Mounjaro and Muscle Loss: How to Preserve Lean Mass on Tirzepatide

One of the most common concerns among patients starting Mounjaro (tirzepatide) for weight loss is whether the medication will cause them to lose muscle along with fat. Muscle loss during weight loss is a legitimate clinical consideration, and understanding the evidence can help you make informed decisions about how to protect your lean mass while benefiting from treatment.

At CutKilo, our doctors monitor body composition as part of your ongoing care. Preserving muscle is not just about appearance; it supports metabolic health, functional strength, and long-term weight maintenance after treatment ends.

Some lean mass loss is expected during any significant weight loss, whether achieved through medication, diet, or surgery. The SURMOUNT-1 DXA substudy, published in Diabetes, Obesity and Metabolism (2025), found that approximately 75 per cent of total weight lost on tirzepatide came from fat mass and 25 per cent from lean mass. This ratio is consistent with what is seen in calorie-restricted diets and is considered clinically acceptable. The key point is that tirzepatide preferentially targets fat, and there are practical steps you can take to minimise lean mass loss further.

The most detailed body composition data come from the SURMOUNT-1 DXA substudy (Look et al., 2025), which followed 160 adults with overweight or obesity over 72 weeks. Participants had a mean baseline weight of 102.5 kg and a BMI of 38.0. After treatment with tirzepatide, the mean total weight reduction was 21.3 per cent. Of that, roughly three quarters was fat mass and one quarter was lean mass. This proportion was consistent across subgroups stratified by age, sex, and degree of weight loss.

A separate post-hoc analysis of the SURPASS-3 trial, published in The Lancet Diabetes and Endocrinology (2025), used MRI to assess muscle composition in patients with type 2 diabetes. The study found that while total lean mass decreased modestly, indicators of muscle quality (including intramuscular fat infiltration) actually improved. This suggests that the muscle you retain may function better than before treatment, even if total lean mass is slightly lower.

A 2025 systematic review published in Cureus, which pooled data from multiple tirzepatide trials, concluded that treatment is associated with reductions in fat mass while maintaining the relative preservation of lean mass. Indicators of muscle composition remained stable or showed signs of improvement across the studies reviewed.

It is important to understand that losing some lean mass during weight loss is a normal physiological response, not a side effect unique to Mounjaro. When the body is in a caloric deficit, it draws energy from both fat stores and lean tissue. This happens whether weight loss is achieved through medication, surgery, or dietary restriction alone.

In fact, the 75:25 fat-to-lean ratio seen with tirzepatide compares favourably with many dietary interventions, where lean mass can account for 30 to 40 per cent of total weight lost in the absence of resistance training. Bariatric surgery patients typically lose a higher proportion of lean mass in the first year post-operatively. The clinical goal is not to eliminate lean mass loss entirely but to maximise the proportion of weight lost that comes from fat.

The evidence supports several practical strategies for preserving lean mass during tirzepatide treatment. These interventions work best when combined, and your prescribing doctor can help you tailor them to your circumstances.

Resistance training is the single most effective intervention for preserving muscle during weight loss. Aim for at least two to three sessions per week, targeting all major muscle groups. You do not need to lift heavy weights; bodyweight exercises, resistance bands, and moderate dumbbell work are all effective. A clinical trial registered on ClinicalTrials.gov (NCT06885736) is currently investigating the specific combination of resistance exercise and increased protein intake during GLP-1 receptor agonist therapy, reflecting growing clinical interest in this approach. For practical guidance on exercise while on Mounjaro, our separate guide covers training recommendations in more detail.

Adequate protein intake is essential. Current evidence suggests aiming for 1.2 to 1.6 grams of protein per kilogram of body weight per day during active weight loss. For a person weighing 90 kg, this means approximately 108 to 144 grams of protein daily. Spreading protein intake across meals (rather than concentrating it in one sitting) optimises muscle protein synthesis. Good sources include lean meat, fish, eggs, Greek yoghurt, legumes, and tofu.

Avoiding excessive caloric restriction is also important. While Mounjaro naturally reduces appetite, eating too little can accelerate lean mass loss. Your doctor or dietitian can help you find the right balance between sufficient caloric deficit for weight loss and adequate nutrition for muscle preservation.

Adequate sleep supports muscle recovery and hormonal balance. Growth hormone, which plays a role in muscle maintenance, is released primarily during deep sleep. Aim for seven to nine hours per night and maintain a consistent sleep schedule.

Standard bathroom scales cannot distinguish between fat loss and muscle loss. If you are losing weight on Mounjaro, the number on the scale tells you very little about what type of tissue you are losing. This is why body composition monitoring is increasingly recommended for patients on GLP-1 receptor agonist therapy.

DEXA London, CutKilo\’s sister service at the same 86 Harley Street clinic, offers body composition scanning specifically for GLP-1 patients. A DEXA scan measures fat mass, lean mass, and bone mineral density with clinical-grade precision. By scanning at baseline and at intervals during treatment, you can confirm that the majority of your weight loss is coming from fat and take corrective action (such as increasing protein or resistance training) if lean mass is declining faster than expected.

The medical community is actively investigating pharmaceutical approaches to preserving lean mass during GLP-1 mediated weight loss. The EMBRAZE phase 2 trial, published in Nature Medicine (2026), tested apitegromab (a myostatin inhibitor) alongside tirzepatide. Participants receiving the combination therapy lost less lean mass compared with those receiving tirzepatide and placebo, suggesting that targeted combination therapies may become available in the future.

These therapies are not yet available outside of clinical trials, but they reflect the growing recognition that lean mass preservation is an important treatment goal alongside fat reduction. For now, the practical strategies of resistance training, adequate protein, and body composition monitoring remain the evidence-based standard of care.

Certain patient groups may be more susceptible to lean mass loss during treatment and should receive closer monitoring. Patients over the age of 65 are at higher risk because age-related sarcopenia (progressive muscle loss) is already occurring. In this group, preserving functional muscle mass is particularly important for maintaining mobility, balance, and independence.

Patients with type 2 diabetes may also warrant closer attention, as insulin resistance can affect muscle protein metabolism. However, the SURPASS-3 MRI data are reassuring, showing that muscle quality actually improved in this population during tirzepatide treatment. Patients who are sedentary, those with very low protein intake, and those losing weight very rapidly (more than 1 kg per week sustained over several months) should also discuss muscle preservation strategies with their doctor.

Mounjaro does cause some lean mass loss alongside fat loss, but the evidence shows that approximately 75 per cent of weight lost comes from fat. This ratio compares favourably with other weight loss methods. The most effective strategies for preserving muscle are resistance training two to three times per week, consuming 1.2 to 1.6 grams of protein per kilogram of body weight daily, avoiding excessive caloric restriction, and monitoring your body composition with DEXA scanning rather than relying on scales alone. At CutKilo, our doctors incorporate these principles into every patient\’s treatment plan to ensure that weight loss is both effective and sustainable.

How much muscle do you lose on Mounjaro? The SURMOUNT-1 DXA substudy found that approximately 25 per cent of total weight lost on tirzepatide was lean mass. For a patient who loses 20 kg, this means roughly 5 kg of lean mass and 15 kg of fat. Resistance training and adequate protein can reduce this proportion further.

Can you build muscle while on Mounjaro? Building significant new muscle while in a caloric deficit is difficult, but maintaining existing muscle mass is achievable with consistent resistance training and sufficient protein intake. Some patients report improved exercise capacity as they lose weight, which can support muscle maintenance.

Is the muscle loss from Mounjaro permanent? No. Lean mass can be rebuilt after weight stabilisation through progressive resistance training and adequate nutrition. The key is to minimise unnecessary lean mass loss during the active weight-loss phase so that less rebuilding is needed.

How do I know if I am losing muscle or fat on Mounjaro? A standard bathroom scale cannot distinguish between fat and muscle loss. The most accurate way to track body composition changes is with a DEXA scan, which measures fat mass, lean mass, and bone density separately. Your doctor may also monitor grip strength or functional fitness markers as indirect indicators of muscle health.

Should I take creatine while on Mounjaro? Creatine is one of the most well-studied supplements for supporting muscle mass and exercise performance. There are no known interactions between creatine and tirzepatide. However, discuss any supplements with your prescribing doctor before starting them, as individual circumstances vary.