GLP-1, GIP, glucagon — the incretin agonist family

GLP-1 (glucagon-like peptide-1) is released from intestinal L-cells in response to nutrient intake. Its receptor (GLP-1R) is expressed on pancreatic β-cells (where it potentiates glucose-dependent insulin secretion), on enteric neurons (slowing gastric emptying), and in CNS nuclei involved in satiety.

GIP (glucose-dependent insulinotropic polypeptide) is released from intestinal K-cells, also nutrient-driven. GIPR is expressed on β-cells, adipocytes, and CNS nuclei. Its role in human obesity has been the subject of decades of debate — historically considered 'pro-adipogenic' in fasting states but insulinotropic and even satiogenic in fed states.

Glucagon is released from pancreatic α-cells under low glucose conditions and acts at the glucagon receptor (GCGR), primarily on hepatocytes (driving hepatic glucose output) but also on adipocytes (lipolysis) and at central sites (energy expenditure).

An incretin agonist's pharmacology — and the experimental questions it can answer — is determined by which subset of these three receptors it engages and with what relative potency.

Semaglutide is a structural analog of native GLP-1(7-37) with two amino-acid substitutions and a C18 fatty-acid attachment that enables albumin binding for a once-weekly plasma profile. It is the most pharmacologically straightforward of the three compounds — GLP-1R only.

In research terms, Semaglutide is the comparison anchor. Effects attributed to 'incretin biology' broadly can be tested against Semaglutide as a single-receptor reference. If Tirzepatide or Retatrutide produces an effect Semaglutide does not, that effect is unlikely to be downstream of GLP-1R alone.

Tirzepatide is a single 39-amino-acid molecule that engages both GIPR and GLP-1R, with biased potency favouring GIPR. Comparative clinical data versus Semaglutide shows greater body-weight and HbA1c effects at equivalent dosing schedules.

The mechanistic interpretation of this is contested in the literature. Three competing hypotheses appear: GIPR agonism adds independent satiogenic / metabolic effects; GIPR agonism is functionally antagonistic (some research groups argue chronic GIPR agonism produces effects more like GIPR knockout); or the additive effect comes from GLP-1R signalling biased differently when delivered alongside GIPR engagement.

Practically: Tirzepatide is the right tool when the research question involves GIPR biology, dual-receptor synergy, or comparative efficacy against GLP-1 mono-agonism.

Retatrutide is a triple agonist at GIPR, GLP-1R, and GCGR. Adding glucagon receptor activity is pharmacologically distinct from adding GIPR — GCGR engagement is expected to increase hepatic glucose output and energy expenditure, which on paper opposes the glucose-lowering goals of GLP-1 agonism but in practice has shown net favourable effects on body weight in early-phase research.

The resolution of that apparent paradox — how a glucagon-receptor-agonist component is compatible with glucose-lowering — is one of the most active questions in current metabolic research. Possibilities include preferential lipid mobilisation over glucose mobilisation at low GCGR doses, central glucagon effects on energy expenditure dominating the peripheral hepatic effect, or net favourable glucose handling because the GLP-1R component over-compensates the GCGR-driven hepatic glucose output.

Retatrutide is the right tool when the question involves glucagon receptor biology, triple-receptor synergy, or whether the body-weight ceiling for incretin agonism extends further than dual-receptor mechanisms allow.

All three are typically supplied lyophilised and reconstituted in bacteriostatic water. Their long-acting fatty-acid-tail designs mean reconstituted solutions are stable for weeks at 2–8°C in most published handling protocols.

Dosing comparisons in animal-model literature use molar equivalences rather than mass equivalences because the three compounds have meaningfully different molecular weights and receptor potencies. Pulling dose-response curves from one compound and applying them to another is a common methodological error.

Receptor desensitisation kinetics differ between GIPR, GLP-1R, and GCGR — chronic-dosing studies should account for receptor downregulation differently for each compound. The literature is fullest for GLP-1R (decades of data) and thinnest for GCGR.