What GLP-1 Actually Is: The Gut Hormone Behind the Drug Class
Before it was a drug class, GLP-1 was a hormone your intestine already makes every time you eat. Here's what it naturally does, and how the drugs extend that effect.
By the GLP Semaglutide editorial team · Last updated April 15, 2025
GLP-1, or glucagon-like peptide-1, is not something pharmaceutical companies invented. It's a hormone the human intestine already produces every time you eat — the drugs in this category are engineered versions designed to mimic and extend an effect your body creates naturally and briefly.
Where it comes from
GLP-1 is released by L-cells, a type of hormone-secreting cell embedded in the lining of the intestine. L-cells are most dense in the colon and rectum, with fewer in the upper small intestine, and they have microvilli — tiny finger-like projections — that extend into the gut to directly sense the food passing through.
The 'incretin effect'
Researchers noticed decades ago that glucose taken by mouth triggers more insulin release than the same amount of glucose delivered directly into a vein, even when blood sugar levels end up matched. That gap is called the incretin effect, and GLP-1 — along with a second hormone, GIP — is responsible for it. Carbohydrate and fat are the strongest triggers for GLP-1 release; protein is a weaker one.
What GLP-1 actually does once released
Natural GLP-1 has several jobs: it tells the pancreas to release more insulin when blood sugar is high, tells the liver (via the pancreas) to stop releasing extra glucagon, slows down how fast the stomach empties into the small intestine, and signals to the brain that you've eaten. All four of these effects are things GLP-1 drugs are designed to reproduce — the appetite and fullness signaling is the piece that got semaglutide and tirzepatide into the weight-management conversation.
Why a drug was needed at all
If the body already makes GLP-1, why inject more of it? Because natural GLP-1 barely survives in the bloodstream — a separate piece we cover on how these drugs are chemically modified to last a week instead of minutes. The short version: an enzyme destroys native GLP-1 almost immediately, so a usable drug has to be built to resist that.
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