Tirzepatide: What the GIP Receptor Adds to GLP-1

How does tirzepatide work? It activates the GLP-1 receptor and the GIP receptor at the same time. GIP contributes to nutrient handling and insulin sensitivity through a pathway distinct from the GLP-1 one, and in trials the combination of the two produced larger weight reductions than mono-agonism.

The molecule is approved by several agencies under the trade names Mounjaro® and Zepbound®, depending on the indication. That approved product and a research compound carrying the same chemical name are not the same thing: they differ in chain of custody, quality control, and legal framework.

What GIP is, and why it was ignored for decades

GIP (glucose-dependent insulinotropic polypeptide) was the first incretin described, before GLP-1. The K cells of the duodenum and jejunum secrete it when they detect nutrients.

Its fall from favor has a concrete explanation: in people with type 2 diabetes, the insulinotropic effect of GIP is markedly reduced. Giving GIP to that population did little. For years that was enough to file it away as the irrelevant sibling of the incretins, and drug development concentrated on GLP-1.

The GIP agonism paradox

The surprise came from combining them. Adding GIP agonism to a GLP-1 agonist produced more effect than GLP-1 alone, even in populations where GIP on its own did almost nothing.

This is where the question of how does tirzepatide work stops having a tidy answer. The mechanism is still under discussion, and saying so is better than offering a clean explanation that has not been established. The hypotheses on the table:

• Direct action on adipose tissue. The GIP receptor is expressed in adipocytes and could influence lipid handling and local insulin sensitivity.
• Complementary central action. GIP receptors in the central nervous system could modulate appetite through a route separate from the GLP-1 one.
• Better tolerability. The GIP component could blunt the nausea associated with GLP-1, allowing higher doses of the GLP-1 component than would be tolerable on its own.

That third hypothesis is interesting because it would imply that part of the added effect comes not from GIP itself but from being able to give more GLP-1. Telling the three explanations apart calls for purpose-built studies, and those have not settled the question yet.

The SURMOUNT program: what it measured, and in whom

SURMOUNT-1 is the reference trial in obesity, published in the New England Journal of Medicine in 2022:

• Population: adults with obesity or overweight, without type 2 diabetes.
• Doses: 5, 10, and 15 mg weekly, subcutaneous.
• Duration: 72 weeks.
• Result: mean reduction between 16.0% and 22.5%, depending on dose and estimand.

That range covers two different analyses of the same trial. The efficacy estimand measures those who completed as planned; the treatment-regimen estimand includes everyone assigned, dropouts included. Quoting the high end of one next to the low end of the other produces false comparisons, and it is a frequent error in the sector's commercial material.

SURMOUNT-5 later supplied something that almost never exists: a head-to-head comparison with semaglutide, at 72 weeks, published in 2025. That is the kind of trial that lets you compare two molecules for real, as opposed to the usual practice of setting numbers from separate studies side by side.

Adverse effect profile versus mono-agonism

Adverse effects with tirzepatide are mostly gastrointestinal (nausea, diarrhea, vomiting, constipation), dose-dependent, and concentrated in the escalation phase.

The interesting question is whether the GIP component improves tolerability relative to GLP-1 alone. The honest answer is that the available data do not allow a clean claim either way: gastrointestinal event rates with tirzepatide are not markedly lower, and the comparison is complicated because equipotent doses of each molecule are not clearly defined.

What is established is the timing pattern, common to the whole class: events cluster during dose escalation and decline on maintenance. The mechanism and the per-trial rates are in the article on gastrointestinal effects, and the reasoning behind the escalation schedule is in the one on titration.

Where tirzepatide stands now that triple agonism exists

With the phase 3 readouts for retatrutide reported, the temptation is to line the molecules up on a ladder. Resist it, for two concrete reasons.

No direct comparison exists. There is no head-to-head trial between tirzepatide and retatrutide. The figures from SURMOUNT-1 and TRIUMPH-1 come from different populations, durations, and estimands.

More receptors means more dropouts. In TRIUMPH-1, discontinuation due to adverse effects at the high dose was 11.3%. A molecule with more average effect and more intolerance is not automatically "better": it is a different profile.

And one difference weighs more than any percentage: tirzepatide is approved and retatrutide is not. An approved product has an evaluated dossier, a label, pharmacovigilance, and a manufacturer who answers for it. That is not an administrative detail. The full picture of the class is in the pillar on mono, dual, and triple agonism, and the analytical data for the research material are in its technical sheet.

What the evidence does not settle

Exactly what GIP contributes. The three hypotheses above have still not been told apart, so part of how does tirzepatide work remains open. That is a notable mechanistic gap in an already approved molecule.

Maintenance after withdrawal. As with the rest of the class, weight regain after discontinuation is the least cited and most relevant figure. It has its own article.

The composition of the weight lost. The proportion of lean mass within the total reduction is a limitation of the entire class and is poorly characterized. It is covered in the article on muscle mass.

These figures come from clinical research protocols conducted under medical supervision. They are not a usage recommendation, nor a protocol applicable outside that context.

This content is strictly educational and does not constitute medical advice, diagnosis or a therapeutic recommendation. The compounds mentioned are research products (Research Use Only) and are not approved by INVIMA, FDA, EMA or ANSM for therapeutic use in humans. Any health-related decision should be made with a licensed medical professional.