What is GLP-1?
GLP-1 stands for glucagon-like peptide-1, a naturally occurring peptide hormone involved in the regulation of blood glucose, digestion and appetite-related signalling. It belongs to a group of gastrointestinal hormones known as incretins, which help coordinate the body's metabolic response to food.
Endogenous GLP-1 has several well-established physiological effects, including glucose-dependent stimulation of insulin secretion, modulation of glucagon release and effects on gastric emptying and satiety signalling. These normal physiological functions should be distinguished from the pharmacology of medicines designed to activate the GLP-1 receptor for much longer periods.
Where GLP-1 is produced
GLP-1 is produced primarily by specialised enteroendocrine L cells located throughout the intestine, with particularly high numbers in the distal small intestine and colon.
After nutrients enter the gastrointestinal tract, L cells release GLP-1 into the circulation. GLP-1-related signalling also occurs within the nervous system, including through neurons in the brainstem.
GLP-1 is generated from the larger precursor protein proglucagon. Tissue-specific processing of proglucagon produces different peptide hormones depending on which enzymes are present within the cell.
GLP-1 receptor
The GLP-1 receptor, abbreviated GLP-1R, is a G-protein-coupled receptor expressed in several tissues, including pancreatic beta cells and regions of the nervous system.
When GLP-1 binds to GLP-1R, it activates intracellular signalling pathways that commonly involve cyclic AMP (cAMP). In pancreatic beta cells, this signalling enhances insulin secretion when glucose concentrations are elevated.
GLP-1 receptor distribution across different tissues helps explain why GLP-1 signalling influences multiple physiological processes rather than functioning solely as a regulator of insulin.
Incretin biology
The incretin effect describes the observation that oral glucose generally produces a greater insulin response than an equivalent amount of glucose delivered directly into the bloodstream.
This occurs partly because nutrients entering the gastrointestinal tract stimulate the release of incretin hormones. GLP-1 and glucose-dependent insulinotropic polypeptide (GIP) are the two principal recognised incretin hormones.
Their signalling helps coordinate nutrient absorption with pancreatic hormone secretion and broader metabolic regulation.
Insulin and glucagon signalling
GLP-1 can enhance insulin secretion from pancreatic beta cells when blood glucose is elevated. This glucose-dependent behaviour is an important feature of normal GLP-1 physiology.
GLP-1 signalling can also reduce glucagon secretion under particular physiological conditions. Glucagon is produced primarily by pancreatic alpha cells and generally acts to increase circulating glucose when required.
The interaction between insulin, glucagon and incretin signalling allows the body to regulate glucose dynamically rather than relying on a single hormone.
Gastric-emptying research
GLP-1 participates in communication between the gastrointestinal tract and the nervous system and can influence the rate at which stomach contents move into the small intestine.
Slower gastric emptying can alter how rapidly nutrients enter the circulation following a meal. This contributes to the broader role of GLP-1 in coordinating digestive and metabolic responses to food.
The magnitude of this effect varies with physiological conditions and with different GLP-1 receptor agonists. It should therefore not be assumed that endogenous GLP-1 and every GLP-1-targeting medicine produce identical gastric effects.
Appetite-related signalling
GLP-1 receptors are present within neural pathways involved in appetite, satiety and food-related behaviour.
Signals originating from the gastrointestinal tract can communicate with the brain through neural and endocrine pathways. GLP-1 forms part of this gut-brain signalling network and contributes to physiological regulation of food intake.
Research into these pathways helped provide the biological foundation for pharmaceutical development of longer-acting GLP-1 receptor agonists. However, endogenous GLP-1 signalling and the sustained pharmacological activation produced by these medicines are not equivalent.
Endogenous GLP-1 vs GLP-1 receptor agonists
Natural GLP-1 has a very short circulating lifetime because it is rapidly degraded, particularly by the enzyme dipeptidyl peptidase-4 (DPP-4).
Pharmaceutical GLP-1 receptor agonists are designed to activate the same receptor while remaining biologically active for substantially longer periods. Molecular modifications can increase resistance to enzymatic degradation and alter absorption, distribution and receptor exposure.
Consequently, evidence concerning a specific GLP-1 receptor agonist should not automatically be attributed to natural GLP-1 or to another drug targeting the same receptor.
Semaglutide, tirzepatide and retatrutide
Semaglutide, tirzepatide and retatrutide are related to GLP-1 research but have different pharmacological profiles.
Semaglutide is a GLP-1 receptor agonist designed to produce prolonged GLP-1R activation.
Tirzepatide is a dual agonist with activity at both GIP and GLP-1 receptors. Its pharmacology therefore cannot be described simply as that of a conventional GLP-1 receptor agonist.
Retatrutide is an investigational multi-receptor agonist designed to activate GLP-1, GIP and glucagon receptors. Its inclusion of glucagon-receptor activity distinguishes it further from semaglutide and tirzepatide.
Clinical findings involving these compounds must be attributed to the exact molecule studied because differences in receptor activity, molecular structure and pharmacokinetics can produce substantially different biological effects.
GLP-1, GIP and glucagon differences
GLP-1, GIP and glucagon are distinct peptide hormones involved in metabolic regulation.
GLP-1 and GIP are incretin hormones released in response to nutrient intake and can enhance glucose-dependent insulin secretion through their respective receptors.
Glucagon is produced primarily by pancreatic alpha cells and has a different physiological role. Among its best-established functions is signalling the liver to increase glucose availability during periods when circulating glucose needs to be maintained.
The three systems interact within metabolic physiology but operate through different receptors. Multi-receptor compounds are specifically designed to engage combinations of these pathways rather than simply reproducing natural GLP-1 activity.
Frequently asked questions
What does GLP-1 stand for?
GLP-1 stands for glucagon-like peptide-1.
Is GLP-1 naturally produced by humans?
Yes. GLP-1 is an endogenous peptide hormone produced primarily by intestinal L cells.
Is GLP-1 the same as semaglutide?
No. GLP-1 is a naturally occurring hormone, whereas semaglutide is a pharmaceutical GLP-1 receptor agonist engineered to produce much longer-lasting receptor activation.
Are semaglutide and tirzepatide the same type of molecule?
No. Semaglutide targets the GLP-1 receptor, whereas tirzepatide activates both GIP and GLP-1 receptors.
How is retatrutide different?
Retatrutide is an investigational compound with activity at GLP-1, GIP and glucagon receptors, giving it a different receptor profile from both semaglutide and tirzepatide.
What does incretin mean?
Incretins are gastrointestinal hormones released in response to nutrients that contribute to the insulin response following food intake. GLP-1 and GIP are the principal recognised incretin hormones.
Does natural GLP-1 remain in circulation for a long time?
No. Endogenous active GLP-1 is rapidly degraded, particularly by DPP-4, giving it a very short biological half-life compared with long-acting pharmaceutical GLP-1 receptor agonists.
Scientific references
Scientific literature on GLP-1 includes extensive research into proglucagon processing, intestinal L-cell secretion, GLP-1 receptor pharmacology, incretin physiology, pancreatic hormone regulation, gastric emptying and gut-brain signalling. Research involving semaglutide, tirzepatide or retatrutide should be attributed specifically to the compound studied because their receptor profiles and pharmacological properties differ substantially.