Keeping muscle on a GLP-1: what the body-composition substudies found and the protein and training evidence that answers them

"Ozempic face", "Ozempic muscle loss": the phrases arrived before the data did. The data have now arrived, from body-composition substudies inside the pivotal trials, and they say something less dramatic and more useful. Weight lost on a GLP-1 is partly lean mass, in about the proportion weight loss always is. That proportion is not fixed; two interventions with decades of evidence behind them shift it. Neither is on the label, because the label is about the drug. Both are on this page.

What the substudies found

STEP 1 (semaglutide 2.4 mg). In the DXA substudy of 140 participants reported within the main STEP 1 publication (Wilding 2021, PubMed 33567185), total fat mass and lean body mass both fell with semaglutide, with a greater reduction in fat mass. The proportion of body weight that was lean mass increased. Lean mass loss as a share of total weight loss was roughly 40 percent in that substudy, which is at the upper end of the range seen in diet-induced weight loss.

SURMOUNT-1 (tirzepatide 5, 10 and 15 mg). The DXA substudy (Look 2025, PubMed 39996356) reported that about three-quarters of the weight lost over 72 weeks was fat mass and about one quarter was lean mass, with the fat-to-lean ratio of the loss similar across doses and, again, an increase in the lean-mass proportion of body weight.

A note on what "lean mass" is. DXA lean mass is everything that is not fat or bone: skeletal muscle, but also organ tissue, and water, including the water lost from a smaller glycogen store and from fat cells themselves. Loss of lean mass therefore overstates loss of muscle. The 2024 review by Neeland and colleagues (PubMed 38937282) makes this point and argues that the clinically important question is function (strength, physical performance) rather than the DXA number alone, while agreeing that preserving lean mass during large weight loss is a legitimate goal, particularly in older adults.

A note on what none of them measured. The pivotal trials did not test a protein target or a training programme. The mitigation evidence comes from the weight-loss literature generally, which is where the rest of this guide goes.

Resistance training

Two systematic reviews frame the evidence. Weinheimer and colleagues (2010, PubMed 20591106) systematically reviewed the trials in middle-aged and older adults and found that energy restriction alone produced weight loss of which a substantial share was fat-free mass, and that adding exercise, resistance training in particular, reduced that share. Sardeli and colleagues (2018, PubMed 29596307) meta-analysed trials in older adults with obesity on calorie-restricted diets and found that resistance training largely prevented the lean-mass loss seen with diet alone, while fat loss was preserved.

The liraglutide trial by Lundgren and colleagues (NEJM 2021, PubMed 33951361) is the closest thing to a direct test in a GLP-1 population: after an 8-week low-calorie diet, participants were randomised to exercise, liraglutide, both, or placebo for a year. The combination produced the greatest maintained weight loss and fat loss, and the exercise groups preserved lean mass and fitness better than liraglutide alone. Liraglutide is an older GLP-1, but the principle generalises.

What the trials used, and what the evidence therefore supports, is progressive resistance training: two or three sessions a week, each covering the major muscle groups (legs, hips, back, chest, shoulders, arms) with a load that makes the last few repetitions hard, and a load that increases over weeks as it gets easier. Machines, free weights or bands all qualify. Ten minutes of body-weight squats is a start; it is not the dose the trials used.

Protein

Muscle is built and maintained from dietary protein, and during an energy deficit the body draws on it. The reviews by Leidy (2015, PubMed 25926512) and Cava (2017, PubMed 28507015) conclude that protein intakes above the general adult RDA of 0.8 g per kg per day protect lean mass during weight loss, with the commonly supported range around 1.2 to 1.6 g per kg per day. Longland and colleagues (2016, PubMed 26817506) took young men into a severe deficit with hard daily training and found that a high protein intake (2.4 g per kg) produced lean-mass gain and greater fat loss than a lower one (1.2 g per kg); it is an extreme protocol, but it shows the direction.

The problem on a GLP-1 is not knowing the target but reaching it on a suppressed appetite. Someone weighing 100 kg aiming for 1.2 g per kg needs 120 g a day, which is roughly 500 g of cooked chicken, or four eggs plus 200 g of Greek yoghurt plus a large piece of fish, spread across a day in which nothing appeals. Practical patterns that work: protein first at every meal, before anything else on the plate; a protein-rich breakfast, since appetite is often best early; liquid protein (milk, yoghurt drinks, a whey or plant shake) on days when solid food is hard; and spreading intake across three or four eating occasions, since the muscle-building response to protein saturates per meal. Kidney disease changes the target and is a reason to set it with a prescriber. The protein target calculator turns a factor into grams and a per-meal split, and the weight loss site goes deeper on food.

Protein without fibre and fluid produces constipation, which is already a label-listed adverse reaction; the hydration guide covers the balance.

Rate of loss

Faster weight loss tends to take a larger share from lean mass; that is a consistent finding of the older weight-loss literature summarised by Cava and by Weinheimer. The labels' escalation schedules exist for GI tolerability, not body composition, but the option they give to delay escalation or settle on a lower maintenance dose (see the dose escalation guide) is also the option that slows loss. The trade-off is a prescriber conversation; the point here is that it is a trade-off and not a failure.

A weekly plan that fits the evidence

Two or three resistance sessions, 30 to 45 minutes, covering legs, push, pull and hinge movements, with the load creeping up. Protein at every meal to a daily target set with your prescriber, tracked for at least the first month because guesses run low. Fibre and fluids alongside. Daily walking for everything else. A grip-strength or sit-to-stand test every month as a functional check that costs nothing, which is closer to what the Neeland review considers the outcome that matters than a smart-scale body-fat estimate.

When to get help

Falls, difficulty rising from a chair, or a noticeable loss of strength are reasons to see a clinician, particularly over 65, where sarcopenia is already a concern and where the Neeland review is most cautious. A referral to a dietitian and to a physiotherapist or qualified trainer who has worked with people losing weight is a reasonable ask of any prescriber running a GLP-1 programme.

Compounded products

Compounded semaglutide and tirzepatide are not FDA approved and are not interchangeable with brand products, and no body-composition data exist for them; the substudies above are of the brand products. The physiology of weight loss, and the protein and training evidence, are the same. FormBlends describes what its pharmacies dispense on its semaglutide and tirzepatide pages.