This New Study Changed My Mind on GLP-1s and Muscle Loss
What if GLP-1s aren’t causing muscle loss at all? New data reveals a massive misinterpretation that could change the entire narrative around weight loss drugs and muscle loss.
A new series of studies was just published in Cell Reports Medicine that legitimately changed my mind on this controversial question:
Do GLP-1 weight loss drugs cause disproportionate loss of lean muscle mass?
Honestly, when I read these new data, I felt like a bit of an idiot for not seeing the solution earlier.
My embarrassment, however, was softened by the fact that almost everyone else, including physicians, scientific researchers, and influencers, were making the same mistake.
In this letter, I’m going to show you what that mistake was, how it reframes the entire conversation around GLP-1-based weight loss drugs, and offer practical insights to minimize functional muscle loss while taking incretin-based weight loss drugs.
And, no, the answer is not simply “eat more protein, bro.”
But first, let’s zoom out and add some context.
The Lean Mass Paradox: Why GLP-1 Data Raised Red Flags
In many randomized controlled trials on GLP-1 weight loss drugs, it appears that a disproportionate amount of weight loss comes from lean mass, on the order of 40% of weight loss as lean body mass.
For example, in a recent study on the GLP-1-GIP-Glucagon triple agonist, retatrutide (a.k.a. “reta” a.k.a. GLP-3), lean mass loss was reported at ~38.2% at higher doses.
*My tweet from March 1, 2026, before these new data were published…
By contrast, lean body mass typically contributes ~25% during weight loss driven primarily by lifestyle interventions.
This has even been termed the “quarter fat-free mass rule.”
So, if GLP-1 interventions cause ~40% of weight loss as lean mass vs ~25% due to lifestyle, then GLP-1-based interventions are at least associated with disproportionate muscle mass loss. Right?
Wrong.
And I’ll explain why.
But if we are to, at this point, assess the controversial question: do GLP-1 receptor agonists cause a disproportionately large amount of lean mass loss?
In a word, the data say “yes.”
But that raises deeper questions.
Is this a unique biochemical property of GLP-1s?
Or is it due to lifestyle differences in the populations taking them?
In other words, are people on GLP-1s exercising less and eating less protein compared to those losing weight through diet and lifestyle?
Until now, the answer hasn’t been clear.
The Critical Mistake: What is “Lean Mass?”
Now, here’s one shocking fact: no study had yet directly explored how incretin-based poly-agonists, tirzepatide (GLP-1-GIP) and retatrutide (GLP-1-GIP-Glucagon) affect skeletal muscle.
“No study.”
Not before this one.
But pause. If you’re well-read, you might be thinking: “That’s not true. I’ve seen studies on tirzepatide or retatrutide and lean body mass loss. In fact, Nick, you’ve even written about them!”
If you’re thinking that you’re partially correct, but there’s a crucial nuance here.
And this nuance changes everything.
Lean body mass is not the same thing as skeletal muscle.
Skeletal muscle is part of lean body mass, but lean body mass also includes organs like the liver and other tissues.
On top of that, muscle itself isn’t just contractile proteins; it also contains substrates in the muscle.
And here’s the key limitation: DEXA scans used in the existing human trial literature can’t distinguish between them.
So, a provocative possibility emerges: what if the “muscle loss” we’ve been worried about… isn’t muscle at all?
Because if that’s true, it doesn’t just tweak the story… it completely breaks it.
This StayCurious Metabolism premium letter dives deep into these new findings, extracting practical insights & challenging what we thought we knew.
We’ll cover:
New human data
How we have been misinterpreting existing evidence
How GLP-1s might even support muscle mitochondria
And how to preserve muscle while losing fat
If you’re committed to grappling with the nuance of health and metabolism, I’d love to have you join the StayCurious Metabolism premium community: the fastest-growing premium newsletter on the platform and a Top 3 Global Bestseller in Science.
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