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 distinction matters more than most people realize when you look at the body composition data that just came out of The Lancet.

Here is the system you need to understand before any of this makes sense. When you lose weight, your body is pulling from two places: stored fat and lean tissue, which includes muscle, water in muscle, and organ mass. Every drug that creates a caloric deficit pulls from both. The question is never whether you lose lean mass. The question is what ratio of your total weight loss comes from fat versus everything else, and how big that total number is. Those two things together tell you what is actually happening to your body.

The new retatrutide substudy scanned 103 people with type 2 diabetes using something called DXA, which is dual-energy x-ray absorptiometry and works by firing two low-dose x-ray beams through your body to distinguish fat mass, lean mass, and bone with more precision than any scale or impedance device. They scanned participants at baseline and again at 36 weeks across 42 centers and the numbers are specific enough to be worth walking through carefully.

At the 12 milligram dose, total fat mass dropped by 23.2 percent. Visceral fat, which is the fat packed around your organs rather than stored under your skin, dropped by 31.4 percent. That visceral fat number is notable because visceral fat is metabolically active in a way that drives insulin resistance and cardiovascular risk, so a disproportionate reduction there is a real benefit.

Now, of the total weight lost, roughly 75 to 80 percent was fat mass and 20 to 25 percent was lean mass. That ratio is not alarming on its own. Tirzepatide, which targets both GIP and GLP-1 receptors, shows essentially the same ratio, about 75 percent fat and 25 percent lean, in its own DXA substudy from SURMOUNT-1. Semaglutide, the original GLP-1 drug, actually performs worse at roughly 60 percent fat and 40 percent lean. So by the ratio, retatrutide looks competitive or better.

But the ratio is not the whole story, and this is where the math starts to matter.

Retatrutide at 12 milligrams produces around 24 percent total body weight loss at 48 weeks in the phase 2 trial. For someone starting at 100 kilograms, that is roughly 24 kilograms gone. Apply the 20 to 25 percent lean mass figure and you are looking at approximately 5 to 6 kilograms of lean tissue lost alongside the fat. That is over 13 pounds. On tirzepatide, which produces less total weight loss, the same ratio produces a smaller absolute lean mass number. The percentage is similar. The actual tissue lost is not.

This is a meaningful distinction because muscle mass is not easily rebuilt, especially in people over 40, and especially under conditions of caloric restriction where the hormonal environment for muscle protein synthesis is already compromised.

There was a theory circulating that retatrutide would handle this differently. The drug is a triple agonist, meaning it activates three receptors: GLP-1, GIP, and the glucagon receptor. The GLP-1 and GIP components work the way they do in tirzepatide, reducing appetite and improving insulin signaling. But the glucagon receptor component was expected to do something additional, specifically to shift the body toward fat oxidation and spare lean tissue. The mechanism proposed was that glucagon receptor activation upregulates energy expenditure by accelerating hepatic amino acid catabolism, meaning the liver burns more nitrogen-containing compounds for fuel, which theoretically would push the body to preferentially pull from fat stores rather than muscle protein. Research in cell models and animal data supported the plausibility of this idea.

The DXA data does not support it in humans. The lean mass ratio at 12 milligrams is not better than tirzepatide. It is comparable. The glucagon receptor may be contributing to the greater total weight loss, which is itself real and significant, but it does not appear to be selectively protecting lean mass in a way that shows up in the body composition scans.

That gap between what the mechanism was theorized to do and what the outcome data actually shows is worth sitting with, because a lot of decisions get made on mechanism alone before the clinical data exists to confirm it.

So what does this mean practically.

If you are taking retatrutide and doing nothing specific to protect lean mass, the drug will reduce your appetite, create a caloric deficit, and your body will lose weight from both fat and lean tissue at roughly a 75 to 25 ratio. Over a 24 kilogram loss, that lean component accumulates into something real. You will be lighter and likely leaner in percentage terms, but you will have significantly less muscle than you started with, and that muscle loss has downstream consequences for metabolic rate, insulin sensitivity, physical function, and how your body responds once the drug is eventually tapered or discontinued.

The two inputs that shift this outcome are resistance training and protein intake. Resistance training sends a signal to preserve and build muscle even in a deficit, because the mechanical load on the muscle tells your body that tissue is needed and should not be cannibalized for fuel. Protein at sufficient intake provides the raw material for muscle protein synthesis to keep pace with breakdown. The combination of those two things does not eliminate lean mass loss entirely during a deficit this large, but the evidence from both surgical weight loss and pharmacological weight loss literature consistently shows it reduces it meaningfully.

The practical target that holds up across the relevant research is somewhere around one gram of protein per pound of goal body weight per day, paired with resistance training three to five times per week using progressive load.

What the retatrutide data actually reveals is that the drug's mechanism does not substitute for the behavioral inputs. A triple agonist with a novel receptor target and the highest weight loss numbers in the class still loses lean mass at roughly the rate you would predict from a drug that causes a large caloric deficit. The biology of muscle preservation under energy restriction has not been outsmarted by the pharmacology.

The drug handles the fat. The training and protein handle the muscle. Those are two separate jobs.

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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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