Insulin is the master hormone of glucose metabolism. It is made by beta cells in the pancreatic islets of Langerhans, secreted in response to rising blood glucose, and acts on muscle, fat, and liver cells to drive glucose uptake, storage, and growth. Counterregulatory hormones oppose its action to keep glucose available during fasting and stress. Understanding the basic biology — how insulin is made, secreted, and acts — helps explain why type 1 and type 2 diabetes behave differently and why specific treatments work.
Where Insulin Is Made
- Beta cells in the islets of Langerhans within the pancreas
- The pancreas contains roughly 1 million islets distributed mostly in the body and tail
- Each islet contains 1,000 to 2,000 cells, with 60 to 80 percent being beta cells
- Other islet cells include alpha cells (glucagon), delta cells (somatostatin), PP cells, and epsilon cells (ghrelin)
- Beta cells communicate within the islet by gap junctions and paracrine signaling
From Gene to Hormone
Insulin Gene and Protein Processing
- The insulin gene (INS) is on chromosome 11p15.5
- Transcription produces preproinsulin mRNA
- Preproinsulin is translated into a 110-amino-acid precursor
- The signal peptide is cleaved in the endoplasmic reticulum, producing proinsulin
- Proinsulin folds, with disulfide bonds linking the A and B chains
- In the Golgi and secretory granules, prohormone convertases PC1/3 and PC2 cleave proinsulin into insulin and C-peptide
- Mature insulin and C-peptide are stored in granules until secretion
What Gets Secreted
| Molecule | Function | Half-Life |
|---|---|---|
| Insulin | Lowers blood glucose | ~5 minutes (endogenous) |
| C-peptide | Marker of endogenous insulin production | ~30 minutes |
| Amylin (IAPP) | Slows gastric emptying, suppresses glucagon, suppresses appetite | Short |
| Proinsulin (small amount) | Low biological activity | Intermediate |
How Beta Cells Sense Glucose
- Glucose enters beta cells through GLUT2 (or GLUT1 in humans) transporters
- Glucokinase phosphorylates glucose, acting as the glucose sensor
- Glucose metabolism raises the ATP-to-ADP ratio
- Rising ATP closes ATP-sensitive potassium (KATP) channels
- The membrane depolarizes
- Voltage-gated calcium channels open, allowing calcium influx
- Calcium triggers exocytosis of insulin granules
This is called glucose-stimulated insulin secretion (GSIS). Sulfonylureas and meglitinides work by closing the KATP channels independently of glucose.
First-Phase and Second-Phase Insulin Secretion
- First phase: rapid burst within minutes of a glucose stimulus, from pre-formed granules near the membrane
- Second phase: sustained, slower release that depends on granule mobilization and new synthesis
- First-phase insulin secretion is lost early in type 2 diabetes — an early functional defect
Beyond Glucose — Other Insulin Secretion Triggers
- Amino acids — leucine, arginine
- Fatty acids — short-term stimulatory, long-term lipotoxic
- Incretin hormones — GLP-1 and GIP from gut
- Parasympathetic stimulation
- Some drugs — sulfonylureas, meglitinides
GLP-1 receptor agonists (semaglutide, liraglutide, tirzepatide) leverage incretin biology to enhance glucose-dependent insulin secretion.
How Insulin Acts on Target Tissues
Insulin Receptor Signaling
- Insulin binds the insulin receptor, a tyrosine kinase
- The receptor autophosphorylates
- Insulin receptor substrate (IRS) proteins are recruited and phosphorylated
- PI3-kinase activates AKT (PKB)
- AKT triggers GLUT4 vesicle translocation in muscle and fat
- AKT also suppresses hepatic gluconeogenesis genes
- A parallel MAPK pathway drives growth and proliferation
What Insulin Does in Each Tissue
| Tissue | Insulin Action |
|---|---|
| Skeletal muscle | GLUT4 to surface; glucose uptake; glycogen synthesis; protein synthesis |
| Adipose tissue | GLUT4 to surface; glucose uptake; fatty acid synthesis; suppress lipolysis |
| Liver | Suppress gluconeogenesis; promote glycogen synthesis; lipogenesis |
| Brain | Modulates appetite and cognition; mostly insulin-independent glucose use |
| Endothelium | Nitric oxide release, vasodilation |
| Pancreatic alpha cells | Suppress glucagon secretion |
Glucose Disposal Distribution
- Skeletal muscle handles roughly 80 percent of insulin-stimulated glucose disposal
- Adipose tissue and liver handle the remainder
- The brain uses approximately 120 g of glucose per day, mostly via insulin-independent transporters
- Red blood cells, lens, kidney medulla also use glucose without insulin
Counterregulatory Hormones
To prevent hypoglycemia during fasting, exercise, or stress, several hormones oppose insulin action.
| Hormone | Source | Key Effects |
|---|---|---|
| Glucagon | Alpha cells | Raises hepatic glucose production |
| Epinephrine | Adrenal medulla | Glycogenolysis, lipolysis, suppresses insulin |
| Cortisol | Adrenal cortex | Gluconeogenesis, peripheral insulin resistance |
| Growth hormone | Pituitary | Counteracts insulin in muscle and fat |
Insulin Resistance vs Insulin Deficiency
Insulin Resistance
- Tissues need more insulin to achieve the same glucose disposal
- Driven by ectopic fat (liver, muscle), inflammation, lipotoxicity, mitochondrial dysfunction
- Initial compensation: beta cells make more insulin
- Hyperinsulinemia is the early signature
- Failure to compensate leads to hyperglycemia
Insulin Deficiency
- Reduced beta-cell mass or function
- Type 1 diabetes: autoimmune destruction
- Late type 2 diabetes: progressive functional decline
- Type 3c: pancreatic damage
- Drug-induced: checkpoint inhibitors, immunosuppressants
Clinical Use of C-Peptide
- Measure of endogenous insulin production
- Low or undetectable suggests type 1 or end-stage type 2
- Normal or elevated suggests preserved beta-cell function
- Useful for classification of unclear diabetes types
- Useful for assessing whether insulin can be tapered
- Not affected by exogenous insulin (which contains no C-peptide)
Exogenous Insulin Preparations
| Insulin Type | Onset | Duration |
|---|---|---|
| Rapid-acting (lispro, aspart, glulisine) | 5 to 15 minutes | 3 to 5 hours |
| Ultra-rapid (fiasp, lyumjev) | 2 to 5 minutes | 3 to 5 hours |
| Short-acting (regular) | 30 minutes | 5 to 8 hours |
| Intermediate (NPH) | 1 to 2 hours | 12 to 18 hours |
| Long-acting (glargine, detemir) | 1 to 2 hours | ~24 hours |
| Ultra-long-acting (degludec) | 1 to 2 hours | ~42 hours |
| Pre-mixed | Combinations | Varies |
Common Misconceptions
- Insulin does not “store fat” any more than any other anabolic hormone — excess calories drive fat gain regardless of insulin
- Insulin is not addictive — type 1 patients need it for life, but the body does not become dependent in the addiction sense
- Starting insulin is not a sign of failure — it reflects the natural progression of beta-cell loss in many forms of diabetes
- Insulin does not cause complications — chronic hyperglycemia does
Related Reading
See our companion pieces on beta cells and diabetes, what is glucose, what causes diabetes, and our prediabetes basics hub.
The Bottom Line
Insulin is made by beta cells in the pancreatic islets, secreted in response to glucose, and binds receptors on muscle, fat, and liver to drive glucose uptake and storage. Counterregulatory hormones — glucagon, cortisol, growth hormone, and epinephrine — oppose insulin to keep glucose available during fasting and stress. Type 1 diabetes is primarily insulin deficiency. Type 2 diabetes is primarily insulin resistance with progressive beta-cell decline. C-peptide is the clinical marker of endogenous insulin production and helps distinguish diabetes types. Understanding insulin biology helps explain why specific treatments — sulfonylureas, GLP-1 agonists, SGLT2 inhibitors, and insulin itself — work the way they do.
