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

Retatrutide is producing the largest weight loss numbers ever recorded in a pharmaceutical trial, and for most people that sounds like a simple win, but the first body composition scan data published in The Lancet changes how you should think about what that weight actually is.

To understand why the data matters, you need the full picture first. When you lose weight on any drug or diet, that weight comes from somewhere, and it is never purely fat. Your body pulls from fat stores, but it also pulls from lean tissue, which includes muscle, bone density, and organ mass. The ratio of fat loss to lean loss tells you the quality of the weight you are losing, not just the quantity. A drug that strips 80 percent fat and 20 percent lean is doing something meaningfully different than one stripping 60 percent fat and 40 percent lean, even if the total weight on the scale looks similar.

That ratio is measured using something called DXA, which stands for dual-energy X-ray absorptiometry and is a body composition scan that distinguishes fat mass from lean mass with much higher precision than the scale or even a DEXA scale at the gym. The Lancet study published in 2025 is the first time this tool has been applied to retatrutide, and it gives us numbers we have never had before.

The study ran 103 people with type 2 diabetes through a 36-week protocol across 42 centers, scanning them at baseline and again at the end. 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 liver and intestines rather than under the skin, dropped by 31.4 percent. Those are large reductions by any standard.

The fat-to-lean ratio came out at roughly 75 to 80 percent fat and 20 to 25 percent lean. When you put that next to the comparison data, it looks like a reasonable outcome. Tirzepatide, from the SURMOUNT-1 body composition substudy published the same year, also lands at about 75 percent fat and 25 percent lean. Semaglutide, from the STEP 1 substudy, runs closer to 60 percent fat and 40 percent lean, which is meaningfully worse in terms of how much lean tissue you sacrifice per unit of fat lost.

So the percentage tells one story and it is a reasonably good one.

But retatrutide is doing something the other drugs are not, and that is where the ratio starts to matter in a different way. The phase 2 trial published in the New England Journal of Medicine in 2023 showed an average total weight loss of 24.2 percent at 48 weeks on the 12 milligram dose. That is not a percentage anyone had seen from a drug before. When total weight loss is that large, even a "good" lean mass ratio produces large absolute numbers.

Run the math. If someone loses 24 kilograms total, and 20 to 25 percent of that is lean mass, you are looking at somewhere between 4.8 and 6 kilograms of lean tissue lost. That is up to 13 pounds of muscle and other lean tissue gone over the course of the trial. The percentage column looks fine. The absolute number is not fine, especially if the person losing that tissue is older, was already carrying lower muscle mass to begin with, or has no resistance training in their routine.

This becomes more significant given the theory that was circulating when retatrutide first drew attention. The drug activates three receptors, GLP-1, GIP, and the glucagon receptor, and the glucagon component generated real optimism. Research on glucagon receptor activation, including work published in Cell Reports Medicine, shows it upregulates amino acid catabolism in the liver and is coupled to increased energy expenditure, which suggested it might push the body to preferentially burn fat for fuel and spare muscle protein. The expectation was that the glucagon receptor would tilt the ratio in favor of fat and protect lean tissue better than single or dual-receptor drugs.

The scan data says that is not what happens in practice. The ratio is roughly equivalent to tirzepatide, which does not have a glucagon receptor. Whatever fat-selective signal the glucagon component is producing at the receptor level, it is not translating into a meaningfully better fat-to-lean ratio in humans over 36 weeks, at least not in this population.

That is the piece the theoretical arguments missed, and it is a good example of why scan data matters. Receptor-level mechanisms do not always produce the expected downstream outcomes in whole-body physiology, and the DXA tells you what actually happened rather than what the mechanism predicted.

What does this mean practically? It means retatrutide will not do the job of preserving lean mass on its own. No current GLP-1 class drug does. The one variable that consistently improves the fat-to-lean ratio across all of these drugs is resistance training combined with adequate protein intake, and the evidence for this is consistent enough that it should be treated as non-negotiable if you are on any of these medications. The target that appears repeatedly in the literature is approximately one gram of protein per pound of target body weight per day, and resistance training three to five sessions per week provides the mechanical signal that tells your body the muscle is being used and should not be broken down for energy.

The drug creates the caloric deficit and shifts appetite and energy balance in ways that produce substantial fat loss. That part it handles well, better than anything available. But the signal that says "keep the muscle" does not come from the drug. It comes from loading the muscle and feeding it protein, and in the absence of those inputs, the deficit gets filled partly by fat and partly by the tissue you spent years building.

The scale going down is not the same thing as the composition improving. And on a drug that can pull 24 kilograms off your frame, the difference between losing it the right way and the wrong way is measured in pounds of muscle, not fractions of a percent.

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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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