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Weight loss sounds exciting until you notice your strength dipping, muscle tone fading, or your body shape changing in ways you didn’t plan. Some Mounjaro users report losing not just fat, but also muscle definition and strength.
So, is muscle loss on Mounjaro real, and more importantly, can it be prevented? In this article, discover more about what research shows and how you can protect your lean muscle while still reaching your weight loss goals.
Why Muscle Loss Happens During Weight Loss
When you lose weight, you’re not just burning fat. The body uses both fat and muscle as fuel when calorie intake drops below energy expenditure. This process is natural, but if the calorie deficit is too large or protein intake too low, the proportion of muscle lost increases significantly.
Studies show that up to 20% to 40% of total weight loss can be lean tissue (including skeletal muscle) if protective strategies aren’t used (NICE, 2023; NHS, 2024).
The risk of muscle loss is higher if you follow very low-calorie diets, lose weight too quickly, or don’t stay active, especially if you skip strength training.
If your body isn’t getting enough nourishment or movement, it may start using muscle for energy. The good news is, with the right balance of protein and exercise, you can protect your strength while losing weight.
Losing muscle isn’t only about appearance. It affects strength, energy, and metabolism. But with the right steps, you can keep your body strong and toned. With less muscle, your body burns fewer calories, making it easier to gain weight back.
Low physical activity, skipping resistance exercise, and insufficient energy or protein intake accelerate muscle loss. Conversely, including regular strength training and sufficient dietary protein can halve the amount of muscle lost during weight reduction (BNF, 2024).
Does Mounjaro Specifically Cause Muscle Loss?
Mounjaro doesn’t directly cause muscle loss, but appetite suppression can lead to eating less, which increases the risk of losing lean mass if not managed properly.
Clinical trials (Jastreboff et al., 2022) show that the majority of weight lost on Mounjaro is from fat mass - roughly three times more fat than lean mass. However, a degree of muscle loss still occurs, particularly in individuals who lose weight rapidly or eat fewer than 1,200 - 1,400 kcal daily without professional supervision.
Muscle loss risk is higher for:
- Older adults (natural sarcopenia risk)
- People consuming <1.0 g/kg protein per day
- Those not engaging in resistance or strength exercise
- People with extended nausea or poor appetite from Mounjaro
The goal is to reduce fat while maintaining strength, not to eliminate all muscle loss (which is unrealistic). The key is maintaining a healthy muscle-to-fat loss ratio by optimising nutrition and movement.
How to Prevent Muscle Loss While Taking Mounjaro
There are a few recommended ways to prevent muscle loss when taking Mounjaro. A few of these include:
Eat Enough Protein
Protein is the single most important nutrient for protecting muscle during weight loss. It provides amino acids that repair and rebuild muscle fibres, and it also increases satiety, helping control appetite while supporting fat loss.
Current UK guidance suggests that adults engaging in weight loss or strength training should consume 1.2 to 1.6 g of protein per kg of body weight per day, and up to 1.8 g/kg for those undertaking regular resistance training (BNF, 2024; NICE, 2023).
That’s roughly:
- Women: 70 to 100 g/day
- Men: 90 to 130 g/day
For comparison, the UK Reference Nutrient Intake (RNI) for adults is 0.75 g/kg/day, which is sufficient for maintenance but too low to preserve muscle during calorie restriction.
Good sources of protein include:
- Lean meats (chicken, turkey, lean beef)
- Fish and seafood (especially oily fish twice a week for omega-3 support)
- Eggs and low-fat dairy (milk, yoghurt, cheese)
- Plant-based options like tofu, tempeh, lentils, beans, and quinoa
- Protein-enriched foods or shakes if appetite is reduced
Distribute protein evenly throughout the day, have around 20 to 30 g per meal, as the body can only use a limited amount of protein at once for muscle synthesis.
Include protein in your first meal of the day to reduce muscle breakdown after the overnight fast, and combine it with resistance activity where possible.
According to the Eatwell Guide (Public Health England, 2016), adults should aim to include at least two portions of fish per week, one of which is oily, and should prioritise lean and plant-based protein options for long-term cardiovascular health.
Strength Train 2x/Week
Do resistance workouts like squats, lunges, push-ups, or bands. Gradually increase the difficulty to keep muscles strong.
The UK Chief Medical Officer’s Physical Activity Guidelines (2019) recommend that adults perform activities to strengthen muscles on at least two days each week, alongside 150 minutes of moderate-intensity aerobic activity (e.g. brisk walking, cycling, swimming).
Examples of effective strength exercises include:
- Squats, lunges, and push-ups (body weight)
- Resistance bands or dumbbell training
- Pilates or yoga for controlled resistance
- Functional tasks like stair climbing or carrying groceries
For best results, aim for 8 to 10 exercises targeting major muscle groups, performing 2 to 3 sets of 8 to 12 repetitions with good form. Progressive overload, which means gradually increasing the weight or difficulty, is essential for ongoing adaptation.
Avoid Extreme Calorie Deficits
Don’t under-eat, aim for steady loss. Severe restriction increases muscle loss and fatigue.
Rest and Sleep
Muscles recover at rest. Get 7–8 hours of sleep and avoid overtraining.
Monitor & Get Advice
If strength drops quickly, speak with a doctor or registered dietitian. Adjustments, protein shakes, or nutrient checks may help.
When to Seek Medical Advice
It’s normal to lose some weight and even a little muscle on Mounjaro, but there are times when it’s worth checking in with a professional. If you notice sudden weakness, significant changes in body shape, or muscle loss in areas like your thighs, arms, or glutes, it may be a sign that your body needs extra support. Ongoing nausea that makes it hard to eat enough is also something you shouldn’t ignore.
In some cases, thyroid or hormonal issues could be contributing to the problem. Speaking with your GP can rule out anything serious, while a dietitian can help you fine-tune your protein, calorie balance, and overall nutrition to keep your strength and energy on track during your weight loss journey.
Take Action! Don’t wait, book an appointment with your GP or a registered dietitian if you notice these changes, and get personalised guidance to protect your muscles and stay healthy on Mounjaro.
FAQs
Does Mounjaro cause loss of muscle mass?
Mounjaro may lead to some muscle loss as part of overall weight reduction, especially if protein intake and resistance training are lacking. Most weight loss comes from fat, but preserving muscle requires lifestyle support.
Can I build muscle while on Mounjaro?
Yes, you can build muscle on Mounjaro with regular strength training and adequate protein intake. The medication doesn’t block muscle growth, it’s your diet and workout routine that matter most.
What are the signs of muscle loss?
Common signs include weakness, reduced strength, fatigue, slower metabolism, and a “softer” body composition, even with weight loss. Regularly tracking strength and energy helps spot early changes.
Can you regain muscle mass?
Yes, lost muscle mass can be regained through consistent resistance exercise, higher protein intake, and recovery. The body responds well once proper nutrition and training are in place.
How much protein do I need?
Most adults require 1.2 to 1.6 g/kg/day. For example, a 70 kg woman needs 85 to 110 g/day, and an 80 kg man needs 95–130 g/day.
Is muscle loss dangerous?
Excessive muscle loss can harm strength, balance, and metabolism, raising risks of frailty and injury. Preserving lean mass is key to long-term health and sustainable weight loss.
Does Mounjaro affect leg muscles?
Mounjaro doesn’t specifically target leg muscles, but without exercise, general muscle loss can occur, including in the legs. Regular lower-body workouts help maintain strength.
How to activate muscle mass?
Combine progressive strength training, high-protein meals, and proper sleep. Activities like weightlifting, resistance bands, and bodyweight exercises are most effective for muscle activation and growth.
Key Takeaways
- Mounjaro users may experience some muscle loss alongside fat loss, especially without enough protein or strength training.
- Weight loss naturally burns both fat and muscle; up to 20–40% of lost weight can be muscle if not managed.
- Rapid weight loss, very low-calorie diets, and inactivity increase the risk of muscle loss.
- Muscle loss reduces strength, slows metabolism, and can make weight regain easier.
- Mounjaro doesn’t directly cause muscle loss, but appetite suppression and rapid fat loss can affect lean mass.
- Most weight lost on Mounjaro is fat, roughly three times more than muscle, but some users report reduced strength or softer body composition.
- Prevent muscle loss by eating 1.2 to 1.6 g protein/kg body weight, spreading intake across meals.
- Follow the Eatwell Guide for balanced, nutrient-dense meals.
- Engage in resistance training at least twice a week, using progressive overload to maintain strength.
- Avoid extreme calorie deficits; aim for steady, sustainable weight loss.
- Prioritise rest and sleep, 7 to 8 hours nightly, to support muscle recovery.
- Monitor strength and energy, and consult a doctor or dietitian if rapid muscle loss, weakness, or persistent nausea occurs.
- Muscle can be regained with proper exercise, protein intake, and recovery.
- Signs of muscle loss include weakness, fatigue, slower metabolism, and softer body composition.
- Mounjaro doesn’t specifically affect leg muscles, but regular lower-body workouts help maintain strength.
- Activate and preserve muscle with progressive strength training, adequate protein, and proper sleep.
Disclaimer: At Weightless, we strive to provide accurate, up-to-date health information that is carefully reviewed by medical professionals. However, the content on this blog is intended for general knowledge only and should not be considered a substitute for medical advice, diagnosis, or treatment. Always speak with a qualified healthcare professional about any questions or concerns you may have regarding your health. Information here should not be used to make decisions about your personal care without consulting a health care professional.
Sources:
https://doi.org/10.1056/NEJMoa2032183
https://doi.org/10.1056/nejmoa1616338
https://doi.org/10.1111/dom.15728
https://link.springer.com/article/10.1007/s11934-023-01180-7
https://pmc.ncbi.nlm.nih.gov/articles/PMC5421125/
https://www.gov.uk/government/publications/the-eatwell-guide
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