What Is GLP-1? A Plain English Guide to the Hormone Behind Ozempic

GLP-1 is a hormone your own gut makes. Every drug in the Ozempic and Wegovy family exists to imitate it.

If you have heard the term thrown around and want to know what it actually is - not a marketing definition, the real biology - this guide covers it: what the letters mean, where the hormone comes from, what it does, why nobody could turn it into a medicine for twenty years, and how the class of drugs finally cracked it.

This is background information, not medical advice. Whether any GLP-1 medicine is appropriate for you is a conversation for you and a healthcare provider.

What does GLP-1 stand for?

Glucagon-like peptide-1.

The name is a bit of a historical accident. In the early 1980s, scientists sequencing the gene for glucagon - the hormone that raises blood sugar - discovered that glucagon is cut out of a much larger precursor protein called proglucagon. Hidden inside that same precursor were two other peptide sequences nobody had known existed [1][10].

Because they sat next to glucagon and resembled it structurally, they were named glucagon-like peptide-1 and glucagon-like peptide-2. The names stuck even though GLP-1 does almost the opposite of what glucagon does.

Graeme Bell reported the hamster, bovine and human proglucagon sequences in 1983 [1]. The peptides were found by reading DNA before anyone had isolated them from tissue - one of the first hormones discovered that way.

Where is GLP-1 produced in the body?

Mainly in your intestines.

Cells called L-cells, scattered through the lining of the lower small intestine and the colon, release GLP-1 within minutes of you eating [1]. They are nutrient sensors: food arriving triggers release.

There is a second, smaller source. A specific population of neurons in the brainstem also makes GLP-1 and uses it as a signaling molecule inside the brain [8]. That matters for how GLP-1 influences appetite.

One curiosity: the same proglucagon precursor is processed differently depending on which cell is doing the cutting. Pancreatic alpha cells cut it to make glucagon. Intestinal L-cells cut it to make GLP-1 and GLP-2. Same raw material, different finished products [1].

What does GLP-1 actually do?

Four main things.

It amplifies insulin release, but only when glucose is high. This is the headline job. When GLP-1 is present, the pancreas responds more strongly to a rise in blood sugar. When blood sugar is normal, the amplification fades. Ozempic’s label describes the drug version the same way: it “stimulates insulin secretion and lowers glucagon secretion, both in a glucose-dependent manner” [2].

It suppresses glucagon. Glucagon tells the liver to release stored sugar. GLP-1 turns that signal down when glucose is high, which reduces how much sugar the liver adds to the bloodstream. Jens Juul Holst’s group in Copenhagen showed this, and it is a property GLP-1 has and the other incretin hormone, GIP, does not [1].

It slows the stomach. Food moves from stomach to intestine more slowly, flattening the post-meal glucose spike.

It signals fullness. Through nerve pathways and through direct action on specific brain regions, GLP-1 contributes to the sense that a meal was enough [8].

What is the incretin effect?

This is the observation that started everything.

If you swallow a specific amount of glucose, your body releases far more insulin than if the same amount of glucose is infused directly into a vein - even when the blood glucose level ends up identical [3]. Somewhere between half and two-thirds of the insulin response to a meal in healthy people comes from this extra gut-driven boost [3].

That gap is called the incretin effect, and GLP-1 and GIP are the hormones responsible.

The word “incretin” was coined in 1932 by Belgian physiologist Jean La Barre, from “intestine secretion insulin” [3][4]. The effect itself was demonstrated independently by McIntyre and by Elrick in 1964, once reliable insulin assays existed, and quantified by Perley and Kipnis a few years later [4].

The incretin effect is blunted in type 2 diabetes. That is a large part of why incretin-based medicines were pursued in the first place.

Why couldn’t GLP-1 itself be used as a drug?

Because it disappears almost instantly.

An enzyme called DPP-4, present all over the body, recognizes an alanine or proline in the second position of a peptide and snips off the first two amino acids. GLP-1 has an alanine there. Its half-life is under two minutes [1][9].

Researchers proved the concept anyway. In 1987, a group led by Kreymann and Bloom showed GLP-1’s insulin-boosting action in human subjects [1]. And in 2002 a Copenhagen group infused synthetic GLP-1 continuously for six weeks and saw improved glucose control, reduced body weight and better beta-cell function, with no tachyphylaxis [1].

It worked. It just required a continuous infusion, which is not a practical treatment.

Two different solutions emerged. One was to block the enzyme: DPP-4 inhibitors such as sitagliptin protect your own GLP-1 rather than supplying a synthetic version [9]. The other was to redesign the peptide so the enzyme cannot touch it - which is the GLP-1 receptor agonist approach that produced semaglutide.

What is a GLP-1 receptor agonist?

A receptor is a lock. An agonist is a key that turns it on.

The GLP-1 receptor is a class B G-protein-coupled receptor - a protein that threads through the cell membrane seven times. When GLP-1 or a GLP-1 drug docks into it, the receptor changes shape, activates a partner protein called Gs, and raises cyclic AMP inside the cell. That cyclic AMP signal is what ultimately drives insulin release and the other downstream effects.

A 2017 cryo-electron microscopy structure gave the first near-atomic view of the activated receptor, showing the peptide clasped between the receptor’s outer domain and its transmembrane core [5]. There is now a published structure of semaglutide itself bound to the receptor in complex with Gs protein [12].

The drugs in this class differ mainly in how they solve the survival problem [11]:

• Semaglutide and liraglutide attach a fatty acid that binds albumin in the blood, creating a slowly released reservoir.
• Dulaglutide fuses the GLP-1 analog to a piece of an antibody, making a molecule too big for the kidney to filter.
• Exenatide is not a human GLP-1 analog at all - it is a synthetic version of exendin-4, a peptide found in Gila monster venom that happens to activate the same receptor and naturally resists DPP-4.
• Albiglutide, no longer marketed, fused the peptide directly to albumin.

Where are GLP-1 receptors found?

Not just the pancreas. Receptor messenger RNA has been detected in lung, brain, kidney, stomach and heart tissue [6]. Protein-level work has located the receptor in pancreatic islets, parts of the gastrointestinal tract, the heart’s sinoatrial node and atria, and smooth muscle in the walls of kidney and lung arteries.

Two things worth knowing about that list. First, human GLP-1 receptor levels are generally low [7]. Second, the distribution is not the same across species - rat and mouse lung and thyroid express far more receptor than human tissue does [7]. That gap is one reason animal findings do not translate straight to people.

Is GLP-1 the same as GIP, or amylin?

No, though they often appear in the same conversation.

GIP (glucose-dependent insulinotropic polypeptide) is the other incretin, released from K-cells higher up in the small intestine. Like GLP-1 it boosts insulin when glucose is high. Unlike GLP-1 it does not suppress glucagon. Tirzepatide activates the GIP receptor as well as the GLP-1 receptor; semaglutide does not touch GIP at all.

Amylin is released from the pancreas alongside insulin and signals fullness through a different brainstem route. Drugs that copy it, such as cagrilintide and petrelintide, are being tested both alongside and against GLP-1 drugs because the two satiety signals appear to add up.

GLP-2, GLP-1’s sibling from the same precursor, is not involved in blood sugar at all. Daniel Drucker showed it is an intestinal growth factor, and it became the drug teduglutide for short bowel syndrome [1].

Why does any of this matter to a patient?

Because knowing what GLP-1 is makes several confusing things simple.

It explains why these drugs rarely cause low blood sugar on their own - the insulin effect is glucose-dependent, so it switches itself off. It explains why nausea and early fullness are so common - the drug is exaggerating a normal post-meal signal. It explains why doses climb slowly rather than starting high. And it explains why the class keeps expanding into new conditions: GLP-1 receptors are not only in the pancreas.

It also explains the naming chaos. Ozempic, Wegovy and Rybelsus all contain the same molecule, semaglutide, at different doses and in different formats. Mounjaro and Zepbound contain a different molecule, tirzepatide. Victoza and Saxenda contain liraglutide. Three brand names spread across five US presentations, one molecule, in the semaglutide family alone.

Understanding the hormone underneath is what makes the product names stop being confusing.

Sources

1. Drucker, Discovery, characterization and clinical development of the glucagon-like peptides - Journal of Clinical Investigation
2. Ozempic (semaglutide) injection label - DailyMed
3. The Origin and Understanding of the Incretin Concept - Frontiers in Endocrinology
4. Incretin-Based Therapies Through the Decades - Medical Sciences
5. Cryo-EM structure of the activated GLP-1 receptor in complex with a G protein - PubMed
6. Tissue-specific expression of the human GLP-1 receptor - PubMed
7. GLP-1 receptor expression in human tumors and normal tissues - Journal of Nuclear Medicine
8. Mechanisms of GLP-1 receptor agonist-induced weight loss - The American Journal of Medicine
9. Therapeutic stimulation of GLP-1 and GIP with DPP-4 inhibitors - PMC
10. The discovery and development of GLP-1 based drugs - PMC
11. Battle of GLP-1 delivery technologies - PMC
12. PDB 7KI0: Semaglutide-bound GLP-1 receptor in complex with Gs protein - RCSB PDB