Lean mass is not muscle
Everyone quotes the same two numbers about weight loss on a GLP-1, and almost nobody says what those numbers actually measured. The distinction decides what you should do about it.
October 28, 2026 · 4min read · Rob Amoroso
You will have seen the numbers. In the semaglutide trial, roughly 39 percent of the weight lost was lean mass. In the tirzepatide trial, roughly 25 percent.
Those figures are real, they come from the manufacturers' own pivotal trials, and you can read them yourself. They are also quoted constantly by people who then say something the trials did not show.
What the scan actually measured
Both figures come from DXA substudies. DXA sorts your body into three buckets: fat mass, bone mineral, and everything else. That third bucket is what gets called lean mass.
Everything else is doing a lot of work in that sentence. Lean mass includes your organs. It includes connective tissue. It includes the water in your body and the glycogen stored in your muscles and liver, which each gram of carries roughly three grams of water along with it.
So when someone drops weight quickly, a meaningful part of the lean mass that disappears from the scan is glycogen and the water bound to it. On a scan that reads as lean mass lost. In your body it is not contractile muscle, and it comes back when you eat carbohydrate again.
That is not a technicality. It is the difference between "the medication is eating your muscle" and "rapid weight loss changes several compartments at once, and one of them is the one you care about."
Why the sloppy version is worse than wrong
If you believe a GLP-1 is stripping muscle at 39 percent, the obvious conclusion is to stop taking it, or not to start.
If you understand that some of that number is water and glycogen, and some of it genuinely is muscle, the conclusion is different and better: the medication is doing its job on appetite, and the muscle part is the part you can act on.
The sloppy version frightens people off something that is working for them. The accurate version tells them what to do on Tuesday.
What you can actually act on
Two things, and the evidence is not seriously disputed on either.
Resistance training. Progressive, covering every major muscle group, two to three days a week. This is the established lever for preserving strength and function during weight loss, and it was established long before anyone was writing about GLP-1 medications. It is the same mechanism in a calorie deficit from any cause.
Adequate protein, spread across the day rather than loaded into one meal, and prioritised on the days when appetite is at its worst. On these medications that is usually the day after a dose, which is exactly when people eat least and most need not to.
Neither of those is exotic. That is rather the point. The interventions are boring and well evidenced, and the hard part was never knowing about them.
The honest answer about building muscle
People want to know whether they can add muscle while losing fat on one of these.
If you are new to lifting, or carrying more body fat to begin with, often yes. If you already train and you are eating in a steep deficit, holding on to what you have is the realistic goal. That is a smaller promise than the one being made elsewhere and it is the true one, and holding your strength through a large weight loss is a better outcome than it sounds.
What I would measure instead
Not the scale, and not a body fat percentage from a machine that changes its mind when you are dehydrated.
Grip strength, with a dynamometer, best of three. A five-times sit-to-stand, timed, same chair. Both are used in sarcopenia consensus criteria, which means two things: they measure function rather than appearance, and your doctor already knows how to read them.
That second point matters more than it looks. A physician cannot interpret an estimated one-rep max from your training log. They can absolutely interpret a grip strength trend, because it is a measure they already use.
The point
Your scan weight is going to fall. Some of what falls will be fat, some will be water and glycogen, and some will be muscle if nothing is done about it. Only the third one is a problem, and it is the only one you have real leverage over.
Train against it, eat enough protein, and measure function rather than mass. Then the weight you lose is the weight you wanted to lose, and what is left when the prescription ends is a body that can do things.
If you are on a GLP-1 medication, or about to start one, here is how I work with people on them.
Sources. Wilding JPH et al., Once-Weekly Semaglutide in Adults with Overweight or Obesity, N Engl J Med, 2021. Jastreboff AM et al., Tirzepatide Once Weekly for the Treatment of Obesity, N Engl J Med, 2022.
I am a coach, not a clinician. I do not advise on dosing, titration, or whether to start or stop a medication. Those decisions belong with your prescriber.