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

Retatrutide is losing weight faster than any other drug available, and for the first time, researchers scanned exactly what that weight is made of.

The Lancet Diabetes and Endocrinology just published body composition data from a 103-person substudy where participants with type 2 diabetes had DXA scans at the start and again at 36 weeks. DXA, which stands for dual-energy X-ray absorptiometry, is the gold standard method for separating fat mass from lean mass in the body, and it is meaningfully more precise than the scale or even DEXA-adjacent methods like bioelectrical impedance. These are the first scans of this kind done on retatrutide, and what they show changes how you should think about the drug.

Before getting into those numbers, it helps to understand what retatrutide actually is and why it was supposed to behave differently than every other GLP-1 drug.

Most weight loss drugs in this class work on one or two receptors. Semaglutide targets the GLP-1 receptor, which reduces appetite and slows digestion. Tirzepatide adds a GIP receptor on top of that. Retatrutide goes one step further and adds a third receptor called the glucagon receptor, which is the part that made researchers excited about its potential to preserve muscle.

Here is why. The glucagon receptor, when activated, increases energy expenditure in the liver and shifts the body toward burning fat for fuel, a process sometimes called fat oxidation. The theoretical logic was straightforward: if the glucagon component is pushing the body to preferentially burn fat, then less muscle should be broken down for energy, and the lean mass losses you normally see with aggressive calorie restriction should be smaller. That theory came from cell and animal research, and some human data on glucagon receptor activation showing it drives up energy expenditure by increasing how the liver processes fuel rather than sparing or consuming muscle directly.

So the prediction going in was that retatrutide's lean-to-fat loss ratio would look better than the other drugs. The DXA data says otherwise.

At the 12 milligram dose, total fat mass fell by 23 percent and visceral fat, which is the fat that accumulates around your organs and is more metabolically harmful than subcutaneous fat, dropped by 31.4 percent. Those are meaningful numbers. But when you look at what fraction of total weight lost was fat versus lean tissue, you get roughly 75 to 80 percent fat and 20 to 25 percent lean. That is comparable to tirzepatide, which also runs about 75 percent fat in its own DXA substudy. Semaglutide is actually less favorable at around 60 percent fat and 40 percent lean.

So retatrutide is not worse than the alternatives. On a percentage basis, it is actually among the better options. That part of the story is genuinely encouraging.

But percentages only tell part of the story, and this is where the math becomes important.

In the phase 2 trial, participants on the 12 milligram dose lost an average of 24.2 percent of their body weight over 48 weeks, which in absolute terms is roughly 24 kilograms for someone starting at 100 kilograms. When 20 to 25 percent of that loss is lean tissue, you are looking at approximately 5 to 6 kilograms of lean mass gone. That is over 13 pounds. The percentage looks reasonable. The absolute number is not small.

This is the core tension with retatrutide. It is the most powerful weight loss drug ever tested in humans, and that power cuts both ways. A favorable lean-to-fat ratio applied to a very large total weight loss still produces a large absolute lean mass loss, in the same way that 25 percent of a large number is still a large number.

The glucagon receptor did not change this arithmetic the way the theory suggested it might. One possible explanation comes from research on what glucagon receptor activation actually does mechanically: it upregulates how the liver breaks down amino acids, the building blocks of protein, for energy. So even though it increases fat oxidation and total energy expenditure, it may simultaneously increase protein catabolism at the liver level, which could offset any lean-sparing benefit. This is still a theoretical framework and the DXA data alone cannot confirm or rule it out, but it fits with why the ratio ended up comparable to drugs without any glucagon component.

What the data makes clear is that the drug is not doing anything structurally different to protect your muscle. The mechanism that was supposed to selectively spare lean tissue is not showing up in the scans.

That brings you to what you can actually do about this.

The two levers that consistently reduce lean mass loss during aggressive weight loss are resistance training and protein intake. Resistance training sends a direct anabolic signal to muscle tissue, telling the body those fibers are being used and should be maintained rather than stripped for energy. Protein intake, particularly at higher thresholds, ensures the amino acid pool available for muscle protein synthesis stays adequate even during a large caloric deficit. The commonly cited target of around one gram per pound of goal body weight is derived from literature on protein needs during weight loss, where demands are higher than in weight-stable individuals because the body is simultaneously dealing with energy restriction and elevated protein turnover.

Three to five resistance training sessions per week combined with consistent protein intake appears to be the range where most of the lean-sparing benefit is captured, though the exact threshold will vary by the individual's training history, age, and rate of weight loss.

The point is not that retatrutide is a bad drug. The fat loss numbers at 36 weeks are genuinely substantial, and losing 31 percent of visceral fat in that timeframe has real metabolic consequences. The point is that the drug handles one side of the equation, which is creating the conditions for large-scale fat loss, and does nothing meaningful for the other side, which is maintaining the muscle underneath.

You are not choosing between the drug's mechanism and your training. You need both operating simultaneously, because the drug's power is exactly the reason the lean mass risk is higher than it would be on a less aggressive compound. The faster and larger the total weight loss, the more important it becomes to actively protect the tissue you want to keep.

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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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