How to Preserve Muscle on Mounjaro: Protein and Strength Training

Medically reviewed by Dr Saira Bano, BMBS, BMedSci, MRCGP, PGCert MedEd, SCOPE-Certified Obesity Specialist

Losing weight is the goal of Mounjaro treatment, but not all of the weight you lose is fat. Some loss of lean muscle is a well-recognised feature of rapid weight loss, whatever method is used to achieve it.

That is worth knowing rather than worrying about. Muscle loss during weight loss is largely manageable with the right nutrition and the right kind of activity, and the steps involved are practical rather than complicated.

This guide explains why lean muscle can be lost during treatment, how much protein is generally recommended, why strength training matters more than cardio for protecting muscle, and how to start training safely when your appetite and energy are lower than usual.

Does Mounjaro cause muscle loss?

Mounjaro does not directly break down muscle tissue. What happens is more indirect: the medication reduces appetite, which means you eat less, and a sustained calorie deficit combined with a lower protein intake is what puts lean muscle at risk.

When weight comes off quickly, a proportion of it is typically lean mass rather than fat. In the body composition substudy of the SURMOUNT-1 trial, roughly 75% of the weight lost was fat mass and around 25% was lean mass. [1] What is genuinely reassuring about that figure is the comparison group: participants taking a placebo who lost weight showed almost exactly the same split, at about 75% fat and 25% lean mass. [1]

In other words, this is a feature of losing weight itself rather than a peculiarity of Mounjaro or of GLP-1 injections generally. Any significant weight loss, whether achieved through dieting alone, bariatric surgery or medication, carries the same pattern.

It matters because muscle does more than move you around. It supports your metabolism, your strength and balance, your blood sugar regulation, and your ability to keep weight off once treatment ends. Losing more lean mass than necessary can make long-term maintenance harder than it needs to be.

Some people are at greater risk than others. Older adults, and anyone who was already inactive before starting treatment, tend to have less muscle in reserve to begin with, and should pay particular attention to protein and resistance exercise from the outset.

How much protein do you need on Mounjaro?

Protein is the single most important nutrient for protecting muscle while you lose weight. It provides the building blocks muscle needs for repair and maintenance, which helps preserve lean tissue while fat comes off.

Research on weight loss during GLP-1 treatment suggests aiming higher than the standard adult recommendation, often somewhere in the region of 1.2 to 1.6 grams of protein per kilogram of body weight per day. [2] Treat that as general guidance rather than a personal target. Individual needs vary considerably with age, activity levels and health circumstances, so it is worth discussing what is appropriate for you with a clinician or dietitian.

The practical challenge on Mounjaro is not knowing the number, it is hitting it on a smaller appetite. Protein needs to be prioritised at each meal rather than left as an afterthought at the end of the day. Spreading it across the day, roughly 20 to 30 grams per meal, tends to be more effective than one large protein-heavy meal, because the body uses protein more efficiently when intake is distributed. [3]

Useful, everyday protein sources include:

• Eggs, Greek yoghurt, cottage cheese and milk
• Chicken, fish and lean beef
• Tofu, lentils, beans and edamame

If your appetite is very low, a protein shake or a high-protein snack can help fill a gap, though these work best supporting meals rather than replacing them entirely. Building meals around protein is covered in more depth in our Mounjaro diet guide. If you are consistently struggling to meet your protein needs, speak to a clinician or dietitian rather than forcing intake or restricting further elsewhere.

Can you exercise on Mounjaro?

Yes. For most people, exercise is both safe and recommended during treatment, and staying active is one of the most effective things you can do to protect muscle mass.

That said, some people feel lower in energy, particularly in the first few weeks or after a dose increase, because they are eating less and may be dealing with side effects such as nausea. [4] That is a normal part of adjusting, and it usually settles as your body adapts.

A few practical adjustments help when energy is lower than usual. Start gently rather than picking up exactly where you left off. Train at the time of day when your energy is highest, which for many people is late morning or early afternoon. Eat something small containing protein and carbohydrate an hour or two beforehand, stay well hydrated, and avoid training on an empty stomach if that tends to worsen nausea.

There are also times to pause and check in with a clinician: if you feel faint or dizzy, if nausea or vomiting is significant, if your food intake has dropped very low, or if you notice any new symptoms during activity. If you were inactive before starting treatment, build up gradually rather than launching into high-intensity sessions.

Why strength training matters more than cardio for preserving muscle

Walking, cycling and other cardio activities are genuinely valuable for heart health and overall energy balance, and they are well worth doing. But they do not send a strong enough signal to the body to hold on to muscle while you are in a calorie deficit.

Resistance training does. Using body weight, resistance bands, free weights or machines tells the body that muscle is being used and therefore needs to be kept, which is why it is the most important type of exercise for anyone losing weight on Mounjaro.

UK guidance recommends strengthening activities that work all the major muscle groups (legs, hips, back, abdomen, chest, shoulders and arms) on at least two days a week. [5] Two to three short sessions a week is a realistic starting point, and none of it requires a gym membership. Simple, accessible options include:

• Squats, or standing up from a chair and sitting back down
• Wall push-ups or push-ups from your knees
• Resistance band rows
• Glute bridges
• Step-ups onto a low step

The principle that drives results is progressive overload, which sounds technical but simply means doing a little more over time. An extra repetition or two, slightly heavier resistance, or an additional set is what keeps the signal strong enough to preserve muscle.

One common worry is worth addressing directly: strength training will not make you bulky. Building or maintaining muscle while fat comes down is what produces a healthier body composition, and it tends to be exactly what people are aiming for. Combining resistance work with regular walking or light cardio is better than choosing one over the other.

Other habits that help protect muscle during weight loss

Sleep does more work than most people realise. Muscle repair happens largely while you are asleep, and poor sleep also tends to increase appetite and sap the motivation to exercise.

Hydration matters for energy, performance and recovery, and thirst is easily mistaken for hunger, which can muddle your sense of how much you have actually eaten.

The pace of weight loss is also relevant. Very rapid loss increases the proportion of lean mass that comes off, which is one reason clinicians aim for steady, sustainable progress rather than the fastest possible result.

Try not to skip meals entirely, even when appetite is low. Small, regular meals containing protein help avoid the long gaps that encourage muscle breakdown. For some people eating smaller amounts overall, a general multivitamin can act as a useful safety net, though it supports a varied diet rather than replacing one.

When to speak to a clinician about muscle loss or weakness

Some reduction in strength can be normal during the early weeks as your body adjusts to eating less. It is worth seeking a review, though, if you notice ongoing weakness, unusual fatigue, difficulty with everyday tasks such as climbing stairs or carrying shopping, or weight coming off unusually quickly.

A clinician can review your dose, talk through your nutrition, and refer you for dietitian support where that would help. SwiftDoctor's clinical team offers ongoing treatment check-ins for exactly this kind of question, so there is no need to wait until something feels serious before raising it.

How SwiftDoctor supports you throughout treatment

Weight loss treatment at SwiftDoctor is doctor-led, and support does not stop once a prescription has been issued. Muscle preservation, nutrition and exercise are exactly the kinds of questions that come up once treatment is underway rather than at the point of prescribing.

Questions about nutrition, exercise and side effects can be raised with the clinical team at any stage of treatment, whichever of our weight loss treatments you are on.

Frequently asked questions

Can you build muscle while taking Mounjaro?

It is possible, but building new muscle can be harder while you are in a calorie deficit. During active weight loss, preserving the muscle you already have is often the main goal, while people who are new to resistance training may still see strength gains. Prioritising adequate protein and training consistently can help.

Is it better to lift weights or do cardio on Mounjaro?

For protecting muscle specifically, resistance training is more effective, because it signals to the body that muscle is needed and should be kept. Cardio remains valuable for heart health and overall fitness, so the ideal approach is both rather than one instead of the other. [5]

Should I take creatine or protein powder on Mounjaro?

Protein powder can be a practical way to fill a gap when appetite is too low to eat enough protein from food, and it is not a problem to use alongside treatment. Creatine is a well-studied supplement for supporting strength training, but as with anything new, it is worth checking with a clinician or pharmacist first, particularly if you take other medication.

Does muscle loss stop when you stop taking Mounjaro?

Muscle loss is driven by the calorie deficit rather than by the medication itself, so it tends to slow once weight loss slows or stops. Stopping treatment does not automatically rebuild lost muscle, though. Continuing resistance training and adequate protein intake is what supports recovery of lean mass over time.

Why do I feel weaker since starting Mounjaro?

Reduced energy intake is the usual explanation, particularly in the early weeks or after a dose increase, and it often improves as your body adjusts. [4] Low protein intake and reduced activity can both contribute as well. If weakness is persistent, worsening, or affecting everyday tasks, it is worth having reviewed by a clinician.

Can I see how much muscle I have lost?

Standard bathroom scales cannot reliably distinguish fat from muscle, and consumer body-composition scales are only estimates. DEXA can estimate lean tissue more precisely, but it does not directly measure skeletal muscle mass. [6] In practice, tracking your strength, how everyday tasks feel, and how your clothes fit gives a more useful picture than chasing a precise number.

Sources

1. Look, M. et al. (2025). Diabetes, Obesity and Metabolism, "Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight", pubmed.ncbi.nlm.nih.gov
2. Mozaffarian, D. et al. (2025). Obesity, "Nutritional priorities to support GLP-1 therapy for obesity: A joint Advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society", pmc.ncbi.nlm.nih.gov
3. Mamerow, M.M. et al., The Journal of Nutrition, "Dietary protein distribution positively influences 24-h muscle protein synthesis in healthy adults", pmc.ncbi.nlm.nih.gov
4. Electronic Medicines Compendium (eMC), "Mounjaro solution for injection: Summary of Product Characteristics" (Eli Lilly), medicines.org.uk
5. NHS, "Physical activity guidelines for adults aged 19 to 64", nhs.uk
6. Cawthon, P.M. (2015). Journal of Clinical Densitometry, "Assessment of Lean Mass and Physical Performance in Sarcopenia", pubmed.ncbi.nlm.nih.gov