GIP was the original incretin. Discovered in 1971, it predates GLP-1 in the scientific literature by more than a decade. For most of that time, GIP was considered an interesting hormone with limited therapeutic potential. Animal data in the 2000s even suggested that blocking the GIP receptor might be the right move for obesity, because GIP-receptor-deficient mice resisted weight gain on high-fat diets. The pharmaceutical industry largely set GIP aside.
Tirzepatide upended that view. A single peptide that activates both the GLP-1 and GIP receptors produced more weight loss in human trials than any GLP-1-only compound had managed. The gap was substantial. SURMOUNT-1 reported 22.5% at 15mg over 72 weeks. STEP 1 reported 14.9% at 2.4mg over 68 weeks. The two trials were not directly comparable, but the head-to-head SURMOUNT-5 trial later confirmed the gap: 20.2% for tirzepatide versus 13.7% for semaglutide at 72 weeks (Aronne et al., NEJM, 2025).
This article covers the receptor biology behind that gap. It explains where GLP-1 and GIP come from, what each receptor does in different tissues, why GIP was misjudged for so long, and what the trial data say about the contribution GIP makes when added to a GLP-1 backbone. For mechanism-level discussion of how the GLP-1 receptor itself works, see How GLP-1 Agonists Work. For the broader question of how triple agonism extends the pattern, see Triple vs Dual Agonism.
What GIP is and where the receptor lives
Glucose-dependent insulinotropic polypeptide is a 42-amino-acid peptide hormone released from K-cells, which sit in the duodenum and upper jejunum. K-cells respond to nutrients passing through the upper small intestine, particularly fats and carbohydrates. GIP is released within minutes of a meal, peaks faster than GLP-1, and is broken down by the same enzyme (DPP-4) over a 5 to 7 minute half-life.
The GIP receptor (GIPR) is a class B G-protein-coupled receptor, like the GLP-1 receptor. It is expressed in a different set of tissues, which is the source of GIP's distinct physiological profile:
- Pancreatic beta cellsInsulin release in response to elevated blood glucose. This is the original mechanism that earned GIP the 'insulinotropic' part of its name. The effect is glucose-dependent, similar to GLP-1, which is why dual GLP-1/GIP agonism does not increase hypoglycaemia risk above GLP-1 alone.
- Pancreatic alpha cellsModest stimulation of glucagon release in low-glucose conditions. This is the opposite of GLP-1, which suppresses glucagon. The two hormones balance each other on the alpha cell, which may be one reason that combining them does not blunt insulin sensitivity.
- Adipose tissueGIP receptors on white adipocytes regulate fat storage and lipid handling. Animal models suggested this drove weight gain, but the human data with tirzepatide point to a more nuanced role: in combination with GLP-1, GIP appears to support energy expenditure rather than accelerate fat accumulation.
- Central nervous systemGIP receptors in the hypothalamus and brainstem appear to amplify GLP-1's appetite-suppressing effects. Recent research suggests GIP agonism modulates the satiety circuits in ways that GLP-1 alone does not reach, which may explain the additive weight loss seen with dual agonism.
- Bone tissueGIP receptor activity has effects on bone turnover, with some evidence of improved bone density. This is mostly a research observation rather than a clinically actionable point, but it is one of the few tissue effects clearly distinct from GLP-1.
The receptor distribution explains why GIP-only therapy has never produced the kind of weight loss that GLP-1 agonists do. GIP lacks the gastric-emptying effect that drives much of the satiety experience with GLP-1 drugs, and its central nervous system reach is narrower. GIP appears to be an amplifier rather than a primary signal. Activating it without GLP-1 in the background does not produce the same downstream effect.
Why GIP was misjudged for two decades
The case against GIP came from rodent genetics. In 2002, a landmark paper from Yutaka Seino's lab (Miyawaki et al., Nature Medicine, PMID: 12068290) showed that mice lacking the GIP receptor were protected against diet-induced obesity. The animals ate the same high-fat diet as wild-type controls but gained less weight and stored less fat. Subsequent papers reinforced the picture. GIP looked like a fat-storage signal, and the obvious therapeutic implication was to block the receptor, not activate it.
Several pharmaceutical programs pursued GIP receptor antagonists in the 2000s and 2010s. None produced clinically meaningful weight loss in humans. Around the same time, a parallel line of research showed that pure GIP agonists did not produce weight gain in human volunteers, contradicting the rodent prediction in the opposite direction. The picture was inconsistent enough that the field began to suspect a species difference between rodents and humans in how GIP affects energy balance.
Tirzepatide settled the question pragmatically. Eli Lilly's dual agonist entered Phase II for type 2 diabetes in 2017, and SURPASS-2 (Frías et al., NEJM, 2021, PMID: 34170647) reported 11 to 13% weight loss as a secondary endpoint, outperforming semaglutide 1mg in the same trial. SURMOUNT-1 (Jastreboff et al., NEJM, 2022, PMID: 35658024, NCT04184622) extended this in a dedicated obesity population, reaching 22.5% at 15mg over 72 weeks. The data were unambiguous: GIP agonism, in combination with GLP-1, increases weight loss in humans rather than decreasing it.
The two incretins side by side
The mechanism for the species difference is still being worked out. One hypothesis is that the rodent knockout effect was driven by developmental compensation rather than adult GIP signalling. Another is that GIP receptor distribution in human adipose tissue and brain differs enough from mouse that the same receptor produces different net effects on energy balance. The clinical bottom line does not depend on which hypothesis turns out to be correct: in humans, dual GLP-1/GIP agonism reliably outperforms GLP-1 alone.
What the receptor selectivity table looks like
Most patients and prescribers do not need to know the molecular details. The relevant question is what each approved or investigational compound does at the receptor level, because that determines weight loss, side-effect profile, and dosing schedule. The table below summarises receptor coverage across the class.
| Compound | GLP-1 | GIP | Glucagon | Top-dose loss |
|---|---|---|---|---|
| Liraglutide | Yes | No | No | 8.0% |
| Semaglutide | Yes | No | No | 14.9% |
| Tirzepatide | Yes | Yes | No | 22.5% |
| Retatrutide | Yes | Yes | Yes | 28.7% |
The pattern is consistent. Each receptor added to the molecule adds roughly 5 to 7 percentage points of additional weight loss. GIP contributes the jump from 14.9% to 22.5%. Glucagon contributes the further jump from 22.5% to roughly 28.7%. Whether this additive pattern continues with a fourth receptor (amylin is the most likely candidate) is an open question being tested in early-stage compounds. For more on what the third receptor adds, see Triple vs Dual Agonism.
Side-effect differences between GLP-1 only and dual agonism
The receptor selectivity also shapes tolerability. GLP-1-only compounds (liraglutide, semaglutide) have a characteristic gastrointestinal profile dominated by nausea, vomiting, and constipation. Adding GIP appears to slightly improve GI tolerability in some analyses, though head-to-head data are limited. Tirzepatide trial participants reported nausea at rates broadly comparable to semaglutide, but severity scores and discontinuation rates from GI symptoms have generally been similar or marginally lower.
Adding glucagon (retatrutide) introduces a new signal that GIP does not produce. Dysesthesia, an abnormal skin sensation, was reported at 20.9% in TRIUMPH-4 12mg participants and 0.7% on placebo. This is a Phase III finding that did not appear at the same magnitude in Phase II. It is attributed to glucagon receptor activity in the central or peripheral nervous system. Lilly has stated that dysesthesia does not generally lead to discontinuation, but the signal does not appear in the GLP-1 + GIP combination, which suggests the third receptor brings its own mechanistic footprint.
The takeaway is that receptor coverage trades efficacy for breadth of effect. More receptors mean more weight loss and more potential off-target signals. The dual GLP-1 + GIP combination is currently the sweet spot for most clinical uses: meaningfully more efficacy than GLP-1 alone, no new side-effect class beyond what GLP-1 already produces. For a direct head-to-head of the two leading agents, see Semaglutide vs Tirzepatide.
Frequently Asked Questions
What is the difference between GLP-1 and GIP?
GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide) are both incretin hormones released from the gut after eating. They both stimulate insulin release from the pancreas. The differences are where they come from, what other tissues they act on, and what they do to body weight. GLP-1 is released from L-cells in the lower small intestine and colon, slows gastric emptying, and reduces appetite through receptors in the brain. GIP is released from K-cells in the upper small intestine, has minimal effect on gastric emptying, and acts on adipose tissue and the central nervous system in ways that are still being mapped.
Why was GIP ignored as a weight-loss target for so long?
Early animal work in the 2000s suggested GIP receptor activation promoted fat storage and weight gain. Knockout mouse studies showed GIP-receptor-deficient mice were resistant to diet-induced obesity, which led to a decade of research focused on GIP receptor antagonism rather than agonism. Tirzepatide reversed this assumption. A dual GLP-1/GIP agonist produced more weight loss than GLP-1 alone, suggesting that GIP agonism in humans does the opposite of what the rodent models predicted. The mechanism for this species difference is still under investigation.
Does GIP work without GLP-1?
Probably not for weight loss. Trials of GIP-only agonists have not produced meaningful weight reduction in humans. The current view is that GIP needs GLP-1 in the background to translate its effects into appetite suppression and weight loss. GIP appears to amplify GLP-1's effects in the central nervous system rather than acting as an independent weight-loss signal. Pure GIP agonists may have a role in glucose control and lipid metabolism, but standalone weight-loss efficacy has not been demonstrated.
How does tirzepatide use the GIP receptor?
Tirzepatide is a single peptide that binds both the GLP-1 receptor and the GIP receptor with high affinity. The molecule is structurally based on native GIP, with modifications that give it GLP-1 receptor binding activity in addition to its native GIP binding. Once injected, it activates both receptors at appropriate doses. SURMOUNT-1 reported 22.5% weight loss at 15mg over 72 weeks, compared with semaglutide's 14.9% at 2.4mg over 68 weeks in STEP 1. The roughly 7-percentage-point gap is attributed to the GIP component, though the precise contribution is hard to isolate.
Is GIP agonism safer than GLP-1 agonism alone?
GI tolerability appears slightly better with dual GLP-1/GIP compounds than with pure GLP-1 agonists, though the data are not definitive. Tirzepatide trial participants reported nausea at rates broadly similar to semaglutide, but the severity profile differs in some analyses. There is no signal that adding GIP introduces a new class of side effects. By contrast, adding glucagon (as in retatrutide) introduces dysesthesia, an abnormal skin sensation reported at 20.9% in TRIUMPH-4 12mg participants and 0.7% on placebo. GIP agonism does not appear to carry an equivalent novel signal.
What about pure GIP agonists in development?
Several pure GIP agonists are in early clinical testing for metabolic indications. None has reported weight-loss data competitive with the multi-receptor compounds. The pharmaceutical interest in GIP-only molecules has shifted toward niche metabolic uses (lipid handling, postprandial glucose) rather than weight management. The dominant strategy in the obesity pipeline is multi-receptor combinations: GLP-1 + GIP, GLP-1 + glucagon, or all three together as in retatrutide.
What is glucagon's role in retatrutide and why is it different from GIP?
Glucagon raises blood glucose by signalling the liver to release stored glycogen. Adding it to a GLP-1/GIP molecule sounds counterproductive for diabetes, but glucagon also increases energy expenditure, which adds a thermogenic component that GLP-1 and GIP lack. Retatrutide's roughly 28.7% weight loss in TRIUMPH-4 reflects the addition of this third mechanism. The trade-off is the dysesthesia signal noted above, which is attributed to glucagon receptor activity. For more on this see triple vs dual agonism.
Should I use a GLP-1 agonist or wait for a triple agonist?
Approved options today are tirzepatide (Zepbound, dual GLP-1/GIP) and semaglutide (Wegovy, GLP-1 only). Both are in routine clinical use. Retatrutide (triple agonist) is investigational and has not been approved by any regulator as of May 2026. Lilly has stated NDA submission is expected late 2026, with possible approval in 2027 or 2028. Waiting two to three years for a slightly more effective drug is generally not recommended when current options produce 14 to 22% weight loss with established safety records. Treatment decisions belong with a healthcare provider.
This article is for informational and educational purposes only and does not constitute medical advice. GLP-1 agonists and dual GLP-1/GIP agonists are prescription medications with a range of indications, contraindications, and side effects. Retatrutide is investigational and has not been approved by the FDA, EMA, MHRA, or any other regulatory agency as of May 2026. Consult a licensed healthcare provider before starting, stopping, or changing any medication.
Last updated: May 2026.