Mounjaro and muscle loss: How to preserve lean mass

If you have spent any time reading about weight loss injections in the last year, you have probably seen the phrase "muscle wasting" attached to them, usually without much explanation of what the numbers actually show.

There is a real effect. It is measurable, it has been studied properly, and it is worth doing something about.

It is also considerably less dramatic than the headlines suggest, and it is largely within your control.

Most people we speak to about muscle loss on Mounjaro are somewhere between two and six months into their treatment.

They feel better, their clothes fit differently, and then they read something that makes them wonder whether they are quietly damaging themselves.

That worry is reasonable, and it deserves a proper answer rather than reassurance.

This guide does exactly that. It explains what is muscle loss, why do people lose muscle on Mounjaro, and how to prevent it.

Before you begin: A little bit about OnlineClinic

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When researchers measure body composition, they usually use a DEXA scan, which measures three things: fat mass, bone mineral density, and lean mass.

Lean mass includes your skeletal muscle. It also includes:

• Water. Both inside and around your cells, and it moves a great deal day to day.
• Glycogen. Your stored carbohydrate, which holds roughly three grams of water for every gram stored.
• Organ tissue. Your liver, kidneys, heart, and gut all count as lean mass.
• Connective tissue. Tendons, ligaments, and the structural tissue that sits within and around fat stores.

So when a study reports a fall in lean mass, it is not reporting a fall in muscle alone.

Some of that drop is water and glycogen leaving as your intake changes.

Some of that is the supporting tissue that surrounds the fat you have lost, which the body no longer needs to maintain.

A larger body carries more of all of it.

None of this makes muscle loss imaginary.

It just means the headline percentage is not a muscle-only figure, and treating it as one overstates the problem.

The best evidence on Mounjaro specifically comes from a substudy within SURMOUNT-1.

A subgroup of 160 participants had DEXA scans at the start of the trial and again at 72 weeks, and the results were published in Diabetes, Obesity and Metabolism in 2025.

Here is what those scans showed for the participants taking Mounjaro (tirzepatide).

Two things stand out in that table.

The first is that fat mass fell three times faster than lean mass. The body was clearly drawing on fat preferentially, which is exactly what you want it to do.

The second is that although lean mass went down in absolute terms, lean tissue made up a larger proportion of participants' bodies at the end than at the start. They were, proportionally, less fat and more muscle than when they began.

The researchers also broke the results down by age, by sex, and by how much weight people lost.

The ratio barely moved.

Men and women, under 50s and over 65s, the people who lost the most and the people who lost the least, all landed at roughly three quarters fat loss and one quarter lean mass loss.

In the same SURMOUNT-1 substudy, participants who received a placebo lost 5.3% of their body weight, and roughly 75% of that was fat loss and 25% was lean mass.

Same split.

The medication changed how much weight came off.

It did not change what the weight was made of.

The pattern matches other weight loss routes.

Losing weight by eating less, by very-low-calorie dieting, or through bariatric surgery all produce a similar or larger lean-mass share.

Wegovy shows a similar picture.

The equivalent DEXA substudy from the STEP-1 trial, published in 2021, found that participants on semaglutide lost 15.0% of body weight, with fat mass down 19.3% and lean mass down 9.7%.

A large real-world analysis published as a preprint in April 2026 suggested that tirzepatide may be associated with slightly more relative lean-mass loss than semaglutide.

That analysis has not yet been peer reviewed, and it reflects routine care rather than a controlled trial.

Mounjaro also produces more total weight loss, which accounts for at least part of the difference.

Muscle matters for reasons that go well beyond how you look, and three of them are directly relevant to whether your weight loss results last.

Muscle sets part of your resting metabolism. Skeletal muscle burns roughly 13 kcal per kilogram per day at rest.

Fat tissue burns roughly 4 kcal.

Lose a few kilograms of muscle, and your body needs slightly less energy every day, which makes the same eating pattern produce less weight loss than it used to.

This is one of the mechanisms behind a stall, which we cover in Mounjaro weight loss plateaus.

It is your insurance against weight regain. Most people regain some weight after stopping a GLP-1 medicine, and regained weight tends to come back as fat rather than muscle.

Someone who protected their muscle on the way down is in a much stronger position if they ever come off treatment.

We go through this in Stopping Mounjaro: What to expect.

It is what lets you do things. Climbing stairs without stopping, carrying shopping, getting up off the floor, staying steady on your feet.

For anyone over about 60, this is the reason that matters most.

Some people tend to lose more muscle than others on Mounjaro. This includes:

• Adults over 60. Muscle mass declines naturally with age, so there is less in reserve to start with and less capacity to rebuild it.
• Anyone already low on muscle. If you have been largely sedentary for years, or you have had periods of illness or immobility, your starting point might be lower.
• People losing weight very quickly. Rapid weight loss consistently costs more lean tissue than gradual loss, whatever the method.
• Anyone struggling to eat much at all. If nausea has made eating difficult and you are eating very little, protein is usually the first thing to disappear from the diet.
• People who have previously had bariatric surgery. Body composition changes tend to be more pronounced in people who have had bariatric surgery in the past.
• Anyone with a history of an eating disorder. Weight loss medication needs careful handling here, and this should be discussed with your prescriber before starting rather than managed alone.

If two or more of these apply to you, mention this at your consultation or your next dose review. It may influence how quickly your prescriber steps you up through the doses.

If you do one thing from this article, make it this one.

The research on protein during weight loss is unusually consistent.

Eating enough of it, particularly alongside some resistance training, reduces how much of your weight loss comes from muscle.

A widely used target during active weight loss is consuming 1.2 to 1.6 grams of protein per kilogram of body weight per day.

For someone weighing 85kg, that works out at roughly 100 to 135 grams daily.

However, Mounjaro works by suppressing appetite, which means the total amount of food you are eating has dropped sharply.

Most people who track their intake for a week are surprised to find they are well under 1g/kg without realising it.

The medication does its job so effectively that protein becomes the accidental casualty.

What tends to actually work:

• Put the protein in first. Build each meal around a protein source and add the rest afterwards, rather than the other way round.
• Front-load the day. Appetite is usually more reliable at breakfast and lunch than at dinner, particularly in the first day or two after your injection.
• Use liquid protein when solid food is difficult. Greek yoghurt, milk, protein shake, or a smoothie all go down more easily on a rough day than a chicken breast will.
• Spread it across the day. Your body uses protein more effectively in reasonable amounts at several meals than in one large serving.
• Keep easy options in the house. Eggs, cottage cheese, tinned fish, pre-cooked chicken, tofu, lentils, and frozen edamame all take minutes to put together.

Our Mounjaro food list explains what to eat and what to avoid, particularly during dose increases.

If nausea is the thing standing between you and eating properly, How to fix nausea on Mounjaro covers 7 easy fixes you can try today.

One note before you make changes.

If you have a history of disordered eating, or if tracking food has been difficult for you in the past, speak to your prescriber before you start logging anything.

There are other ways to approach this that do not involve counting.

The second best thing to protect your muscles is to consistently work on resistance training.

A pooled analysis of six randomised trials in older adults on reduced-calorie diets found that adding resistance training three times a week prevented 93.5% of the lean-mass loss the diet would otherwise have caused.

Fat loss carried on as normal.

The training did not slow the weight loss down, it changed what the weight was made of.

The signal your muscles need is simply resistance, applied regularly.

What that resistance comes from matters far less than most people assume.

• Two to three sessions a week is the target. More is fine if you enjoy it, but this is where most of the benefit sits.
• Bodyweight counts. Press-ups against a wall or a worktop, sit-to-stands from a chair, step-ups, lunges, and glute bridges all work.
• Resistance bands count. They cost very little, take up no space, and are genuinely effective for upper-body work.
• Weights count, at any size. Dumbbells, kettlebells, or two full water bottles will all do the job at the start.
• Cover the whole body. Move your legs, back, chest, shoulders, and arms, rather than repeating one favourite exercise.
• Add something over time. A little more weight, another repetition, or a slightly harder variation. Progression is what keeps the stimulus working.

Walking is good too, but on its own, it does not provide enough load to preserve muscle.

Treat your steps and your resistance work as two separate things.

One practical note on timing.

Some people feel flat or slightly nauseous for a day or two after their injection.

If that happens to you, plan your training sessions later in the week when you feel more like yourself.

Your bathroom scale cannot answer this question.

It reports one number and tells you nothing about what that number is made of.

Here are better signals.

The things you can check yourself:

• Strength in your usual activities. If the weights you were lifting, or the stairs you were climbing, have become noticeably harder over a few weeks, that is worth paying attention to.
• Getting up from a chair without using your arms. A simple, sensitive test of leg strength. If it has become a push where it used to be automatic, take note.
• Grip. Jars, bags, and bottle tops. A meaningful drop in grip strength is one of the earliest signs of muscle loss.
• Where your clothes have loosened. Weight loss around the waist is expected. If your shirts have gone noticeably slack across the chest, shoulders, and upper arms, more of the loss may be coming from muscle.
• Everyday energy. Sustained fatigue that does not improve, especially alongside very low food intake, is worth mentioning to your prescriber.

If you want an actual measurement:

• DEXA scan. The most accurate option available to patients. Private scans are offered by a number of UK clinics, and one at the start and another six to twelve months after your first injection gives you a genuine comparison. It is the same technology used in the trials described above.
• Bioimpedance scales. The smart scales sold for home use estimate body composition by passing a small current through you. The absolute numbers are not reliable, because they are heavily affected by hydration, but the trend over many weeks, measured at the same time of day under the same conditions, is somewhat informative. Read the direction, not the decimal places.
• Tape measurements. Waist, hips, thigh, and upper arm, taken monthly. Crude, but a waist that keeps shrinking while your arms and thighs hold steady is a good sign.

There is a genuinely interesting line of research, and even though none of it is available yet, things look promising for the near future.

Bimagrumab is an antibody that acts on the muscle-growth pathway, originally developed for age-related muscle loss.

In the BELIEVE trial, a phase 2b study in 507 adults published in Nature Medicine in 2026, researchers combined it with semaglutide.

The results were striking.

Participants on semaglutide alone lost 7.4% of their lean mass.

Those on the combination lost 2.9% lean mass, and around 93% of their total weight loss came from fat.

Bimagrumab, on its own, actually increased lean mass while still reducing weight.

The important caveats, stated plainly:

• It is not licensed anywhere. Bimagrumab is an investigational medicine and cannot be prescribed for weight management in the UK.
• We do not supply it. No UK pharmacy does, and anywhere claiming otherwise should be treated with real suspicion.
• It was given by infusion in the trial, not by pen, and it produced its own side effects, including muscle spasms and transient changes in liver and muscle blood markers.
• Trials combining it with tirzepatide are still to report.

In the meantime, protein and resistance training remain the interventions that are available to you today, and they are considerably more effective than most people give them credit for.

For more on separating the evidence from the noise on GLP-1 medicines generally, see GLP-1 myths and facts.

Bring it up at your next dose review if any of the following apply.

• You are over 60, or you have limited muscle to begin with. A slower titration through the doses may suit you better. Our Mounjaro dose schedule guide explains how the steps normally work.
• You are losing weight faster than roughly 1 to 2kg a week over a sustained period. Slowing the rate protects lean tissue.
• Nausea is preventing you from eating properly. This is treatable, and it should not be endured for months. Check out our guide on managing nausea on Mounjaro
• Your strength has visibly declined. Worth acting on rather than monitoring indefinitely.
• You have hit a weight loss plateau and suspect muscle loss is behind it. The two are frequently connected.

Our process is straightforward. From start to finish, this is what happens when you order with us:

1. Fill in the medical questionnaire. It takes 5 to 10 minutes. The questions are thorough on purpose.
2. A GMC-registered prescriber reviews your answers. If they need more details or want confirmation from the clinician managing your heart, they will message you through your account.
3. If treatment is approved, it is dispensed and dispatched. Next-day delivery, with needles, a sharps bin, and the full patient information leaflet included.
4. If it is not approved, the prescriber explains why. You are not charged for treatment that is not issued.

Our Mounjaro prices start at £139.99, and every price includes the prescription, the consultation, the packaging, and delivery. There are no hidden fees.

We prescribe Mounjaro for weight management following an online consultation.