August 29, 2026
Research and educational content only. The compounds discussed are supplied strictly for Research Use Only (RUO) — not for human or veterinary use, diagnosis, or treatment. This article summarizes published, peer-reviewed research; it is not medical advice, and nothing here describes or endorses human use of any research compound.
Almost every headline about modern metabolic peptides — semaglutide, tirzepatide, retatrutide — traces back to a single, century-old idea: that the gut talks to the pancreas. That idea has a name, incretin, and a surprisingly long research history. This primer defines the term from the ground up and lays out the physiology that the more specialized articles in our library build on. Think of it as the plain-language hub: the GLP-1 vs. GIP comparison and the what-is-a-GLP-1-receptor-agonist overview both elaborate on the concepts introduced here.
An incretin is a gut-derived hormone released after nutrients are eaten that amplifies the pancreas’s insulin response to those nutrients. The term is old: the Belgian physiologist Jean La Barre introduced “incrétine” in 1932 to describe a gut-mucosa substance that lowered blood glucose without stimulating the pancreas’s digestive (exocrine) secretions (Diapedia, Incretin physiology and its history).
A modern working definition was later formalized. As summarized in that same historical review, Creutzfeldt proposed that to qualify as an incretin, a hormone must (1) be released from gut endocrine cells in response to nutrients — especially glucose, (2) stimulate insulin secretion at concentrations the body actually reaches after a meal, and (3) do so in a glucose-dependent way, meaning it acts mainly when glucose is elevated. Why does that third criterion matter so much? Because glucose dependence is the trait that made this whole hormone family attractive to study in the first place.
The incretin effect is the observation that an oral glucose load triggers a substantially larger insulin response than the same amount of glucose delivered intravenously. In other words, the route matters: swallowing glucose engages gut hormones that an IV drip bypasses entirely.
The phenomenon was pinned down in 1964, when two independent groups — Elrick and colleagues, and McIntyre and colleagues — showed that orally administered glucose evokes a greater insulin response than intravenously administered glucose, and proposed that gut-derived factors were potentiating insulin release (Diapedia). How big is the gap? According to StatPearls, the incretin hormones together account for an estimated 25% to 70% of the insulin secreted after a meal — a range wide enough to signal just how central this gut-to-pancreas axis is to normal glucose handling. That single, reproducible discrepancy between oral and IV glucose is the empirical seed of everything that follows.
The two principal incretins are glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), and they come from different cells in different parts of the gut. GIP is released from enteroendocrine K cells concentrated in the upper small intestine; GLP-1 is released from L cells that are more abundant further down, in the distal small intestine and colon (Diapedia; Baggio & Drucker, Physiological Reviews, 2007).
Both were established as genuine incretins through the same kind of experiment: infusing each hormone alongside IV glucose, at concentrations matching those seen during an oral glucose test, and watching insulin secretion climb. They share a defining mechanism — glucose-dependent stimulation of insulin from pancreatic beta cells — but they are not interchangeable. GLP-1 also suppresses glucagon and slows gastric emptying, while GIP behaves differently in fat and bone. Those divergences are exactly what our companion piece on GLP-1 vs. GIP unpacks in detail.
DPP-4 matters because it is the enzyme that switches the incretin signal off — rapidly. Dipeptidyl peptidase-4 cleaves both GIP and GLP-1 shortly after they are released, and for GLP-1 the effect is dramatic: the intact hormone has a circulating half-life on the order of only one to two minutes (Baggio & Drucker, Physiological Reviews, 2007).
So here is the design problem in a single sentence: how do you study, or build tools around, a hormone that disappears almost as fast as it appears? Native GLP-1’s fragility is not a footnote — it is the constraint that shaped the entire research program that followed. Two broad strategies emerged in the literature: slow the enzyme down (DPP-4 inhibition), or engineer a molecule that activates the same receptor but resists the enzyme’s cut. That second path is the one traced in our GLP-1 receptor agonist overview.
The connection is a well-replicated finding: the incretin effect is markedly reduced or absent in type 2 diabetes. As stated in a 2023 Diabetologia review, the greater insulin-secretory response after oral versus matched intravenous glucose “is markedly reduced or absent” in people with the condition.
The two incretins are not affected equally, and this is where GIP’s reputation was made — and later remade. The same review reports that when synthetic hormone is infused, insulin responses are “uniformly much smaller” in type 2 diabetes than in healthy individuals, with one comparison finding the GLP-1 response preserved at roughly 70% of healthy controls while the GIP response fell to about 40%. That blunted GIP action — often called GIP resistance — helped earn GIP a long-standing label in the literature as “the neglected incretin,” the one many researchers assumed was a therapeutic dead end. Was that assumption correct? The next generation of multi-receptor peptides suggests it deserved another look — a reversal our GLP-1 vs. GIP article covers in full.
Incretin biology is the direct foundation of the modern GLP-1-based research-peptide class: the receptors these molecules target, and the degradation problem their designs solve, both come straight from the physiology above. A GLP-1 receptor agonist is simply a molecule built to activate the GLP-1 receptor while resisting the DPP-4 cleavage that destroys the native hormone — which is why semaglutide and related peptides signal for hours rather than minutes.
From there the story is one of adding receptors. Rehabilitating GIP led to dual GIP/GLP-1 designs such as tirzepatide, and layering in the glucagon receptor produced the investigational triple agonist class. Each step is a variation on the same incretin theme first described in 1932. Note the regulatory distinction that runs through all of it: some branded products in this space are FDA-approved human medicines, but that status applies only to those specific products — it does not extend to research-grade material, supplied strictly for Research Use Only.
If you’re sourcing these compounds for laboratory research, every product we supply is Research Use Only, with its lot, purity and testing laboratory published on the product page or stated plainly as pending. Explore the research-compound library → the PureLab research-peptide catalog
All products are sold for Research Use Only and are not for human or veterinary consumption, diagnostic, or therapeutic use. Content is educational; no medical or performance claims are made or implied.
