What Is GIP? The Second Incretin Behind Tirzepatide

GIP stands for glucose-dependent insulinotropic polypeptide. It is a hormone made by intestinal neuroendocrine cells that, like its better-known cousin GLP-1, is released after you eat and signals the pancreas to release insulin [2, 8]. GIP matters for weight-loss drugs because tirzepatide is built on a GIP-based peptide that activates both the GIP receptor and the GLP-1 receptor at the same time [5, 8]. In clinical trials, that dual action produced greater weight loss and better glucose control than the GLP-1-only drug semaglutide [8, 9].

GIP is also at the center of one of the strangest open questions in obesity medicine. Researchers have found that adding GIP receptor activation to a GLP-1 drug improves results — and so does blocking the GIP receptor. Both approaches work, and nobody has fully explained why [2, 4]. This page covers what GIP is, what it does, what the human evidence shows, and what remains unsettled.

What is the difference between GIP and GLP-1?

GIP and GLP-1 are both short-acting peptides made and secreted by intestinal neuroendocrine cells ^[8]. Both act in a nutrient-dependent way — meaning they respond to food — to stimulate insulin secretion. That shared job is called the “incretin” effect: insulin released because of gut signals, not just rising blood sugar ^[8].

The two hormones work through separate receptors. GIP acts on the GIP receptor, abbreviated GIPR, and GLP-1 acts on the GLP-1 receptor, abbreviated GLP-1R [2, 9]. In healthy volunteers, GIP is described as the major incretin — the bigger contributor to that after-meal insulin signal — but its effects are blunted in people with type 2 diabetes ^[8].

The practical difference for consumers is which drugs touch which system. Semaglutide is a GLP-1 receptor monoagonist with no GIP activity ^[8]. Tirzepatide activates both receptors [4, 8]. And at least one experimental drug takes the opposite approach: blocking the GIP receptor while activating the GLP-1 receptor [2, 9].

What does GIP do in the body?

GIP’s best-established job is stimulating insulin release after meals, working together with GLP-1 ^[8]. Beyond that, GIP has known effects that can improve insulin sensitivity, including modulating blood flow in fat tissue and changing rates of lipolysis, the process by which fat cells release stored fat ^[8].

Researchers who argue for activating the GIP receptor as a drug strategy point to a broader list of actions: enhancing insulin secretion, improving insulin sensitivity, reducing inflammation in fat tissue, and effects in the brain that reduce food intake and dampen aversive responses ^[4].

Some of this evidence comes from animals and some from people, and the distinction matters. In mice, GIP receptor activation requires receptors in the central nervous system to lower body weight ^[4]. Separately, GIP receptor activation alone has been reported to reduce food intake and body weight both in preclinical models and in humans ^[4].

How did GIP become a drug target?

Tirzepatide started life as LY3298176, a molecule developed specifically to test one question: does GIP’s metabolic action add to the established benefits of selective GLP-1 receptor agonists in type 2 diabetes? ^[5]

The molecule is a fatty-acid modified peptide with dual GIP and GLP-1 receptor activity, designed for once-weekly subcutaneous injection ^[5]. It is described as a GIP-derived peptide that acts at both the GIP and GLP-1 receptors ^[8]. In mice given chronic dosing, it potently decreased body weight and food intake, and those effects were significantly greater than those of a GLP-1 receptor agonist ^[5].

The Phase 1 program (registered as NCT02759107) had three parts: a single-ascending dose study of 0.25–8 mg, a 4-week multiple-ascending dose study of 0.5–10 mg in healthy subjects, and a 4-week Phase 1b proof-of-concept study of 0.5–15 mg in people with type 2 diabetes, with dulaglutide as a positive control ^[5]. A total of 142 human subjects received at least one dose of LY3298176, dulaglutide, or placebo ^[5].

The early human results were notable. In people with type 2 diabetes, fasting serum glucose fell versus placebo by 49.12 mg/dL at the 10 mg dose (95% CI −78.14 to −20.12) and by 43.15 mg/dL at the 15 mg dose (95% CI −73.06 to −13.21) ^[5]. Weight also dropped:

Group	Dose	Weight change vs. placebo
Healthy subjects, 4 weeks	1.5 mg	−1.75 kg
Healthy subjects, 4 weeks	4.5 mg	−5.09 kg
Healthy subjects, 4 weeks	10 mg	−4.61 kg
Type 2 diabetes, 4 weeks	10 mg	−2.62 kg
Type 2 diabetes, 4 weeks	15 mg	−2.07 kg

All figures are from the Phase 1 program ^[5]. The most frequent side effects were gastrointestinal — vomiting, nausea, decreased appetite, diarrhea, and abdominal distension — and were dose-dependent and mild to moderate in severity ^[5].

Later clinical trials found tirzepatide superior to semaglutide for both weight and glucose control ^[8]. A head-to-head comparison of tirzepatide versus semaglutide for obesity was published by Aronne and colleagues in the New England Journal of Medicine in 2025 (volume 393, pages 26–36) ^[9].

What does the GIP part of tirzepatide actually contribute?

Since tirzepatide hits two receptors, a fair question is what the GIP half brings. Three lines of evidence point to real contributions beyond GLP-1 alone.

First, a 2024 paper in Cell Metabolism is titled “Tirzepatide modulates the regulation of adipocyte nutrient metabolism through long-acting activation of the GIP receptor” — a title-level statement that tirzepatide affects how fat cells handle nutrients through long-acting GIP receptor activation ^[6].

Second, a study in the Journal of Clinical Investigation is titled “GIPR agonism mediates weight-independent insulin sensitization by tirzepatide in obese mice” — a mouse study whose title claims that GIP receptor activation mediates insulin sensitization by tirzepatide independently of weight ^[7]. That is animal data, so it cannot be assumed to hold identically in people.

Third, a prior clamp study comparing tirzepatide with semaglutide found that tirzepatide improved both insulin sensitivity and insulin secretion more than semaglutide did ^[8]. Because semaglutide has no GIP activity ^[8], the difference between the two drugs is a clue that the GIP component does something — though a clamp study alone cannot fully separate the contributions of each receptor.

Is there a GIP-only drug, and what does it do in people?

Yes, at least in research. Macupatide is a long-acting selective GIP receptor agonist studied by Eli Lilly ^[8]. A Phase 1b randomized, double-blind trial tested macupatide alone, dulaglutide (a GLP-1 receptor agonist) alone, or both together in people with type 2 diabetes taking metformin, using hyperinsulinemic euglycemic clamps — a research technique that measures insulin sensitivity directly — at baseline and after 12 weeks of treatment ^[8]. The results were published in Diabetes, Obesity and Metabolism, first published 02 June 2026 (volume 28, issue 8, pages 7196–7206) ^[8].

The trial measured three things: M value (insulin sensitivity), insulin secretion rate (ISR), and the clamp disposition index (cDI), which combines the two. Baseline means across participants were an M value of 5.8 mg·kg⁻¹·min⁻¹, an ISR of 116.8 pmol·min⁻¹·m⁻², and a cDI of 0.3 pmol·m⁻²·L·min⁻²·kg⁻¹ ^[8]. Here is what changed over 12 weeks:

Measure (12-week change)	Macupatide	Dulaglutide	Combination
M value (insulin sensitivity)	+1.3 (33.1%)	+0.2 (10.4%)	+2.0 (38.3%)
ISR (insulin secretion)	+37.6 (32.2%)	+191.1 (163.6%)	+225.3 (192.9%)
cDI (combined index)	+0.2 (63.2%)	+0.5 (181.0%)	+1.0 (321.5%)

Macupatide alone significantly increased the M value, and produced a smaller but still significant increase in ISR ^[8]. The combination was numerically greater than either drug alone on all three indices ^[8]. The study’s conclusion: chronic GIP receptor activation in people with type 2 diabetes leads primarily to increased insulin sensitivity, and combining GIP and GLP-1 receptor activation can augment both insulin secretion and insulin sensitivity compared with GLP-1 therapy alone ^[8].

The most frequent adverse events in that trial were injection site reactions and diarrhea; discontinuations due to treatment-emergent adverse events were uncommon, and there were no deaths during the study ^[8].

What is the GIP paradox?

The GIP paradox is this: both activating and blocking the GIP receptor appear to add metabolic and weight benefits on top of GLP-1 receptor agonism, and the mechanisms behind this are unknown [2, 4]. On its face, that should not be possible — turning a receptor on and turning it off are opposites.

The journal Diabetes ran a formal point-counterpoint on this question in August 2025 (volume 74, issue 8, pages 1320–1338). One article, by Rosenkilde, George, Véniant, and Holst, made the case for blocking the GIP receptor (pages 1334–1338); a companion article made the case for activating it (pages 1326–1333); and a reconciliation piece by Campbell and Drucker sat between them (pages 1320–1325) [2, 4].

The case for blocking rests on several lines of evidence. Global and central nervous system knockout of the GIP receptor protects mice fed a high-fat diet from obesity and insulin resistance ^[2]. Human genetics point the same way: genome-wide association studies link GIP receptor variants with reduced function to lower BMI ^[2]. And pharmacology studies in mice and monkeys found that GIP receptor antagonism enhances GLP-1-induced weight reduction, with a phase 1 study providing proof of principle that the effect extends to humans ^[2].

The case for activating the receptor rests on different evidence: GIP receptor activation alone reduces food intake and body weight in preclinical models and in humans, and mouse studies show this requires GIP receptors in the central nervous system ^[4]. The confusion is stated plainly in the Diabetes perspective: both gain-of-function and loss-of-function strategies decrease body weight ^[4].

A real-world example of the antagonist approach is maridebart cafraglutide (MariTide, from Amgen), a once-monthly agent whose Phase 2 obesity trial results were published in the New England Journal of Medicine in 2025 (volume 393, pages 843–857) ^[9].

How can activating and blocking the same receptor both help?

The newest attempt to resolve the paradox comes from a Nature Metabolism study published in 2026, which asked whether the two strategies work through different parts of the brain ^[9].

Researchers knocked out the GIP receptor gene in either the area postrema (a structure in the brainstem) or the hypothalamus of mice, creating two strains: GiprAP-KO and Giprhypo-KO ^[9]. The results split cleanly:

• Mice missing GIP receptors in the area postrema showed partial protection from diet-induced obesity, reduced appetite suppression in response to an acyl-GIP compound, and a reduced ability of acyl-GIP to prevent avoidance behavior triggered by peptide YY. But liraglutide’s weight loss was unchanged in these mice, and a GIPR antagonist peptide still added similar weight loss ^[9].
• Mice missing GIP receptors in the hypothalamus showed normal appetite suppression from acyl-GIP, but enhanced weight loss on liraglutide — and the synergy from adding a GIPR antagonist was abolished. That loss of synergy was not mediated by preproglucagon neurons in the nucleus tractus solitarius ^[9].

The study’s conclusion: the area postrema carries the appetite-suppressing effects of GIP receptor activation, while GIP receptors in the hypothalamus underlie the ability of GIP receptor blockade to enhance weight loss from GLP-1 and amylin receptor agonists ^[9]. GIPR antagonism and hypothalamic GIP receptor knockout also sensitized mice to weight loss from cagrilintide ^[9]. A companion commentary in Nature Reviews Endocrinology, titled “Appetite-suppressing effects of GIPR agonism and antagonism work through different brain regions,” discussed the findings ^[10].

Earlier work had already suggested separate mechanisms: a Nature Metabolism paper from 2025 (volume 7, pages 1282–1298) reported that GIPR agonism and antagonism decrease body weight and food intake via different mechanisms in male mice [9, 11]. A related study found that a GIPR-antibody/GLP-1 peptide conjugate requires brain GIPR and GLP-1R for additive weight loss in obese mice ^[9].

All of these findings are from mice. They offer a plausible explanation for the paradox, but the question of how the two strategies compare in people is not settled.

Can GIP make GLP-1 drugs easier to tolerate?

There is evidence pointing that way, though it is not conclusive. In mice, GIP receptor activation in hindbrain GABAergic neurons produces antiaversive effects against a range of noxious or aversive stimuli, including GLP-1 receptor activation ^[4]. Notably, the neuron populations carrying these antiaversive effects are distinct from the ones carrying anorectic (appetite-suppressing) signals ^[4].

Two mechanisms have been proposed from this animal work: either inhibitory tone from GIPR-positive neurons dampens the GLP-1R neurons that transmit aversive signals, or GIP acts downstream of those GLP-1R neurons ^[4]. In mice, deleting the GIP receptor in GABAergic neurons actually enhanced GLP-1 receptor agonism’s reduction of food intake and body weight, and those neurons are critical to the weight-loss advantage of dual incretin agonists over GLP-1 alone ^[4].

The effects appear to translate to healthy human participants given a single dose of a long-acting GIP analog together with liraglutide ^[4]. The underlying human trial, by Knop and colleagues, was published in Diabetes, Obesity and Metabolism in 2024 under a title stating that a long-acting GIP receptor agonist improves the gastrointestinal tolerability of GLP-1 receptor agonist therapy ^[12].

Which drugs target the GIP system?

Mechanistically, the landscape looks like this:

Drug	GIP system action	Other action
Tirzepatide	Activates GIPR	Also activates GLP-1R [4, 8]
Maridebart cafraglutide (MariTide, Amgen)	Blocks GIPR	Combined with GLP-1R agonism; once-monthly; Phase 2 obesity results published 2025 [2, 9]
Macupatide (Eli Lilly)	Activates GIPR selectively	Long-acting; studied alone and with dulaglutide [8]
Semaglutide	None	GLP-1 receptor monoagonist only [8]

Dosing frequency has also evolved across the GLP-1 drug class generally: exenatide was taken twice daily, liraglutide once daily, and semaglutide once weekly ^[2].

Can you raise GIP with food or supplements, or test it in a blood panel?

GIP itself is a peptide, and the medicines built on it are engineered, long-acting analogs — fatty-acid modified molecules designed for once-weekly or once-monthly injection — not something you can eat [5, 8]. None of the drug versions described here are dietary or supplemental products.

As for testing: no routine clinical GIP blood test has been described. GIP is measured in research settings, not in standard care ^[8]. Note that the clamp studies described above measured insulin sensitivity, insulin secretion, and related indices — not GIP levels themselves ^[8]. GIP is not part of a standard blood panel.

What is still unresolved?

The central mystery — why both activating and blocking the GIP receptor can add weight-loss benefit on top of GLP-1 drugs — is explicitly described as unresolved, with mechanisms unknown [2, 4]. The Diabetes perspective states that resolving the question will require additional experimentation and the results of forthcoming clinical trials ^[4]. The newest brain-region data, published in Nature Metabolism in 2026, offers one explanation from mouse studies, but it has not settled the question in humans ^[9].

This page is general information, not medical advice. Decisions about starting, stopping, or switching any medication belong with a clinician who knows your health history.

Sources

1. MDPI. The Premise of the Paradox: Examining the Evidence That Motivated GIPR Agonist and Antagonist Drug Development Programs.
2. American Diabetes Association. GIP Receptor Antagonists in the Pharmacotherapy of Obesity: Physiologic, Genetic, and Clinical Rationale. Diabetes, August 2025.
3. Cell Metabolism. The glucose-dependent insulinotropic polypeptide (GIP) regulates body weight and food intake via CNS-GIPR signaling.
4. American Diabetes Association. Therapeutic Targeting of the GIP Receptor—Revisiting the Controversies. Diabetes, August 2025.
5. PubMed Central. LY3298176, a novel dual GIP and GLP-1 receptor agonist for the treatment of type 2 diabetes mellitus: From discovery to clinical proof of concept.
6. Cell Metabolism. Tirzepatide modulates the regulation of adipocyte nutrient metabolism through long-acting activation of the GIP receptor.
7. Journal of Clinical Investigation. GIPR agonism mediates weight-independent insulin sensitization by tirzepatide in obese mice.
8. Diabetes, Obesity and Metabolism. Insulin Sensitivity and Beta Cell Function With Macupatide Alone or With Dulaglutide in Type 2 Diabetes: A Phase 1b, Randomised Controlled Trial. 2026.
9. Nature Metabolism. Distinct brain regions mediate regulation of food intake in response to GIPR agonism or antagonism.
10. Nature Reviews Endocrinology. Appetite-suppressing effects of GIPR agonism and antagonism work through different brain regions.
11. Nature Metabolism. GIPR agonism and antagonism decrease body weight and food intake via different mechanisms in male mice.
12. Diabetes, Obesity and Metabolism. A long-acting glucose-dependent insulinotropic polypeptide receptor agonist improves the gastrointestinal tolerability of glucagon-like peptide-1 receptor agonist therapy. 2024.
13. Eli Lilly and Company. Mounjaro (tirzepatide) injection, for subcutaneous use: prescribing information. Initial U.S. approval 2022.
14. Eli Lilly and Company. Clinical Development Pipeline. August 5, 2026.