Where this page came from. It began as my write-up of FDN’s GLP-1 Practitioner Week (14–18 September 2026). The event has finished. Three questions it prompted come up with clients all the time, so I’ve kept them here, each checked against the source. I am an FDN practitioner and an FDN affiliate; the one affiliate link on this page is marked.
Three claims, and where each stands
Graded, with sources
The same standard I would hold a supplement company to. These are the three claims most likely to come up when you work with someone on a GLP-1.
1. “One in eight American adults is currently taking one”
Evidence: good survey data, and newer than it looks. KFF Health Tracking Poll released 14 November 2025, fielded 27 October to 2 November 2025, 1,350 US adults, margin of error ±3 percentage points: 12% say they are currently taking a GLP-1, 18% say they have ever taken one.
The trap. An earlier KFF poll from May 2024 found that one in eight adults had ever taken one, with 6% currently taking. Both produce a “one in eight” headline and they mean different things. If anyone challenges you, the citation is the November 2025 poll, not the 2024 one. It is also self-reported, and it is a US figure — UK prevalence is materially lower.
2. “These drugs cause muscle loss”
Evidence: a real effect, and the usual framing of it is wrong. The SURMOUNT-1 DXA substudy scanned 160 participants at baseline and week 72. On tirzepatide, body weight fell 21.3%, fat mass 33.9% and lean mass 10.9%. Roughly 75% of the weight lost was fat and 25% was lean.
But here is the part that gets left out: the split was about 75/25 in the placebo arm too. So the honest statement is not “GLP-1s strip muscle” — it is that losing weight costs you lean mass at roughly that ratio however you do it, and these drugs make people lose a great deal of weight. That is still a strong argument for resistance training and adequate protein alongside. It is a much weaker argument for the drug being uniquely catabolic.
Caveats worth carrying: n = 160 out of 2,539 in the parent trial, tirzepatide specifically rather than GLP-1 agonists as a class, 72 weeks, and DXA cannot distinguish muscle from other lean tissue. The substudy was funded by Eli Lilly, which makes tirzepatide, and most authors are Lilly employees. Based on an article retrieved from PubMed — Look M et al., Diabetes, Obesity and Metabolism 2025;27(5):2720–2729, DOI 10.1111/dom.16275.
3. “Genetics can tell us about weight regulation”
Evidence: association at population level, poor prediction for the individual in front of you. Common variants associated with body weight are real and replicated, but each contributes very little, and polygenic scores built from them explain only a modest fraction of variation between people. They are useful for understanding biology and close to useless for telling one client what will happen to them.
So if a genetic test is offered to you as a way of predicting how a client will respond to a GLP-1, that is further than the data goes.
If you are in the UK, read this bit
Because most of the scope advice online is written for America
A lot of practitioner training talks about accessing functional labs without a medical licence. That framing is a US one, and it does not transfer cleanly.
In Britain, “nutritionist” is not a protected title and “dietitian” is. Direct-to-practitioner lab access exists and I use it every week — but what you may then say about a result is governed by advertising and consumer-protection rules, not by whether a lab will sell you a kit. You do not diagnose, you do not treat disease, and you do not tell someone to change a prescribed medication. Those are the lines, and they are not the same lines American training describes.
The clinical content transfers. The regulatory content does not. Take the first and check the second against where you practise.
What this means in practice
For practitioners, not prescribing advice
- Protein and resistance training are the practitioner’s territory. Losing a lot of weight costs lean mass whatever the method; these drugs cause a lot of weight loss. That is the argument, and it is strong enough without overstating it.
- Medication decisions stay with the prescriber. You do not advise starting, stopping or changing the dose, in the UK or anywhere else.
- Treat genetic “responder” claims as unproven until someone shows prediction in individuals, not association in populations.
If you are not a practitioner and you are on a GLP-1 yourself, the prescribing conversation belongs with your doctor. If you want the nutrition side worked through properly, that is what a consultation is for.
If you want the lab reasoning in writing
Mine, not an affiliate link
Reading blood chemistry functionally, working a case from markers rather than symptoms — is what I have written down properly in the TDG Clinical Manual: thirteen modules, around eight hours of reading, £97. The £97 comes off the TDG Five-Test Programme if you enrol as a client within 60 days of buying the manual. It isn’t credited against single tests or combined with the retest discount.
Module 01 is free and complete.
Not an affiliate link — this one is mine
Occasional notes for practitioners, written from the UK
GLP-1s, labs, UK scope, and corrections when the evidence moves or I change a position. Not an automated sequence: I send something when there is something worth sending, and you can leave at any time.