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

Retatrutide produces more total weight loss than any GLP-1 drug currently available, and that single fact changes how you have to think about everything that follows.

At 48 weeks on the 12 milligram dose, people lost an average of 24.2 percent of their body weight. For someone starting at 100 kilograms, that is roughly 24 kilograms gone. That number is larger than what tirzepatide produces, larger than what semaglutide produces, and it comes from a drug that hits three receptors instead of two, targeting something called the glucagon receptor in addition to the GLP-1 and GIP receptors that tirzepatide already activates.

The glucagon receptor piece is where the theory got interesting, and where the scan data got complicated.

The working assumption in the field was that adding glucagon receptor activation would shift the body toward burning fat preferentially. Glucagon does several things in the body, but one of its known functions is increasing something called thermogenesis, which is heat production driven by fat oxidation. The idea was that a drug hitting the glucagon receptor would essentially tell your metabolism to reach for fat stores first, protecting your lean tissue in the process.

The Lancet just published the first DXA data on retatrutide that lets us test that assumption directly.

DXA, which stands for dual-energy X-ray absorptiometry, is the current gold standard for separating fat mass from lean mass in the body. It uses two different X-ray energies to distinguish bone, fat, and everything else, and it gives you compartment-level data that a standard scale cannot. The substudy scanned 103 people with type 2 diabetes at baseline and again at 36 weeks.

At the 12 milligram dose, total fat mass dropped by 23.2 percent and visceral fat, which is the metabolically dangerous fat that accumulates around the organs inside your abdomen, dropped by 31.4 percent. Those are substantial reductions in the compartments that actually drive metabolic risk.

Now here is where the ratio comes in.

Of the total weight lost, roughly 75 to 80 percent was fat mass and 20 to 25 percent was lean mass. When you read that, your first instinct might be to think that sounds reasonable, and you would not be wrong. Tirzepatide produces almost exactly the same split, around 75 percent fat to 25 percent lean according to the SURMOUNT-1 body composition substudy. Semaglutide is actually worse on this metric, losing closer to 60 percent fat and 40 percent lean, meaning a larger fraction of what you lose on semaglutide is tissue you wanted to keep.

So on a percentage basis, retatrutide looks comparable to tirzepatide and better than semaglutide.

But percentages can hide what absolute numbers reveal.

When you are losing 24 kilograms total and 20 to 25 percent of that is lean mass, you are losing somewhere between 4.8 and 6 kilograms of lean tissue. That is over 13 pounds. The ratio looks fine on paper because both the fat loss and the lean loss are large, but the lean loss in absolute terms is higher than you would see on a drug producing less total weight loss, even if that drug had a worse fat-to-lean ratio.

Think of it this way. If you have a factory running at higher capacity, even an efficient factory produces more waste than a smaller factory running at the same efficiency level. The efficiency ratio does not tell the whole story when the scale of the operation changes.

This is also where the glucagon receptor hypothesis runs into the data. The assumption was that glucagon activation would shift the ratio, not just maintain it. The scan data shows the ratio is comparable to tirzepatide, which does not activate the glucagon receptor at all. The glucagon component does appear to increase energy expenditure, and research published in Cell Reports Medicine showed that glucagon receptor activation upregulates amino acid catabolism in the liver, meaning the body is breaking down amino acids for fuel at a higher rate, which may actually work against lean mass preservation rather than for it.

That is a meaningful distinction. More energy expenditure sounds like a good thing, and for fat loss it is. But if the pathway driving some of that expenditure involves burning amino acids, you need more dietary protein in the system to offset what is being broken down, not less.

The practical implication is direct.

If you are on retatrutide and you are not doing resistance training at least three to five times per week, the lean mass loss that shows up in those scans is going to come substantially from muscle. Lean mass includes muscle, connective tissue, water, and organ mass, and the drug has no mechanism that preferentially protects the muscle fraction of that. The drug does not know the difference between skeletal muscle you have built over years of training and the lean tissue your body finds easiest to break down.

Protein intake matters here for a specific reason. When the glucagon receptor is activated and hepatic amino acid catabolism goes up, the body has a higher demand for amino acids as a substrate, and if dietary protein is insufficient, it will pull from muscle protein to meet that demand. Targeting at least one gram of protein per pound of goal body weight gives the body a substrate pool to work with before it reaches into your muscle.

The resistance training piece works through a different mechanism. Mechanical loading of muscle tissue creates what is called an anabolic signal, essentially a message to the muscle cells to preserve and rebuild protein, and that signal competes directly with the catabolic signals driven by caloric restriction. Without that signal, caloric restriction tilts the balance toward breakdown.

Retatrutide does not turn off the part of the system that breaks down muscle during weight loss. It accelerates the rate of weight loss, and that acceleration applies to every compartment, including lean mass, unless you give your body a specific reason to protect that compartment.

The glucagon receptor was supposed to be that reason. The scan data says it is not sufficient on its own.

The drug handles the scale. What happens to your body composition at that scale is still largely determined by what you do in the gym and on your plate.

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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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