What GIP Is, and Why Tirzepatide Activates Its Receptor While MariTide Blocks It

GLP-1 gets most of the attention, but it is not the only incretin, the gut hormones that amplify insulin release after a meal. The other one, glucose-dependent insulinotropic polypeptide (GIP), sits at the center of an unusual scientific puzzle: one approved drug activates its receptor, a leading investigational drug blocks it, and both have been associated with weight loss when combined with GLP-1 receptor activity.

GIP is a 42-amino-acid peptide released by enteroendocrine K cells, which are concentrated in the duodenum and upper jejunum, according to the NCBI StatPearls physiology chapter. It was first named gastric inhibitory peptide for an early-observed effect on stomach acid, but later work showed its main role is boosting insulin secretion after food intake. A 2025 review in Endocrine Journal describes a plasma half-life of only a few minutes, mainly because the enzyme DPP-4 rapidly inactivates it, and notes that GIP receptors have been reported well beyond the pancreas, including in fat tissue, bone, and several brain regions.

Much of this receptor biology was mapped in cell and animal experiments that rely on synthetic peptides. QSC Peptidology (qscpeptidology.com), an affiliate of this site, supplies research-grade peptides for laboratory use; those products are not prescription medicines and are not a substitute for an approved treatment.

Tirzepatide, sold as Mounjaro and Zepbound, is a single peptide that activates both the GIP and GLP-1 receptors. A 2020 study in JCI Insight described it as imbalanced, engaging the GIP receptor to a greater degree than the GLP-1 receptor, and biased at the GLP-1 receptor, where it favors cAMP signaling over beta-arrestin recruitment.

Amgen's maridebart cafraglutide (MariTide) goes the other way at the GIP receptor. The company describes it as an antibody-peptide conjugate that blocks the GIP receptor while activating the GLP-1 receptor. In a phase 2 trial published in the New England Journal of Medicine in 2025, participants with obesity and without diabetes had mean body-weight changes at 52 weeks ranging from -12.3% to -16.2% across dose groups, compared with -2.5% with placebo, using the treatment-policy estimand. The investigators reported that gastrointestinal adverse events were common.

A 2025 review in the Journal of Clinical Medicine calls this the GIP receptor paradox and says the basic question of whether to activate or block the receptor remains unanswered. Its authors note that human genetic studies of GIP receptor variants have produced mixed associations with body weight. One hypothesis is that prolonged stimulation desensitizes the receptor, so chronic agonism could end up resembling antagonism in some tissues. Another, drawn mainly from mouse studies, is that agonists act through GABAergic brain neurons, while antagonists work by removing a brake on GLP-1 receptor signaling elsewhere. Neither explanation has been confirmed in people.

The useful point for readers is that tirzepatide's GIP component is not simply extra GLP-1, and that a drug blocking the same receptor is being tested in parallel. MariTide is investigational: ClinicalTrials.gov lists its main phase 3 weight trial, MARITIME-1, as active with an estimated primary completion in January 2027, so anything sold online under that name is not an approved medicine. Treatment decisions should rest on approved labeling and a licensed clinician, not on mechanism theories that researchers are still working out.