Does Retatrutide Preserve Muscle? What the Body Composition Data Shows

The Lean Mass Question

Every weight loss drug raises the same basic concern: how much of the weight lost is fat, and how much is lean tissue, mainly skeletal muscle? The question has become more pressing as GLP-1 receptor agonists reach levels of weight loss once seen only with bariatric surgery, where preserving lean mass has long been a clinical concern.

For retatrutide, which produced 24.2% mean weight loss at the highest dose in Phase 2, body composition matters even more. Losing nearly a quarter of your body weight always involves some loss of lean mass. The useful question is not whether lean mass is lost (it is with almost every way of losing weight) but whether the share is acceptable and whether it harms function and metabolism.

The Phase 2 Body Composition Substudy

The Phase 2 retatrutide trial in type 2 diabetes included a body composition substudy, published in The Lancet Diabetes & Endocrinology (Coskun et al., 2025). It used dual-energy X-ray absorptiometry (DEXA) scans to measure changes in fat mass and lean mass separately. DEXA is the clinical standard for body composition and reliably tells apart bone mineral, fat tissue and lean soft tissue.

Of 189 people enrolled in the substudy, 103 finished treatment with scans at the start and at week 36. Total fat mass fell by 15.2% on 4 mg, 26.1% on 8 mg and 23.2% on 12 mg, against 4.5% on placebo and 2.6% on dulaglutide. Lean mass also fell. The authors concluded that the share of weight lost as lean mass was similar to that seen with other obesity treatments. No body composition results have been published from the Phase 2 obesity trial, so these figures come from people with type 2 diabetes after 36 weeks.

Putting the Ratio in Context

To read that conclusion, it helps to compare it with known benchmarks from other ways of losing weight.

With calorie restriction alone (dieting without exercise), the typical split is about 75% fat and 25% lean mass. Adding resistance exercise to calorie restriction improves this a little, to roughly 80-85% fat loss. Bariatric surgery, despite greater total weight loss, usually shows 70-80% fat loss depending on the procedure and time frame.

Among drugs, semaglutide 2.4 mg (Wegovy) has shown fat making up around 75% of total weight lost. Tirzepatide’s SURMOUNT program reported similar figures, with about 75-80% of weight lost being fat at the highest doses.

The substudy’s abstract does not give a single fat-to-lean ratio for retatrutide, so it cannot be ranked precisely against these drugs. The data do not suggest that retatrutide causes more lean mass loss than other drugs in the class.

The Glucagon Receptor Hypothesis

One question of particular interest is whether retatrutide’s glucagon receptor component, the part that sets it apart from dual agonists like tirzepatide, gives any advantage for body composition. The idea rests on glucagon’s known metabolic effects.

Glucagon promotes fat burning in the liver and elsewhere, steering the body to use fat stores for energy first. In theory, this could tilt weight loss toward fat while relatively sparing lean tissue. Glucagon also raises energy expenditure. That may reduce metabolic adaptation (the drop in resting metabolic rate that usually comes with weight loss) and so ease the pressure on the body to break down muscle for energy.

The Phase 2 body composition data fit this hypothesis but do not prove it. The fat-to-lean loss ratio falls within the range seen with other highly effective drugs, so the available data cannot credit any specific advantage to the glucagon component. Showing that glucagon receptor agonism helps preserve lean mass would take controlled comparisons, ideally a head-to-head trial against a GLP-1/GIP dual agonist with matched total weight loss.

Absolute Versus Relative Lean Mass Loss

Discussions of body composition often miss the difference between relative and absolute changes in lean mass. Even if lean mass makes up 20-25% of the total weight lost, the actual amount of lean tissue lost has to be judged against the person’s overall body composition.

Take a participant starting at 120 kg who loses 24% of body weight (about 29 kg). If 22% of that loss is lean mass, they lose roughly 6.4 kg of lean mass. For someone with a starting lean mass of about 55-65 kg (typical at this weight), that is a 10-12% reduction in lean tissue.

That is not trivial, but it comes alongside a far larger drop in fat mass (about 22.6 kg in this example). The net effect is a big improvement in the ratio of lean to fat tissue. The person ends treatment with a higher percentage of lean body mass, even though they lost some lean tissue in absolute terms.

This matters because it puts lean mass loss in its place: as part of an overall improvement in body composition, not a separate harm.

Functional and Metabolic Implications

How much lean mass loss matters depends on what it does to function. Skeletal muscle is the main driver of resting metabolic rate, strength, mobility and independence, especially in older adults. Losing too much lean mass could in theory harm these, undermining the long-term benefits of weight loss.

The available evidence suggests that drug-induced weight loss on the scale seen with retatrutide does not cause clinically meaningful loss of function in most patients. Phase 2 participants did not show a disproportionate loss of physical function, though the trial was not designed or powered to measure functional endpoints.

The link between the amount of lean mass and physical capacity is also not a straight line. A modest loss of lean mass along with a large loss of fat may actually improve things like mobility, exercise tolerance and daily activities, because the physical load of excess fat falls by more than the muscle’s capacity to do those activities.

What Longer Trials Could Add

Larger body composition analyses, in more diverse populations and over longer treatment (the Phase 3 trials ran 68 to 80 weeks, against 36 weeks in the Phase 2 substudy), would address several open questions.

First, whether the fat-to-lean loss ratio stays the same or shifts as treatment continues past 36 weeks. It is plausible that as weight loss slows later in treatment, the share of lean mass lost falls, as some bariatric surgery follow-up studies have found.

Second, whether particular groups, such as older adults, people with lower lean mass at baseline or people with sarcopenic obesity, see different body composition results. This will be key for judging individual risk and for deciding whether extra measures such as resistance exercise or protein supplements should be formally recommended with retatrutide.

Third, whether body composition changes line up with functional tests, metabolic rate measurements and patient-reported outcomes in ways that can guide clinical decisions.

The Practical Takeaway

The Phase 2 body composition data show that the weight lost on retatrutide is mostly fat, in a ratio similar to other highly effective weight loss drugs. The lean mass loss is real but modest next to the overall improvement in body composition. The glucagon receptor component may in theory favor fat burning, though comparative studies have not shown this conclusively.

For clinical practice, these data support treating lean mass loss on retatrutide as something to manage rather than a reason not to treat. That is especially true when patients eat enough protein and do resistance exercise, both of which have been shown to reduce lean mass loss across weight management approaches.