Natural insulin is produced by beta cells in the pancreas from a larger precursor called proinsulin. Pharmaceutical insulin is manufactured using recombinant DNA technology, in which the human insulin gene is inserted into bacteria or yeast that then produce the hormone in large fermentation tanks. Insulin analogs are made by the same process after small changes to the gene sequence.
Where Insulin Comes From in the Body
Insulin is a peptide hormone produced exclusively by beta cells in clusters called the islets of Langerhans, scattered throughout the pancreas. According to the NIDDK, the average adult pancreas contains about a million islets, each containing several hundred to a few thousand beta cells.
Insulin’s main job is to let glucose enter cells from the bloodstream, where it can be used for energy or stored as glycogen and fat. Without insulin — or with the insulin resistance seen in type 2 diabetes and prediabetes — glucose accumulates in the blood, leading to the complications of diabetes.
The Biological Pathway: Proinsulin to Insulin
The making of a single insulin molecule in a beta cell involves several steps:
- Transcription: The insulin gene (INS) on chromosome 11 is transcribed into messenger RNA.
- Translation: Ribosomes read the mRNA and produce preproinsulin, a single long chain of about 110 amino acids.
- Signal cleavage: A signal peptide is removed as the protein enters the endoplasmic reticulum, leaving proinsulin.
- Folding: Proinsulin folds into a U-shape and forms three disulfide bonds that stabilize the structure.
- Cleavage in secretory granules: Enzymes PC1/3, PC2, and carboxypeptidase E cut out the connecting C-peptide, leaving the mature two-chain insulin molecule (A chain and B chain).
- Storage: Mature insulin is packaged with zinc into crystalline hexamers inside secretory granules.
- Release: When blood glucose rises, ATP-sensitive potassium channels close, calcium flows in, and the granules fuse with the cell membrane to release insulin — and C-peptide — into the bloodstream.
C-peptide is released in equal amounts to insulin and is often measured in blood tests to estimate how much insulin a person’s own pancreas is making.
How Pharmaceutical Insulin Is Made
The Historical Path: Animal Insulin
From 1922 through the early 1980s, insulin was extracted from the pancreases of cattle and pigs collected from slaughterhouses. Pig (porcine) insulin differs from human insulin by a single amino acid; cow (bovine) insulin differs by three. Animal insulin was life-saving but less pure and sometimes caused allergic reactions.
The Modern Path: Recombinant DNA
The first recombinant human insulin, Humulin, was approved by the FDA in 1982 — the first drug ever approved using genetic engineering. The basic process has not changed dramatically since:
- Gene insertion: The human insulin gene (or a modified version for analogs) is inserted into a plasmid, a small circular piece of DNA.
- Host transformation: The plasmid is introduced into a host organism — typically Escherichia coli bacteria (used by Eli Lilly) or Saccharomyces cerevisiae yeast (used by Novo Nordisk and Sanofi).
- Fermentation: The engineered microbes are grown in sterile fermentation tanks under controlled conditions of temperature, pH, and nutrients. They produce proinsulin (or insulin chains) as they multiply.
- Harvest and lysis: After fermentation, cells are collected and broken open to release the proinsulin they have produced.
- Purification: A series of chromatography and filtration steps removes contaminants. Proinsulin is then cleaved by enzymes to produce mature insulin.
- Formulation: The purified insulin is combined with stabilizers, preservatives, and sometimes zinc, then filled into vials, cartridges, or pre-filled pens.
- Quality control: Each batch undergoes extensive testing for potency, purity, and sterility before release.
The FDA regulates insulin as a biological product, which affects how biosimilars are developed and approved.
How Insulin Analogs Are Engineered
Insulin analogs are made the same way as human insulin, but with small engineered changes to the amino acid sequence that alter how the molecule behaves after injection.
| Analog | Brand | Change from Human Insulin | Effect |
|---|---|---|---|
| Lispro | Humalog | B28 proline and B29 lysine swapped | Faster onset |
| Aspart | Novolog / Fiasp | B28 proline replaced with aspartate | Faster onset |
| Glulisine | Apidra | B3 asparagine and B29 lysine replaced | Faster onset |
| Glargine | Lantus / Toujeo | A21 asparagine replaced; two arginines added to B chain | Long, flat duration |
| Detemir | Levemir | B30 removed; fatty acid attached to B29 | Binds albumin for slow release |
| Degludec | Tresiba | B30 removed; fatty acid attached to B29 | Ultra-long duration (>42 h) |
These tiny changes in sequence produce big changes in clinical behavior — allowing clinicians to match insulin profiles to meal timing and basal needs.
Quality Control and Sterility
Because insulin is injected, manufacturing must meet strict sterility standards. Facilities operate as cleanrooms, with filtered air, gowned workers, and continuous environmental monitoring. Batches are tested for:
- Correct amino acid sequence and folding.
- Potency measured in international units (1 mg of pure human insulin = 28 IU).
- Absence of bacterial endotoxins and host-cell protein residues.
- Stability under refrigeration and in-use temperatures.
Biosimilar and Generic Insulins
In March 2020, U.S. regulations moved insulin from the drug pathway (New Drug Application) to the biologics pathway (Biologics License Application), enabling biosimilar competition. Products like Semglee (biosimilar to Lantus) and Rezvoglar offer equivalent clinical performance at typically lower prices. Authorized generics of Humalog and Novolog now provide additional options.
Why This Matters for Treatment
Understanding how insulin is made clarifies a few practical points:
- Modern insulins are not “unnatural” — they are biologically identical or carefully modified human insulin.
- Allergic reactions are rare because impurities from animal sources are no longer present.
- Generics and biosimilars work through the same mechanism — pharmacist-selected switches are generally safe when approved as interchangeable.
- Cold-chain storage matters because the three-dimensional folding of insulin is sensitive to heat and freezing.
Affordable, reliable insulin supply is central to diabetes treatment, particularly for people with type 1 who cannot live without it.
The Future of Insulin Production
Research is moving toward smarter insulins and alternative delivery:
- Glucose-responsive insulins that self-regulate based on blood sugar are in preclinical and early clinical development.
- Oral insulin formulations aim to survive the digestive tract — a long-standing goal, still unrealized at scale.
- Weekly basal insulins like insulin icodec are moving through regulatory review.
- Stem cell-derived beta cells could eventually produce insulin inside the body instead of requiring injection.
The Bottom Line
Insulin is a small molecule with a big story — produced naturally by beta cells in a multi-step maturation process, and manufactured commercially by engineering bacteria or yeast to grow it in fermentation tanks. Modern insulin, including every analog, relies on recombinant DNA technology that is now more than 40 years old. Knowing how insulin is made helps explain why storage matters, why biosimilars work, and why newer analogs behave so differently from the cloudy NPH bottles of decades past.
