On a weight-loss injection like Ozempic, Wegovy or Mounjaro, part of the weight you lose is lean mass, and a large part of that is muscle. In the large tirzepatide trial where people were scanned, it was about a quarter of everything they lost. Muscle loss on Ozempic and the other GLP-1 medications can be slowed down with strength training under real load and enough protein. The medication itself is decided and managed by your doctor. My part is the training.
Why you lose muscle too
The injection takes away a lot of your appetite. You eat much less, and the body covers the gap from two places: fat and lean tissue. That happens with any big diet. The injection just makes the deficit bigger and easier to keep.
The best numbers come from trials where people had a body composition scan before and after. In the SURMOUNT-1 trial, 124 people on tirzepatide (Mounjaro) were scanned over 72 weeks. They lost 33.9% of their fat mass and 10.9% of their lean mass, which is about 15.9 kg of fat and 5.6 kg of lean mass. Lean mass was 26% of the weight they lost (Look et al., 2025). In the same trial, the placebo group lost far less weight, but with almost the same split.
For semaglutide (Ozempic, Wegovy) the STEP 1 trial scanned 95 people over 68 weeks. Body weight went down 15.0%, fat mass 19.3% and lean mass 9.7% (Wilding et al., 2021).
Two honest notes. First, the share changes a lot from study to study. A 2024 review found trials where lean mass was 40 to 60% of the weight lost, and others where it was around 15% or less (Neeland et al., 2024). Second, lean mass is not only muscle. It also includes organs, fluids and the water inside fat tissue. So the lean mass number can make the muscle loss look bigger than it is. It still goes in the wrong direction.
Why it matters
The first thing I notice on the floor is strength. The scale goes down nicely, and then the weight on the bar stops moving or goes back. You need that same muscle to carry the shopping or to get up from the floor.
Then metabolism. Each kilo of muscle burns about 13 kcal a day at rest, against about 4 kcal for a kilo of fat (Neeland et al., 2024). That is small per kilo, and I won’t tell you muscle burns fat all day. But it is one of the reasons weight loss slows down over time.
The bigger reason is what happens if the weight comes back. In a study of women who regained weight after a diet, for every kilo of fat they lost they also lost 0.26 kg of lean mass. For every kilo of fat they regained, only 0.12 kg of lean mass came back (Beavers et al., 2011). That was a small diet study, not an injection study, and the difference between the two numbers was not statistically certain. Still, it points one way: lost muscle doesn’t simply come back with the weight. I wrote more about this in what happens when you stop Ozempic.
How we measure it
The scale can’t tell you what you lost. Two people can both drop 10 kg, one mostly fat and one with a lot of muscle in it, and the number looks the same.
We use two things. A body composition scan shows fat mass and lean mass, for the whole body and for each arm, leg and the trunk. We do one at the start and one in week 12, so you see in kilos what changed. If you want to know how the scan works, I explained it in what a body composition scan is. The scan uses a very small dose of radiation, so in Switzerland each one has to be justified. Following a risk of muscle loss is one of those reasons.
The second thing is the logbook. Every set is logged, so your coach knows exactly what you lifted last week. If the scan says lean mass is holding and the logbook says you are getting stronger, the plan works. If both go down, we change the plan straight away.
What slows it down: strength training
Strength training is the part that does most of the work. A meta-analysis of six trials in older adults with obesity looked at diets with and without resistance training. Training three times a week for 12 to 24 weeks prevented 93.5% of the lean mass loss that the diet alone caused (Sardeli et al., 2018). Those people were dieting, not on an injection, but the mechanism is the same: the body keeps the muscle it is using.
Walking is good for you, but it is not enough here. In STEP 1, people got monthly advice to reach 150 minutes of activity a week, like walking, and that did not stop the loss of lean mass (Gross and Brinkmann, 2024). A joint advisory from four US obesity and nutrition societies recommends strength training at least three times a week, plus at least 150 minutes of moderate cardio, for people on these medications (Mozaffarian et al., 2025).
This is how I set it up for someone on an injection:
- The first one or two sessions are an assessment: your health history, the medication you take, a few simple tests, and learning to brace and to hinge before we load anything.
- The basic movements every session: a squat, a hinge like a deadlift or hip thrust, a push and a pull. Big muscles, real load.
- We find your starting weights with the two-set method. You do a set, tell me how many more reps you could have done, and we adjust the second set from that.
- The heaviest exercise goes first, while you are fresh. Energy is often lower and less predictable on these medications.
- Over the weeks the load goes up, a rep or a bit more weight at a time, and every set is written down. On a bad day I would rather you do one clean set less than grind through a bad one.
Come to the session having eaten something and with enough to drink. If you feel dizzy or sick during a session, tell your coach. We adjust the day. If it keeps happening, that is a question for your doctor. If you also take insulin or other diabetes medication, ask your doctor how to time meals and training, because the risk of low blood sugar is higher.
Protein when you are not hungry
The advisory I mentioned cites 1.2 to 1.6 g of protein per kilo of body weight a day, a target that has been proposed during active weight loss, or simply 80 to 120 g a day, which is easier to follow. It also says protein alone is probably not enough to keep muscle without strength training (Mozaffarian et al., 2025). A German study (not randomised) with 267 patients on a 800 to 1,000 kcal formula diet found the same thing: 1.5 g/kg against 1.0 g/kg made no difference to lean mass after 12 weeks (Stein et al., 2024).
So protein is there to support the training.
With little appetite, the practical rule is to eat the protein part of the meal first. Greek yoghurt, eggs, fish, cottage cheese or a shake are easy when you are not hungry. If you want a plan for this, our nutrition coaching with Roberta Burla (MAS ETH in Nutrition and Health) is where it belongs.
What we don’t do
The medication is decided and managed by your doctor. That includes the dose, when to start and when to stop. We don’t give advice on any of that, and we don’t replace your doctor. If a scan or a training number shows something that looks medical, we tell you to take it to your doctor.
Keep your muscle on weight-loss injections: the 12-Week Transformation
For people on Ozempic, Wegovy or Mounjaro, we do this in our 12-Week Transformation. It already has everything this article talks about:
- A body composition scan in week 0, so we know your fat mass and lean mass before the plan begins.
- Strength training 1:1 with your coach, written down and progressed every session.
- Nutrition coaching with Roberta Burla, so the protein gets in even when you are not hungry.
- A second scan in week 12. We sit down together, look at what changed in fat mass and lean mass, and plan the next block from there.
You find the whole program on the 12-Week Transformation page.
It starts with a free trial session at VAULT Training, Brandschenkestrasse 150, 8002 Zürich (Enge). First we talk about your goals and the medication you are on. Then we train together, so you see how we work before you decide anything. Book your free trial here.
Frequently asked questions
Does Ozempic cause muscle loss?
Part of the weight you lose on Ozempic or Mounjaro is lean mass, and a large part of that is muscle. The reason is the large calorie deficit the injection creates, the same as with any big diet. In the SURMOUNT-1 scans, lean mass was 26% of the weight lost on tirzepatide (Look et al., 2025).
How much muscle do you lose on Wegovy or Mounjaro?
In STEP 1, people on semaglutide (Wegovy) lost 9.7% of their lean mass over 68 weeks. In SURMOUNT-1, people on tirzepatide (Mounjaro) lost 10.9%, about 5.6 kg, over 72 weeks. These are averages and individual results vary a lot (Wilding et al., 2021; Look et al., 2025).
Is eating more protein enough to keep muscle on a GLP-1 medication?
Protein on its own is probably not enough. A joint advisory of four obesity and nutrition societies says protein needs to go with structured strength training, and cites 1.2 to 1.6 g per kilo a day as a target proposed during weight loss (Mozaffarian et al., 2025).
How often should I do strength training on Ozempic?
The same advisory recommends strength training at least three times a week plus 150 minutes of moderate cardio. If you are starting from zero, we build up to that. What matters most is that the load goes up over time and that you keep going.
If you are on a weight-loss injection and want to keep your muscle, book a free trial and we start with where you are today.