GLP-1 → GIP → Glucagon:
How the Weight-Loss Evidence Changed

Semaglutide reached 14.9% mean weight loss in STEP-1. Tirzepatide reached 20.9% at its highest dose in SURMOUNT-1. Retatrutide reached 24.2% at its highest dose in a Phase 2 obesity trial. One receptor target, then two, then three. It is an unusually memorable sequence.

It is also easy to overread. These were separate studies, not three arms of one experiment. They tested different molecules, doses and durations. The results tell us what each drug achieved in that setting; subtracting the percentages does not tell us how much weight loss GIP or glucagon contributed.

The interesting story is not simply that more receptors mean more weight loss. It is how multi-receptor drugs expanded the field, and where the evidence stops short of the neat explanation.

Semaglutide activates the GLP-1 receptor. Its effects include reduced appetite and glucose-dependent insulin secretion. Delayed gastric emptying is part of GLP-1 pharmacology, but reducing long-term weight loss to a permanently “slow stomach” misses the role of appetite regulation.

STEP-1 randomized 1,961 adults with overweight or obesity, without diabetes, to weekly semaglutide 2.4 mg or placebo alongside lifestyle intervention. At 68 weeks, mean body-weight change was −14.9% versus −2.4%. Gastrointestinal effects were common, and nausea and diarrhea were usually transient.

That result made a persuasive case for GLP-1-based obesity treatment. It did not identify a biological ceiling for the receptor. A result at one dose, in one study, cannot establish the maximum possible effect of every GLP-1 drug or formulation.

Tirzepatide is a single molecule that activates both GIP and GLP-1 receptors. GIP is another incretin hormone involved in glucose-dependent insulin secretion. How its activation interacts with GLP-1 signaling in the brain and other tissues is more complicated than “store less fat.”

SURMOUNT-1 enrolled 2,539 adults with overweight or obesity and no diabetes. At 72 weeks, mean weight loss was 15.0%, 19.5% and 20.9% with weekly 5, 10 and 15 mg doses, respectively, versus 3.1% with placebo. These are treatment-regimen-estimand results, intended to estimate treatment effects regardless of discontinuation.

That estimand is not simply a raw average of everyone who started. Missing outcomes still require statistical handling. It also differs from an efficacy estimand, which asks about the effect under specified treatment-adherence assumptions.

Gastrointestinal events, especially during dose escalation, remained prominent. SURMOUNT-1 supports the efficacy of tirzepatide as a dual agonist; it does not isolate GIP’s contribution or prove a better nausea-to-weight-loss trade-off than semaglutide in a different trial.

We no longer need STEP-1 versus SURMOUNT-1 alone to compare these two drugs. SURMOUNT-5 directly randomized 751 adults with obesity and without type 2 diabetes to maximum tolerated tirzepatide or semaglutide doses for 72 weeks.

Mean weight change was −20.2% with tirzepatide and −13.7% with semaglutide, a difference of 6.5 percentage points. The open-label trial used tirzepatide 10 or 15 mg and semaglutide 1.7 or 2.4 mg.

This is stronger evidence of a drug-level difference than comparing unrelated placebo-controlled trials. It still is not an experiment in which investigators added GIP activity to an otherwise identical molecule. Dose, receptor balance and other pharmacological differences remain bundled together.

Retatrutide activates GLP-1, GIP and glucagon receptors. Glucagon can increase hepatic glucose output and influence energy metabolism, making it an intriguing but not automatically beneficial addition. A multi-agonist has to balance these signals rather than simply bolt a fat-burning switch onto an appetite drug.

In the 338-participant Phase 2 obesity trial, the highest maintenance dose, 12 mg weekly, produced 24.2% mean weight loss at 48 weeks versus 2.1% with placebo. Gastrointestinal events were dose-related, and dose-dependent increases in heart rate peaked during treatment before declining.

That was a striking result in a shorter trial. It did not establish that glucagon caused the difference from tirzepatide, or that the same person would necessarily lose more with retatrutide. There was no tirzepatide comparison arm.

The evidence has since moved beyond a topline-only description: TRIUMPH-1 was published in the New England Journal of Medicine in September 2026. The historical three-number sequence here deliberately retains the Phase 2 result; it should not be presented as a summary of every current retatrutide study.

Retatrutide remains investigational, not an approved treatment. A research-market vial is not interchangeable with the study product.

GLP-1 provides a well-established route to appetite reduction and glucose-dependent insulin release. GIP adds a second incretin signal. Glucagon introduces additional metabolic effects, including a rationale for increased energy expenditure. Structural research confirms that retatrutide can engage all three receptor targets.

Those facts support the design of a triple agonist. They do not allocate a clinical weight-loss percentage to each receptor. A binding structure cannot tell us how many kilograms came from reduced intake versus increased expenditure in trial participants.

A randomized retatrutide liver-fat substudy also reported substantial reductions in liver fat. That is a measured clinical finding, but it is not proof that glucagon alone caused it: weight loss and the drug’s other receptor actions are potential contributors.

“Eat less plus burn more” is useful shorthand for the development strategy. It becomes misleading when used as a complete explanation of the observed human results. Receptor potency, exposure and signaling balance matter alongside the number of targets.

Keep the population, duration, dose, comparator and estimand attached to every headline percentage. Do not mix a treatment-regimen result for one drug with an efficacy result for another and call it a fair ranking. These doses describe research protocols, not dosing recommendations.

TRIUMPH-5 is the registered direct comparison of retatrutide and tirzepatide. As reviewed on October 29, 2026, its ClinicalTrials.gov record showed no posted results. Until comparative evidence is available, a higher result in another trial remains suggestive rather than a demonstrated win over tirzepatide.

The move from GLP-1 agonism to dual and triple agonism has delivered important weight-loss results. The early trials make that progression easy to see. SURMOUNT-5 adds direct evidence that tirzepatide outperformed the tested semaglutide regimen.

The remaining question is not whether the three drugs activate different receptors. They do. It is how much each receptor contributes within a particular molecule, and whether a difference in weight loss comes with benefits or trade-offs that matter beyond the scale.

Keep the 14.9%, 20.9% and 24.2% figures. Keep the study names and time points beside them. Drop the implication that subtracting one from another has solved the pharmacology.