn’t the only thing that nudges beta cells into action. Gut hormones released during digestion (called incretins) amplify the response, which is why eating a meal triggers more insulin than an equivalent amount of sugar delivered straight into a vein. Signals from the nervous system, amino acids from protein, and fatty acids all contribute as well. Even during fasting, beta cells maintain a low, steady output of insulin to keep background metabolism running smoothly.
How Insulin Reaches Your Organs
Once released, insulin doesn’t spread evenly throughout the body right away. It flows first into the portal vein, the large blood vessel that carries blood from the digestive organs directly to the liver. The liver is the first organ to see that fresh wave of insulin, and it clears 40% to 80% of it on the first pass. This means the liver is always exposed to a much higher concentration of insulin than the rest of the body, roughly 2.2 times higher in humans. That design makes sense: the liver is the central hub for storing and releasing glucose, so it needs the strongest insulin signal.
Whatever insulin survives that first pass enters the general circulation and travels to muscle, fat tissue, the brain, and other organs, where it signals cells to take up glucose and store energy.
The First Insulin From Animals
Before the 1920s, a diagnosis of type 1 diabetes was a death sentence. The breakthrough came in Toronto, when Frederick Banting and Charles Best prepared extracts from animal pancreases and injected them into diabetic dogs. In November 1921, an extract lowered a dog’s blood sugar dramatically. By January 1922, a more potent extract made from adult cow pancreases was used to treat Leonard Thompson at Toronto General Hospital, the first successful clinical use of insulin.
Banting’s insight had roots in his upbringing on a farm. Knowing that cattle were often bred before being fattened for slaughter, he realized fetal calf pancreases would be available at slaughterhouses and might be a rich source of the hormone. Fetal calf tissue was never scaled up for clinical production, but the early experiments proved the concept. For the next six decades, virtually all medical insulin came from pig or cow pancreases collected at meatpacking plants. Pork insulin was especially popular because its structure differs from human insulin by only a single amino acid, making allergic reactions less common than with beef insulin.
How Insulin Is Made Today
The modern era began on October 28, 1982, when the FDA approved Humulin, the first biosynthetic human insulin and the first medical product of any kind made through recombinant DNA technology. Rather than extracting insulin from animal organs, scientists designed the process to use genetically modified bacteria.
The approach was elegant. Researchers worked out the exact DNA sequences that code for insulin’s two protein chains (called A and B), then chemically built those gene sequences from scratch, choosing DNA letters that bacteria read efficiently. Each synthetic gene was inserted into a different strain of E. coli bacteria, spliced into the middle of an existing bacterial gene so the bacteria would produce it as part of a larger fused protein. The bacteria were then grown in large fermentation tanks, churning out the fusion protein in bulk. After harvesting, the insulin chain was chemically cut free from the bacterial protein, purified, and the separate A and B chains were mixed together under conditions that allowed them to link up through the same chemical bonds found in natural insulin.
Some manufacturers use yeast instead of bacteria, which can fold and process the insulin molecule in ways more similar to human cells. Both methods produce insulin that is structurally identical to what your pancreas makes.
Insulin Analogues: Engineered Variations
Standard biosynthetic insulin matches the human molecule perfectly, but it doesn’t always behave ideally when injected under the skin. Natural insulin molecules tend to clump together, which slows absorption. Starting in the 1990s, manufacturers began tweaking the amino acid sequence to change how the molecules interact with each other and how quickly they enter the bloodstream.
Rapid-acting analogues are designed to clump less, so they absorb faster. Insulin lispro, for instance, simply swaps the positions of two amino acids near the end of one chain. Insulin aspart replaces one amino acid with a different one at a similar spot. These small changes let the molecules separate more quickly after injection, reaching peak blood levels in about half the time and at roughly twice the concentration compared to regular human insulin. That makes them better suited for covering meals.
Long-acting analogues take the opposite approach, using structural changes that slow absorption and create a steady, low-level supply of insulin over 12 to 24 hours. This mimics the background insulin your pancreas releases between meals and overnight. Most people with type 1 diabetes use a combination of both types: a long-acting injection once or twice daily for baseline coverage, plus a rapid-acting injection before each meal.
Animal Insulin vs. Modern Insulin
Animal-derived insulin kept people alive for decades, but it had drawbacks. Pork and beef insulin could trigger immune reactions, and the purification process was imperfect, sometimes leaving trace contaminants that caused injection-site irritation. Supply also depended on the meatpacking industry, which made it vulnerable to shortages.
Biosynthetic production solved these problems. Bacterial and yeast fermentation can be scaled up reliably, the product is chemically identical to human insulin (or intentionally modified in the case of analogues), and purity is far higher. Animal insulin has largely disappeared from the market in most countries, though a small number of patients who switched from animal to human insulin reported changes in their ability to sense low blood sugar, and a handful of manufacturers still produce pork insulin for those who prefer it.