Understanding GIP biology

The forgotten incretin

For most of the modern history of type 2 diabetes and obesity pharmacology, the incretin hormone that captured everyone's attention was GLP-1 (glucagon-like peptide-1). GLP-1 was the target for the whole first generation of incretin drugs — exenatide, liraglutide, semaglutide, dulaglutide — because it produced robust insulin-secretion enhancement in a glucose-dependent way, delayed gastric emptying, and reduced appetite through central pathways. The other incretin hormone, GIP (glucose-dependent insulinotropic polypeptide), was largely dismissed. Multiple studies in type 2 diabetes patients had shown that GIP infusion in that population did not produce meaningful insulin-secretion enhancement — a phenomenon called 'GIP resistance in type 2 diabetes' — and academic reviews sometimes suggested that pharmacologically-active GIP agonism might be an actively bad idea because of theoretical effects on adipose tissue. The clinical success of tirzepatide changed this framing completely. The GIP story is a case study in how received wisdom about a receptor can turn out to be substantially wrong.

GIP physiology — what the K cells do

GIP is a 42-amino-acid peptide secreted by K cells in the small intestine (predominantly in the duodenum and proximal jejunum) in response to nutrient ingestion, particularly fat and glucose. Its physiological role is to prepare the pancreas and adipose tissue for the arrival of absorbed nutrients. GIP potentiates glucose-dependent insulin secretion — the beta cell responds more strongly to a given glucose level in the presence of GIP than in its absence. GIP also has direct effects on adipose tissue: promoting nutrient uptake and storage, particularly through lipoprotein lipase activation and free-fatty-acid clearance from circulation. In healthy people, GIP is quantitatively the more important of the two incretins for the postprandial insulin response — it accounts for a larger share of the incretin effect than GLP-1 does. That role gets attenuated in type 2 diabetes, which is where the 'GIP resistance' story came from.

Why GIP was dismissed

The critical experimental observation was that GIP infusion in type 2 diabetes patients did not produce the strong insulinotropic response seen in healthy subjects — beta cells appeared to have become less responsive to GIP over the course of hyperglycaemic disease. GLP-1 responsiveness, meanwhile, appeared to be relatively preserved. The straightforward interpretation was that GIP was clinically irrelevant in the target diabetic population and that developing GIP-based drugs would therefore be futile. A parallel concern was theoretical: since GIP promotes fat storage in adipose tissue, sustained GIP receptor activation might promote obesity, which would be counterproductive in a metabolic-disease drug. Multiple academic reviews through the 2000s and early 2010s took this position. It felt like settled science. The clinical trials of tirzepatide — a dual GIP/GLP-1 receptor agonist — produced glucose-lowering and weight-loss effects substantially larger than semaglutide (a pure GLP-1 agonist) at head-to-head-comparable doses, which contradicted the received wisdom directly and forced a reassessment.

The tirzepatide reversal

The SURPASS-2 head-to-head trial of tirzepatide vs semaglutide in type 2 diabetes (Frías 2021 NEJM) is the key data point. At the maximum tirzepatide dose (15 mg weekly), HbA1c reduction was 2.30% vs semaglutide 1 mg at 1.86%. Weight loss was 11.2 kg vs 5.7 kg. Effects at other tirzepatide doses were smaller but consistently larger than semaglutide across the trial. In SURMOUNT-1 (obesity without diabetes), tirzepatide 15 mg produced mean weight loss of ~22.5% at 72 weeks. These are clinically meaningful differences that cannot be explained by GLP-1 activity alone — tirzepatide has approximately fivefold selectivity for GIP receptor over GLP-1 receptor. The most plausible interpretation is that GIP receptor agonism, when combined with GLP-1 receptor agonism, produces effects on glucose metabolism, weight regulation, and possibly other pathways that are not simply additive. GIP agonism appears to contribute meaningfully to the clinical response rather than being either irrelevant (the pre-tirzepatide view) or actively harmful (the theoretical concern).

The mechanism debate is still open

Why GIP receptor agonism helps rather than harms in the tirzepatide context is an active research question. Several hypotheses exist. Perhaps chronic GLP-1 receptor activation restores GIP responsiveness in beta cells (removing the 'GIP resistance' phenomenon). Perhaps central GIP receptor activity contributes to appetite regulation in ways parallel to GLP-1's central effects. Perhaps the adipose-tissue effects of GIP are more nuanced than the older 'GIP promotes obesity' framing captured. Perhaps some combination of all three is at work. The bottom line is that tirzepatide's clinical effect exceeds what pure GLP-1 agonism would predict, and GIP receptor activation is doing something clinically valuable — even if the exact mechanism is still being clarified. Retatrutide extends this thinking by adding glucagon receptor agonism to the GIP+GLP-1 combination; the phase-2 results (Jastreboff 2023 NEJM) suggest triple agonism produces even larger weight loss, but phase-3 confirmation is ongoing.

Common questions

References

• Tirzepatide versus semaglutide once weekly in patients with type 2 diabetes (SURPASS-2)· Frías JP, Davies MJ, Rosenstock J, et al. · 2021
• Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1)· Jastreboff AM, Aronne LJ, Ahmad NN, et al. · 2022
• Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial· Jastreboff AM, Kaplan LM, Frías JP, et al. · 2023

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