What Reta Actually Does To Your Muscle (First DXA Data)

Retatrutide produces more total weight loss than any other drug currently in trials, which sounds like the headline. But the first DXA scan data published in The Lancet tells a more complicated story underneath that number, and if you are using this drug, you need to understand what is actually happening to your body composition.

Start with the full chain so you have the map.

When you lose weight, you are not just losing fat. You are losing a mix of fat mass and lean mass, and lean mass includes muscle, water, connective tissue, and organ tissue. The ratio between those two determines whether the weight you lost was the weight you actually wanted to lose. A drug that drops 20 kilograms but takes 8 of those from muscle has done something very different than a drug that drops 20 kilograms and takes 5 from muscle, even though the scale shows the same number. That ratio is what body composition scans measure, and it is the only way to know what the weight loss is actually made of.

DXA, which stands for dual-energy X-ray absorptiometry and works by sending two low-dose X-ray beams through the body to distinguish fat tissue from lean tissue and bone, is the gold standard for this measurement. It is not perfect, but it is far more precise than scale weight alone.

The retatrutide substudy scanned 103 people with type 2 diabetes at baseline and again at 36 weeks, which gives the first real window into what this drug does beyond total weight.

At the 12 milligram dose, total fat mass dropped by 23.2 percent and visceral fat, which is the metabolically dangerous fat stored around the abdominal organs rather than under the skin, dropped by 31.4 percent. Those are meaningful reductions, and the visceral fat number is particularly relevant because visceral fat is more strongly associated with cardiovascular and metabolic disease than subcutaneous fat.

Now here is where the ratio matters.

Of the total weight lost, roughly 75 to 80 percent came from fat and 20 to 25 percent came from lean mass. That ratio is actually comparable to tirzepatide, which shows approximately the same 75 to 25 split in the SURMOUNT-1 body composition substudy. Semaglutide performs worse on this metric, with roughly 60 percent fat and 40 percent lean mass, meaning for every kilogram lost on semaglutide, more of it is coming from muscle. So on a percentage basis, retatrutide looks reasonable.

But percentage is not the whole picture, and this is the part that changes how you should think about it.

Retatrutide at 12 milligrams produces around 24 kilograms of total weight loss at 48 weeks in the phase 2 trial. Apply the 20 to 25 percent lean mass figure to 24 kilograms and you arrive at roughly 5 to 6 kilograms of lean tissue lost. That is over 13 pounds of lean mass gone over the course of the treatment. The ratio looks acceptable in a table. The absolute number is significant in a body.

This is what scale math conceals. Two drugs can share the same fat-to-lean ratio and still produce very different absolute outcomes if one of them moves total weight further, and retatrutide moves total weight further than anything else currently available.

There was a theory about why retatrutide might outperform on muscle preservation, and it is worth addressing directly because it has circulated widely.

Retatrutide is a triple agonist, meaning it activates three receptors: GLP-1, GIP, and glucagon. The GLP-1 and GIP components drive satiety and insulin response. The glucagon receptor component was theorized to preferentially shift the body toward fat oxidation, meaning it would direct the energy deficit toward burning fat rather than breaking down protein, which would produce a better lean mass ratio than drugs without glucagon activity.

The mechanism for this theory is real. Research on glucagon receptor activation does show it upregulates hepatic amino acid catabolism and shifts energy expenditure, which on paper could theoretically spare muscle by driving the deficit more toward fat. The theory had a biological basis.

The DXA data does not support it in practice. The lean mass ratio on retatrutide is not meaningfully better than tirzepatide, which has no glucagon component. Whatever the glucagon receptor is doing to energy expenditure, it is not producing a clinically meaningful advantage in muscle preservation over the comparator that shares two of the three receptor targets.

This matters because it closes a door that was still open. People assuming the glucagon component would protect their muscle should now treat that assumption as incorrect.

What the data does confirm is that the drug produces substantial fat loss, including preferential reduction in visceral fat, but it will not protect your lean mass beyond what the overall fat-to-lean ratio allows, and that ratio at the scale of weight loss retatrutide produces still results in a meaningful absolute lean mass reduction.

The practical translation is direct. Resistance training at three to five sessions per week creates a mechanical signal that tells the body to preserve and build muscle tissue, and dietary protein, at least one gram per pound of goal body weight per day, provides the raw material to do that. Neither of those things can be replaced by the pharmacology. The drug reduces appetite and creates the caloric deficit, but it has no mechanism to selectively route that deficit away from your muscle. That requires the training stimulus and the protein intake.

The number to keep in mind is 6 kilograms. That is roughly what the average person at this dose loses from lean tissue if they are not actively working against it.

The drug produces the most weight loss of anything in its class. That is also exactly why the muscle loss number is the one worth watching.

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References

1. Retatrutide Body Composition Substudy, 2025, The Lancet Diabetes & Endocrinology, Vol 13(8), 674-684. Phase 2, 42 centers, 103 DXA-completed participants with T2D. Fat mass reduction: 23.2% at 12mg. Lean mass: 20-25% of total weight lost. Visceral fat: -31.4% at 12mg.
2. SURMOUNT-1 Body Composition Substudy (Tirzepatide), 2025, Lancet Diabetes Endocrinol. PMC11965027. ~75% fat / 25% lean mass ratio.
3. STEP 1 Body Composition Substudy (Semaglutide), PMC8089287. ~60% fat / 40% lean mass ratio.
4. Jastreboff et al., 2023, NEJM. Retatrutide phase 2 trial (N=338): 24.2% weight loss at 48 weeks (12mg). DOI: 10.1056/NEJMoa2301972.
5. Sherwood et al., 2022, Cell Reports Medicine. GCGR activation upregulates hepatic amino acid catabolism, coupled to energy expenditure increase. PMC9729826.

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