How can blocking and activating a receptor both cause weight loss?

That is the paradox. Proposed explanations include receptor desensitisation under chronic agonism, opposing roles in different tissues, and effects mainly on tolerability of the GLP-1 component.

What is new about the 2026 study?

It compared both directions in one model under matched conditions, rather than inferring across separate trials of separate compounds.

Does it resolve the question?

No. It is a mouse study, and rodent incretin physiology differs from human. It narrows the hypotheses rather than choosing between clinical strategies.

GLP-1 & Incretin Science

Blocking and Activating the Same Receptor, in One Model

JMWritten & reviewed by Jack Muncaster · Founder, UK PeptidesLast reviewed 2026-08-234 cited sources

Davies and colleagues published a metabolic comparison of GIP receptor agonism against antagonism in male mice in Diabetes, Obesity and Metabolism in February 2026 — the first time the two opposing strategies have been tested side by side in one model rather than inferred across separate human trials.

Key facts

Direct comparison: Davies 2026 (PMID 41287212)
Model: Male mice
Agonism example: Tirzepatide (with GLP-1 agonism)
Antagonism example: Maridebart cafraglutide
Case for agonism: Samms 2025 (PMID 40521890)
Human question: Still unresolved

Why the paradox exists at all

Tirzepatide activates the GIP receptor and produces substantial weight reduction. Maridebart cafraglutide blocks the same receptor and also produces substantial weight reduction. Both are paired with GLP-1 receptor agonism. Opposite manipulations of one target apparently producing the same directional outcome is the kind of result that indicates something in the model is wrong.

Why the existing evidence could not settle it

Every comparison until now was across trials — different compounds, different formats, different doses, different populations, different durations. Cross-trial comparison cannot attribute a difference to the receptor when everything else differs too. That is the same structural problem that makes head-to-heads necessary in the clinical literature.

Research material referenced

Retatrutide 10mg — third-party HPLC tested

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What a matched preclinical comparison adds

One species, one model, both directions, conditions held constant. Davies and colleagues compared the metabolic consequences of agonism and antagonism in male mice, which is the design that can actually isolate the variable. It is the right experiment for the question even though it is not in humans.

The candidate explanations it can discriminate between

Several have been proposed. That sustained agonism desensitises the receptor, making chronic agonism functionally equivalent to blockade. That the two act in different tissues — central versus adipose — with opposite roles. That GIP receptor manipulation in either direction mainly modulates tolerability of the GLP-1 component rather than driving weight loss itself. A matched comparison produces data these hypotheses predict differently.

The case for agonism, stated by its proponents

Samms and colleagues published a contemporary rationale for GIP receptor agonism in obesity in Diabetes in August 2025. Reading it alongside the antagonist clinical programme is the honest way to hold this question — two serious arguments, both with supporting data, and no resolution yet.

What a mouse study does not settle

The human question. Rodent metabolic physiology differs from human in ways that matter for incretin biology specifically, and compounds behave differently across species. This narrows the hypothesis space; it does not tell you which clinical strategy is better, and only a human head-to-head could.

Extended research context

The GLP-1 & Incretin Science deep dive

Deep dive: the two routes to a bigger effect

Every compound trying to beat GLP-1 alone has taken one of two routes. The first adds more receptors from the same hormone family — GIP in tirzepatide, GIP and glucagon in retatrutide. The second adds a non-incretin satiety hormone, which in practice means amylin: CagriSema combines cagrilintide with semaglutide, and amycretin engages both receptors from one molecule. Both routes work, because they recruit signalling pathways that do not fully overlap. Neither has escaped the constraint that binds all of them, which is that gastrointestinal tolerability worsens as effect size grows.

Deep dive: why a percentage is not a result

The most-quoted numbers in this field are the least comparable. REDEFINE 1 reported 22.7% and 20.4% for the same compound in the same trial — the first among participants who adhered to treatment, the second across everyone randomised. TRIUMPH-1 reported 28.3% in an uncomplicated obesity population while TRIUMPH-3 reported up to 22.6% in adults with established cardiovascular disease, using the same compound. Before any two figures can be compared they have to match on estimand, population, duration, comparator and whether the number is placebo-adjusted. Most published comparisons match on none of them.

Deep dive: what happens after the trial stops

Every headline figure describes weight while treatment continues. The STEP-1 extension found that a year after semaglutide was stopped, participants had given back roughly two-thirds of what they lost, moving from 17.3% mean reduction to a net 5.6% — though average weight remained below baseline and nearly half stayed at least 5% down. Meta-analysis puts regain at around 0.8 kg per month. This is why maintenance studies such as TRIUMPH-6 matter more to the field's future than another two points of peak reduction.

Research applications

▸Comparing incretin and amylin compounds on a like-for-like basis
▸Interpreting estimands, thresholds and placebo-adjusted figures in trial reports
▸Tracking the obesity pipeline across sponsors and jurisdictions
▸Understanding receptor pharmacology behind GLP-1, GIP, glucagon and amylin
▸Distinguishing licensed medicines from investigational compounds

Handling checklist

✓Identify which estimand a quoted percentage comes from before citing it
✓Check the trial population and baseline BMI against the comparison you are making
✓Confirm the duration and whether the reduction curve had plateaued
✓Read discontinuation rates alongside efficacy figures
✓Verify every NCT identifier against ClinicalTrials.gov rather than secondary reporting

Common research-handling mistakes

Learnt from thousands of researcher orders across our UK labs.

✗ Comparing headline percentages across different trials

Fix: Population, duration, estimand and comparator all differ; the numbers are not interchangeable.

✗ Quoting the larger of two figures from the same trial

Fix: Name the estimand. Efficacy and treatment-policy answer different questions.

✗ Treating peak reduction as a durable outcome

Fix: Substantial regain follows cessation across the class; peak figures describe a maintained state.

✗ Assuming an oral route means a weaker mechanism

Fix: Route and receptor count are independent. Orforglipron is weaker because it hits one receptor, not because it is a tablet.

✗ Reading investigational compounds as available treatments

Fix: Most of this pipeline holds no authorisation anywhere; mazdutide is approved only in China.

Continue researching

Peer-reviewed guides, comparators and matched reference materials.

Frequently asked questions

How can blocking and activating a receptor both cause weight loss?: That is the paradox. Proposed explanations include receptor desensitisation under chronic agonism, opposing roles in different tissues, and effects mainly on tolerability of the GLP-1 component.
What is new about the 2026 study?: It compared both directions in one model under matched conditions, rather than inferring across separate trials of separate compounds.
Does it resolve the question?: No. It is a mouse study, and rodent incretin physiology differs from human. It narrows the hypotheses rather than choosing between clinical strategies.

Primary sources & clinical trials

Peer-reviewed research and registered trials from PubMed, ClinicalTrials.gov, PubChem, FDA and NIH. All links open in a new tab and point to the primary source, so every claim can be verified at origin.

Written and reviewed by

Jack Muncaster · Founder, UK Peptides

Jack founded UK Peptides in Manchester after repeatedly receiving research compounds with missing or recycled paperwork. He is responsible for supplier selection, batch release decisions and the content published in this research library. Every article here is sourced to primary literature and every product page to a signed third-party certificate.

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