On Ozempic or Mounjaro? Lift Weights and Eat Protein, or Lose Muscle Too

These drugs are very good at moving the number on the scale. That's the point of them. But the scale can't tell you what you lost, and that matters more than most people are told when they get the script.

Lose fat and you're healthier. Lose muscle along with it and you end up lighter but weaker, with less of the tissue that handles your blood sugar and keeps you moving well as you age. The good news is that the fix is cheap, well understood and entirely in your hands. It just doesn't happen by itself.

What are GLP-1 medications?

GLP-1 medications copy a gut hormone (glucagon-like peptide-1) that tells your brain you're full and slows how fast your stomach empties. You feel less hungry, you eat less, and you lose weight. They were developed for type 2 diabetes and are now also prescribed for weight management. The "family" includes newer drugs that act on a second hormone as well, like tirzepatide.

Everything in this article applies to all of them. They all work mainly by making you eat less, and that's where the muscle problem comes from.

Do GLP-1 medications cause muscle loss?

They cause lean mass loss, and quite a lot of it. In the STEP 1 trial of semaglutide, scans of a subgroup showed that about 40% of the weight lost was lean mass. In the SURMOUNT-1 trial of tirzepatide, it was about a quarter.

Two things keep that in proportion. First, "lean mass" isn't just muscle. It also counts water, organs and connective tissue, so not every kilo of it is muscle you'd miss. Second, some lean loss happens with any weight loss, because a lighter body needs less to carry it around. A 2024 review of GLP-1 body-composition studies put the range at roughly 25–40% of weight lost, which is in the same ballpark as dieting. The drug isn't attacking your muscle. You're just losing weight fast, on very little food.

That's the real issue. On these medications people commonly eat far less than they did before, and protein drops along with everything else. Activity drops too. A study presented at ENDO 2026, the Endocrine Society's annual meeting, tracked Fitbit data from 753 adults who started a GLP-1. Their daily steps fell from about 5,000 to about 4,500, and their moderate-to-vigorous activity fell from 28 to 22 minutes a day. Less food, less protein and less movement is close to the perfect recipe for losing muscle.

Why muscle matters more than the number on the scale

• Strength and getting around. Muscle is what gets you out of a chair, up the stairs and through the shopping. Losing it makes everyday life harder, and the effect gets bigger with age.
• Blood sugar. Muscle is where most of the sugar from a meal ends up. More muscle means more room to put it, which matters if you're on these drugs for type 2 diabetes or to prevent it.
• Keeping the weight off. Muscle doesn't burn as many calories as the internet claims, but it does help, and people who keep training after weight loss hold on to more of it.
• How you look and feel. Lose fat and keep muscle and you look leaner and feel stronger. Lose both and you get the "smaller but softer" result a lot of people are disappointed by.

What happens when you stop taking a GLP-1?

Most of the weight comes back. In the STEP 1 extension, people who came off semaglutide regained about two-thirds of the weight they'd lost within a year.

That's the strongest reason to take muscle seriously from day one. If you lose muscle on the way down and stop training, the weight that comes back is unlikely to bring the muscle back with it. You can end up near your old weight with less muscle than you started with. Training gives you something that lasts whatever happens with the medication. A Danish trial found that people who exercised after GLP-1-assisted weight loss kept more weight off and lost more body fat than people relying on the drug or exercise alone.

Don't stop, start or change your dose without talking to your prescriber. That decision is theirs and yours, not ours.

How much strength training do you need on a GLP-1?

Less than you'd think, but it has to be strength training. Walking and cardio are great for your heart and mood, but on their own they don't protect muscle the way lifting does. In a trial of older adults losing weight by dieting, the groups that lifted weights lost noticeably less lean mass than the group that only did aerobic exercise. The 2025 joint advisory on nutrition for GLP-1 users says that eating more protein on its own is unlikely to be enough to keep muscle without structured strength training.

The minimum that works:

• Two sessions a week, full body, 30–45 minutes each. Three is better if you can manage it.
• Cover the big movements every session: a squat (goblet squat, leg press), a hinge (Romanian deadlift, hip thrust), a push (push-up, dumbbell press), a pull (row, lat pulldown) and some trunk or carry work.
• 2–3 sets of 6–15 reps, stopping 1–3 reps before you'd fail. You don't need to grind every set to the end. Training to failure costs more than it gives, especially when you're eating less.
• Get slowly harder over time. Add a rep, then a little weight. That's progressive overload, and it's the signal that tells your body the muscle is worth keeping.
• On flat days, do less, not nothing. Low appetite often means low energy. Drop a set or lighten the load, but turn up. One easy session beats one skipped one.

If you've never lifted, this is a good time to start, not a bad one. And no, lifting won't make you bulky. On a calorie deficit you'll be working hard just to keep what you have.

How much protein should you eat on Ozempic or Mounjaro?

We suggest 1.6–2.0 g of protein per kilogram of body weight per day while you're losing weight.

That's above the 1.2–1.6 g/kg the 2025 joint advisory from four obesity and nutrition societies recommends, and we think it's worth aiming higher. Treat their number as the floor. Protein needs go up when you're eating less, and a large meta-analysis found the benefit of protein for building muscle with training levels off at around 1.6 g/kg on average, with some people still benefiting at higher intakes. In one trial, people in a large calorie deficit who ate 2.4 g/kg and trained hard actually gained lean mass while losing fat, while the group on 1.2 g/kg didn't. A target of 1.6–2.0 g/kg covers most people without being impossible on a small appetite.

Which body weight? If you have a lot of weight to lose, base it on your goal weight (or a healthy weight for your height) instead of your current weight. Otherwise the number gets unrealistic fast. If you're within 10–15 kg of your goal, your current weight is fine.

Split it across 3–5 meals and snacks of about 25–40 g each, rather than trying to fit it all into dinner.

One important exception: if you have kidney disease, which is more common with type 2 diabetes, talk to your doctor or a dietitian before raising your protein.

How to eat enough protein when you're not hungry

This is the hard part. When the drug is working, a normal plate of food can feel like a challenge. A few things make it easier:

• Protein first. Eat the protein part of the meal before the rest, while you still have room.
• Every meal and snack has some. Small and often beats one big serve you can't finish.
• Drinks count. Milk, high-protein milk, a yoghurt smoothie or a protein shake go down more easily than a steak when you're not hungry. Shakes aren't required, but they're a handy tool.
• Choose dense foods. Pick foods with a lot of protein per mouthful.

Two side effects are worth planning for. Constipation is common, so get some fibre (vegetables, fruit, oats, legumes) and drink enough water, especially if you're upping protein. Nausea tends to be worse after big or greasy meals, which is one more reason to go small and often. If you genuinely can't eat enough, tell your prescriber. It's useful information for them, not a failure on your part.

How FitFor handles this

FitFor builds a strength program around the days you actually have, from one or two sessions a week up. At low frequencies every session covers a squat, a hinge, a push, a pull and trunk work, so two sessions still train your whole body. Loads step up week by week and ease off on deload weeks, and the app shows what you did last time, so a flat day is just a lighter set rather than a guess. The nutrition screen sets your daily protein target at 1.8 g per kilogram of body weight, the middle of the range above.

FitFor is a training app, not a medical service. It doesn't know about your medication, and it doesn't replace advice from the person prescribing it.

Quick answers

Does Ozempic cause muscle loss?

It causes lean mass loss along with fat loss. In the STEP 1 trial about 40% of the weight lost on semaglutide was lean mass. Part of that is water and other tissue, not just muscle, and strength training plus enough protein reduces it.

How much protein should I eat on Mounjaro?

We suggest 1.6–2.0 g per kilogram of body weight per day while you're losing weight, based on your goal weight if you have a lot to lose. The 2025 joint advisory's 1.2–1.6 g/kg is a sensible minimum.

Can you build muscle on a GLP-1?

Beginners often can, especially early on, if they lift regularly and eat enough protein. For most people the realistic goal is to keep the muscle you have while you lose fat. That alone is a big win.

Is walking or cardio enough to protect muscle?

No. Cardio is good for your heart, mood and fitness, but strength training is what protects muscle during weight loss. Do both if you can, and make the lifting non-negotiable.

Do I need protein shakes on Ozempic?

No, but they help. When your appetite is low, a shake or high-protein milk is often the easiest way to reach your target. Whole foods work just as well if you can eat enough of them.

The short version

• GLP-1s make you eat less, and a big share of the weight lost can be lean mass.
• Lift weights at least twice a week, full body, stopping a rep or two short of failure.
• Aim for 1.6–2.0 g of protein per kg of body weight a day (goal weight if you have a lot to lose).
• Eat protein first, little and often, and use drinks when food is hard.
• The muscle you keep stays with you whatever happens with the medication.

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FitFor builds a strength program around the days you have, progresses it week by week and sets you a protein target, so the muscle you have stays where it is. Download it free.