GLP-1 & Incretin Science
The Method Determines the Number
Body composition is inferred rather than observed. Dual-energy X-ray absorptiometry and bioimpedance rest on different assumptions and produce different answers, and they diverge most during active weight loss when hydration is shifting.
Key facts
- DXA principle
- Differential X-ray attenuation
- Bioimpedance principle
- Electrical resistance of tissue
- Bioimpedance depends on
- Hydration status
- Worst agreement
- During active weight change
- Comparison study
- Vermeiren 2021 (PMID 32917962)
- Neither is
- A direct measurement of muscle
Why nothing here is measured directly
Determining tissue composition directly would require dissection. Every practical method infers composition from a physical property that correlates with it, using assumptions about how tissues behave. Those assumptions are reasonable in stable individuals and get less reliable exactly when body composition is changing — which is when these measurements are wanted.
How DXA works and where it strains
Two X-ray energies pass through the body and are attenuated differently by bone, fat and lean tissue, and composition is calculated from the differential. It is the most widely accepted method and it still assumes a fixed hydration fraction for lean tissue. When hydration changes, that assumption introduces error into the lean mass figure.
Research material referenced
Retatrutide 10mg — third-party HPLC tested
How bioimpedance works and why it is more fragile
A small current is passed through the body and resistance measured. Lean tissue conducts well because it holds water and electrolytes; fat conducts poorly. The method is therefore measuring body water and inferring lean mass from it. Anything altering hydration — a meal, exercise, time of day, a diuretic — moves the reading without any tissue changing.
What the comparison studies show
Vermeiren and colleagues compared bioimpedance spectroscopy against DXA for assessing body composition changes during weight loss in the European Journal of Clinical Nutrition in 2021. Studies of this kind consistently find the methods agree less well for change than for a single snapshot, which matters because change is what weight-loss trials report.
Why this undermines cross-trial comparison
A trial using DXA and a trial using bioimpedance are not producing interchangeable numbers, and a difference between them may reflect instrumentation rather than biology. When lean mass percentages from different trials are placed side by side, the method used is as relevant as the compound — and is usually not mentioned.
What would actually settle a functional question
Function. Grip strength, gait speed, stair climbing and similar measures test whether anything consequential changed, and they do not depend on an inference about tissue compartments. A trial reporting preserved lean mass without functional measures has reported a proxy.
Quick reference
| DXA | Bioimpedance | |
|---|---|---|
| Measures | X-ray attenuation | Electrical resistance |
| Infers lean mass from | Tissue density | Body water |
| Hydration sensitivity | Moderate | High |
| Availability | Clinical/research | Widely available |
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.
Related questions researchers ask
- Which weight-loss compound produces the largest reduction?
- What is the difference between CagriSema and amycretin?
- What is an amylin receptor agonist?
- How much weight is regained after stopping a GLP-1?
- Why does CagriSema report two different percentages?
- Why is orforglipron less effective than retatrutide?
Frequently asked questions
- Is DXA a direct measurement of muscle?
- No. It infers composition from differential X-ray attenuation and assumes a fixed hydration fraction for lean tissue.
- Why is bioimpedance more variable?
- It measures body water and infers lean mass from it, so meals, exercise, time of day and hydration all move the reading.
- Can I compare lean mass figures across trials?
- Cautiously at best. Different methods do not produce interchangeable numbers, and they agree least on change.
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.
- PubMedVermeiren E et al., Comparison of bioimpedance spectroscopy and dual energy X-ray absorptiometry for assessing body composition changes during weight loss — Eur J Clin Nutr 2021 (PMID 32917962)pubmed.ncbi.nlm.nih.gov
- PubMedDubin RL et al., GLP-1 agents and weight loss composition — Diabetes Obes Metab 2024 (PMID 39344838)pubmed.ncbi.nlm.nih.gov
- PubMedStefanakis K et al. — Metabolism 2024 (PMID 39481534)pubmed.ncbi.nlm.nih.gov
- TrialClinicalTrials.gov · TRIUMPH-1 (NCT05929066) — Retatrutide pivotal obesity trialclinicaltrials.gov
- TrialClinicalTrials.gov · TRIUMPH-6 (NCT06859268) — Maintenance of weight reductionclinicaltrials.gov
- RefNovo Nordisk · CagriSema REDEFINE 1, published in NEJMprnewswire.com
- PubMedAmycretin phase 1b/2a subcutaneous study — PubMed (PMID 40550231)pubmed.ncbi.nlm.nih.gov
- RefTrajectory of weight regain after GLP-1 cessation — eClinicalMedicinethelancet.com
- PubMedOrforglipron: A Comprehensive Review — Int J Mol Sci 2026 (PMID 41683830)pubmed.ncbi.nlm.nih.gov
- EMAICH E9(R1) — estimands in clinical trials (EMA)ema.europa.eu
- GuidelineGoogle — Creating helpful, reliable, people-first contentdevelopers.google.com
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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- What Is Actually Registered, and at What PhaseA Phase 3 in 160 adolescents aged 12–17, and a Phase 1 safety study in 28 children aged 6–11. The distinction matters for what results will mean.
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