A body composition substudy of the SURMOUNT-1 trial found that when participants lost around 21% of their total body weight on tirzepatide (Mounjaro), roughly 25% of that weight loss came from lean mass, with the remaining 75% from fat — a favourable ratio that's broadly in line with what's expected during successful weight loss by any method. For every 1kg of lean mass lost, participants lost between 2.9 and 6.1kg of fat mass. The two most evidence-backed ways to protect muscle during treatment are adequate protein intake (around 1.2 to 1.6g per kilogram of body weight daily) and regular resistance training, ideally at least twice a week.
A body composition substudy within the SURMOUNT-1 trial used dual-energy X-ray absorptiometry (DXA) scans — a precise imaging method for measuring fat and lean mass separately — in 160 participants (124 on tirzepatide, 36 on placebo) out of the trial's 2,539 total enrollees. Participants on tirzepatide who lost around 21% of their total body weight over 72 weeks saw approximately 25% of that weight loss come from lean mass, with the remaining 75% from fat mass. This pattern is described by researchers as proportional and predictable for significant weight loss, and broadly consistent with what's seen during successful weight loss achieved through other methods, including bariatric surgery and intensive dieting.
A separate systematic review of tirzepatide studies using validated imaging methods reached a similar conclusion: treatment is associated with reductions in fat mass while relatively preserving lean mass, with muscle composition remaining stable or even showing signs of improvement in some measures.
Worth knowing
It's worth being clear that Mounjaro doesn't directly cause muscle wasting. Some lean mass reduction during weight loss reflects a normal physiological adaptation to a smaller body carrying less overall mass, not a toxic effect of the medicine on muscle tissue specifically. The clinically important question is the ratio between fat loss and lean loss, and the evidence consistently shows fat loss dominates.
When your body is in a significant calorie deficit — which is exactly what Mounjaro's appetite-suppressing effect creates — it draws on stored energy from multiple sources, not exclusively fat. Muscle tissue is metabolically active and requires energy to maintain, so during periods of reduced calorie intake, particularly if protein intake and physical activity aren't actively supporting muscle retention, some lean mass loss is a normal and expected part of the process. This isn't specific to GLP-1 medicines — it happens with any method that produces significant, sustained weight loss.
Prioritise Protein Intake
Protein provides the building blocks your muscles need to repair and maintain themselves, and requirements increase — not decrease — during a calorie deficit. Clinical guidance for GLP-1 treatment generally recommends 1.2 to 1.6g of protein per kilogram of body weight daily, notably higher than the standard 0.8g/kg general recommendation. Since Mounjaro's appetite suppression often means eating less overall, deliberately prioritising a protein source at each meal becomes more important, not less. See our dedicated nutrition guide for a full breakdown of protein targets and food sources.
Resistance Training, Not Just Cardio
- Aim for at least two resistance training sessions a week, focusing on compound exercises that work multiple muscle groups (squats, presses, rows) rather than isolated single-muscle exercises
- If you're new to strength training, starting with bodyweight exercises is a reasonable and effective entry point — you don't need a gym or equipment to begin
- Resistance training sends your body a genuine signal to retain and build muscle, even in a calorie deficit, which cardio alone doesn't provide to the same degree
Avoid an Overly Aggressive Calorie Deficit
A calorie deficit of roughly 10–25% is generally considered sustainable without disproportionately compromising lean mass. Since Mounjaro can suppress appetite quite significantly, some people end up in a much larger deficit than intended simply because they're not hungry — aiming for weight loss of around 0.5 to 2lbs a week is a reasonable target, and eating deliberately even when appetite is low, rather than skipping meals, helps avoid an unnecessarily steep deficit.
Don't Under-eat on Purpose
It might seem logical that eating even less would speed up weight loss, but severely restricting calories on top of Mounjaro's own appetite suppression tends to accelerate muscle loss disproportionately, without meaningfully accelerating fat loss. If you're struggling to eat enough due to reduced appetite, a protein shake or nutritionally complete drink between meals can help you reach your protein target without needing to eat a large volume of food.
| Strategy | Practical action |
|---|---|
| Protein | Include a protein source at every meal; consider a protein shake if appetite is very limited |
| Resistance training | At least 2 sessions weekly, focusing on major muscle groups |
| Calorie deficit | Avoid restricting further than your appetite naturally suggests |
| Weight loss pace | Aim for roughly 0.5–2lbs a week rather than the fastest possible rate |
| Hydration | Stay well hydrated, since this supports both muscle function and recovery |
Important
If you're new to resistance training or have any existing health conditions, particularly joint or cardiovascular issues, it's worth speaking to your GP or a qualified trainer before starting a new exercise programme. Building up gradually is safer and more sustainable than starting too intensively.
It's worth understanding why muscle preservation is worth actively thinking about, beyond aesthetic reasons. Muscle is metabolically active tissue, meaning it burns calories even at rest, and maintaining more of it supports your metabolic rate as you lose weight. Muscle also plays a central role in mobility, balance, and long-term functional independence, particularly as people age. For older adults especially, unintentional muscle loss during rapid weight loss carries genuine long-term risks around frailty and reduced physical function, which is exactly why the preservation strategies in this guide matter as much as the weight loss itself.
Timing Protein and Exercise Around Your Mounjaro DoseSide effects tend to be most noticeable in the days following a dose increase — see our dedicated Mounjaro side effects guide for the full picture — which can make both eating enough protein and exercising feel harder during that window. Rather than abandoning your routine entirely during a difficult few days, many people find it helpful to focus on smaller, more frequent protein-rich meals, and to keep exercise lighter (a walk rather than a heavy session) until symptoms ease, then return to your normal resistance training routine as you start to feel better. Consistency over the following weeks matters more than any single session. For the full titration timeline, see our dedicated Mounjaro dose schedule guide.
Tracking Progress Beyond the ScaleSince the scale can't distinguish between fat loss and muscle loss, relying on weight alone can be misleading — someone losing weight with a favourable fat-to-muscle ratio and someone losing an unfavourable ratio can show identical numbers. If protecting muscle matters to you, consider tracking additional measures alongside weight: how your clothes fit, strength progress in the gym (are you maintaining or improving your lifts, even as weight drops?), and if accessible, periodic body composition measurements. Strength that holds steady or improves during weight loss is a genuinely reassuring sign that muscle is being protected.
Protect Your Muscle, Not Just Your Weight
Our GPhC-registered pharmacy team can talk through practical nutrition and lifestyle strategies to help you protect muscle mass while taking Mounjaro.
Start Your Free Online Consultation → Happy Pharmacy is a GPhC-registered pharmacy (No. 9012585) — Superintendent Pharmacist: Palvinder Deol — Rated 4.8★ Excellent on Trustpilot- Effects of Tirzepatide on Skeletal Muscle Mass in Adults: A Systematic Review. Cureus, 2025.
- Does Tirzepatide Cause Muscle Loss? SURMOUNT-1 body composition substudy analysis. Diabetes, Obesity and Metabolism.
- Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). N Engl J Med. 2022.
- GLP-1 and Muscle Loss: Semaglutide, Tirzepatide and Lean Mass. Clinical review, 2026.
- Do you lose muscle mass on Mounjaro? Tirzepatide muscle loss review. Second Nature, 2024.