Tirzepatide and the Dual Incretin Mechanism

Tirzepatide is an approved molecule that carries both incretin hormones in a single peptide, and the trial record that followed shows what that pairing does to weight, glucose, and the biology of GIP itself.

I · Two gut hormones and the effect they share

The incretin story begins with a meal, and it begins decades before any of the current drugs existed.

The observation that set this field in motion is the incretin effect, the finding that glucose taken by mouth produces a larger insulin response than the same glucose given by vein, which means the gut itself must be sending signals that amplify the pancreas. ¹ The first of those signals to be named was GIP, discovered as gastric inhibitory polypeptide and later renamed glucose-dependent insulinotropic polypeptide once its insulin-amplifying role came into focus, and Michael Wolfe’s 2025 synthesis in Endocrine Reviews credits GIP with 60% to 80% of the postprandial insulin response. The second was GLP-1, the hormone that regulates appetite and caloric intake, and between them the two incretins frame the metabolic arc of a meal, one hormone governing how much of the meal gets stored and the other governing how quickly the meal stops being sought.

That storage emphasis matters for the rest of this article, because GIP’s classical biology points in a direction a weight-loss drug would normally avoid, and the molecule that eventually pressed GIP into service did so through a mechanism that researchers still argue about.

“Under normal conditions, GIP plays a major role in nutrient deposition and storage.”

Wolfe, Endocrine Reviews, 2025

II · One molecule engineered to hold both keys

Tirzepatide was engineered to engage both incretin receptors at once, and the FDA-approved label states the design in one declarative sentence.

The label describes tirzepatide as a GIP receptor and GLP-1 receptor agonist that selectively binds and activates both targets, and it attributes the molecule’s long presence in circulation to a C20 fatty diacid that enables albumin binding and prolongs the half-life. ² The design intent is visible in the phrase the label uses for the second receptor, since the label notes that the addition of GIP may further contribute to the regulation of food intake, which frames the GIP component as an active contributor. ³ One peptide turning two locks is the easy part of the story, because the harder question is what each lock does once turned, and that is where GIP’s classical biology makes the story interesting.

That balance matters because Willard and colleagues’ 2020 paper in JCI Insight characterized tirzepatide as an imbalanced and biased dual agonist, with greater in vitro potency at the GIP receptor than at the GLP-1 receptor and a signaling bias relative to the native ligands, so the molecule is a deliberate departure from the two hormones it mimics. ⁴ Structural work published in PNAS in 2022 mapped the determinants of that dual engagement, showing how a single peptide chain accommodates both receptor interfaces, which explains why the molecule behaves differently from a simple mixture of the two native hormones. ⁵

“Tirzepatide is a GIP receptor and GLP-1 receptor agonist. It contains a C20 fatty diacid that enables albumin binding and prolongs the half-life. Tirzepatide selectively binds to and activates both the GIP and GLP-1 receptors, the targets for native GIP and GLP-1.”

FDA, Zepbound Prescribing Information, 2026

The design question that follows is what the GIP half of the molecule is doing, since GIP’s classical biology runs in the opposite direction from weight loss, and that paradox is the subject of the next section.

III · The GIP paradox in the weight-loss story

The same physiology that makes GIP the principal incretin also makes it an awkward partner for a weight-loss drug, because its classical role is nutrient deposition.

Michael Wolfe’s 2025 review describes GIP as a hormone of nutrient deposition and storage, so the physiology runs in the opposite direction from fat loss, and the mechanistic literature explains the paradox through a desensitization model, the idea that chronic stimulation eventually turns the signal around. ⁶ The leading statement of that model comes from Killion and colleagues, whose 2020 paper in Nature Communications showed that chronic GIP receptor agonism desensitizes the receptor on adipocytes, so the sustained signal comes to function like a GIP receptor antagonist. ⁷

Jonathan Campbell and colleagues added a localization twist in their 2022 Diabetes paper, finding that GIP receptors in white adipose tissue sit predominantly on pericytes and mesothelial cells, which complicates any simple story about GIP acting directly on fat cells. ⁸ The two findings fit together, since a receptor that desensitizes under sustained stimulation and a receptor that mostly lives on supporting cells both point away from the old picture of GIP as a straightforward fat-storage switch.

“Paradoxically, both enhanced and reduced GIP receptor signaling reduce adipose tissue mass and attenuate weight gain in response to nutrient excess.”

Campbell, Diabetes, 2022

The paradox resolves into a testable hypothesis: chronic GIP receptor stimulation flips a deposition signal into a limiting one, and the obesity trials that followed provided the largest-scale tests of that idea yet conducted.

IV · The 72 weeks that produced two famous numbers

The largest-scale test arrived in 2022, when Ania Jastreboff and colleagues published the 72-week SURMOUNT-1 trial in the New England Journal of Medicine.

The trial randomized 2,539 adults with obesity or overweight, and the intention-to-treat analysis reported mean weight reductions of 15.0% at 5 mg, 19.5% at 10 mg, and 20.9% at 15 mg against 3.1% with placebo at 72 weeks, with a P value below 0.001 for every dose. ⁹ Those are the figures carried in the NEJM abstract and the FDA label, and they show that 57% of participants in the 15 mg group lost at least 20% of their body weight, compared with 3% on placebo, while the proportions losing at least 5% reached 85%, 89%, and 91% across the three doses, against 35% with placebo. ¹⁰

The frequently quoted 22.5% figure comes from the same trial through a different analysis, since the ClinicalTrials.gov results module reports the on-treatment efficacy estimand, which counts only participants who stayed on treatment, and at 15 mg that analysis showed a 22.5% mean weight reduction with 62.9% of participants losing at least 20%. ¹¹ The same results module reported on-treatment means of 16.0% at 5 mg and 21.4% at 10 mg, against 2.4% with placebo, so the on-treatment figure runs more negative at each active dose. The distinction is honest and worth stating plainly: the intention-to-treat analysis includes everyone who was randomized, including those who stopped treatment, so it describes the average experience of the group the trial assigned, whereas the on-treatment analysis describes the people who remained on the drug, which is why the two figures differ by roughly one and a half percentage points at the top dose.

“% change from baseline (least-squares mean): placebo -3.1%; 5 mg -15.0%; 10 mg -19.5%; 15 mg -20.9%.”

FDA, Zepbound Prescribing Information, 2026

Both numbers answer a real question, so the useful habit is to state which analysis is being quoted, and that is the discipline the rest of the trial record follows.

V · Racing the dual agonist against the single one

The cleanest test of a dual mechanism is a head-to-head race against the best single agonist, and the SURMOUNT and SURPASS programs provided two such races.

The SURMOUNT program runs from SURMOUNT-1 through SURMOUNT-5, a five-trial series that includes SURMOUNT-4, and the fifth trial in the series, reported by Louis Aronne and colleagues in the New England Journal of Medicine in 2025, randomized 751 adults with obesity but without type 2 diabetes to the maximum tolerated dose of tirzepatide (10 or 15 mg) or semaglutide (1.7 or 2.4 mg) for 72 weeks. ¹² The least-squares mean weight reduction was 20.2% with tirzepatide versus 13.7% with semaglutide, with a P value below 0.001, and waist circumference fell by 18.4 cm against 13.0 cm, so the margin held across measures, with tirzepatide participants more likely to achieve 10%, 15%, 20%, and 25% weight reductions. ¹³

The trial was open-label, which means both participants and investigators knew the assignment, and that design limitation is disclosed in the paper itself, so the comparison should be read with that transparency in mind. The diabetes-side head-to-head came earlier, when Juan Frías, Melanie Davies, and Julio Rosenstock led SURPASS-2, published in NEJM in 2021, which compared tirzepatide with once-weekly semaglutide 1 mg in people with type 2 diabetes. ¹⁴ The estimated mean change in HbA1c was -2.01 percentage points at 5 mg, -2.24 at 10 mg, and -2.30 at 15 mg, against -1.86 with semaglutide 1 mg, and tirzepatide was noninferior and superior to semaglutide on glycemic control at every dose tested, with weight differences favoring tirzepatide by 1.9, 3.6, and 5.5 kg. ¹⁵

The two trials measure different populations, people with type 2 diabetes in SURPASS-2 and people with obesity without diabetes in SURMOUNT-5, so weight numbers from one should be kept separate from the other, a discipline the literature itself maintains.

VI · What the weight loss is made of and what the heart trial showed

The headline numbers describe how much weight came off, so the DXA substudy and the cardiovascular outcomes trial exist to describe what that weight loss is made of and how the heart fared along the way.

A DXA substudy of SURMOUNT-1, published in 2025 with 160 participants, reported that approximately 75% of the weight lost was fat mass and approximately 25% was lean mass, a ratio matched by the placebo group, and the FDA label states that tirzepatide produces greater fat mass loss than lean mass loss. ¹⁶ The substudy’s raw numbers carried that composition: a 21.3% reduction in body weight built from a 33.9% fall in fat mass and a 10.9% fall in lean mass, against placebo changes of 5.3%, 8.2%, and 2.6%. ¹⁷

Because the substudy enrolled 160 of the 2,539 randomized participants, the ratio carries a small-sample caveat, and a 2025 commentary argues that the trial’s model may underestimate fat-free mass loss, so the honest reading is that fat loss predominates while the exact lean share stays debated. ¹⁸

“Tirzepatide lowers body weight with greater fat mass loss than lean mass loss. Tirzepatide decreases calorie intake.”

FDA, Zepbound Prescribing Information, 2026

The cardiovascular question was answered by SURPASS-CVOT, reported in NEJM at the end of 2025, which randomized 13,299 participants with type 2 diabetes to tirzepatide or dulaglutide, and the primary composite of cardiovascular death, myocardial infarction, or stroke occurred in 12.2% of the tirzepatide group versus 13.1% of the dulaglutide group. ¹⁹ The hazard ratio of 0.92, with a 95.3% confidence interval of 0.83 to 1.01, met the noninferiority threshold with a P value of 0.003, whereas the superiority test returned a P value of 0.09, so the correct phrasing is that tirzepatide was noninferior to dulaglutide on major adverse cardiovascular events, with more gastrointestinal adverse events in the tirzepatide group. ²⁰

The cardiovascular readout clears the bar the trial was built to clear, and the composition data describe a fat-predominant loss, which together leave the mechanism debate as the last open question.

VII · The open arithmetic of the dual mechanism

After five years of trial data, the question that remains open is arithmetic, whether the GIP component adds to GLP-1 or multiplies it.

Khaled El, Jonathan Campbell, and their colleagues showed in Nature Metabolism in 2023 that tirzepatide requires the GIP receptor for hormone secretion from human islets, which means the GIP component is functionally required for the molecule’s insulinotropic effect. ²¹ The weight-loss arithmetic is where the debate lives, since a 2025 review by Yan Jiang, Haofei Zhu, and Feng Gong documents that GIP/GLP-1 dual agonism outperforms GLP-1 mono agonism across the clinical trials, whereas a mouse study published in Nature Metabolism in 2025 describes additive weight loss from combined receptor agonism. ²²

“Nonclinical studies suggest the addition of GIP may further contribute to the regulation of food intake.”

FDA, Zepbound Prescribing Information, 2026

The FDA label itself stays inside that modest register, and the gap between the label’s language and the trial record is the honest summary of the field, since the clinical data show what dual agonism achieves while the mechanism data explain part of why. The open questions are specific: whether the GIP contribution is additive or synergistic at the whole-body level, where the receptor acts given its localization on nonadipocyte cell types, and how the lean mass share should be read, questions the next round of mechanistic and factorial studies will have to answer.