FM Coaching · Weight Loss & GLP-1

Mounjaro Muscle Loss: What New 2026 Studies Reveal About Fat, Lean Mass and Strength

Major 2026 reviews find that some lean mass is lost during GLP-1 weight reduction but cannot yet establish what this means for strength. FM Coaching explains the findings, limitations and practical support.

By FM Coaching Editorial Team · Published 27 October 2026 at 19:59 UK

Mounjaro and glp1 weight loss jab — a Mounjaro weight loss jab and a glp 1 jab FM Coaching · Original image — authorised for FM Coaching use

New 2026 reviews are adding detail to a question affecting people using Mounjaro and Wegovy: how much of the weight lost is fat, how much is lean tissue, and what does that mean for actual strength? The short answer is that most measured weight loss is fat. Lean tissue also tends to fall, but the popular claim that “a quarter of weight lost is muscle” goes further than the evidence allows.

By FM Coaching Editorial Team · Fitness News / Weight Loss & GLP-1 · Research checked 27 October 2026.

A systematic review published 12 September 2026, spanning 60 publications, found reductions in lean or fat-free mass among GLP-1 users but described the certainty of the muscle evidence as low. The researchers could not establish that these changes caused meaningful losses of muscle strength or physical performance. A separate 2026 review of 35 studies likewise found considerable variation in the measurements used and noted that none of the included studies reported objective physical-function outcomes.

That distinction matters when a weight-loss medicine is helping someone achieve a substantial change on the scales: preserving independence, strength and adequate nutrition can matter as much as reaching a particular number.

AI-generated illustrative editorial image about medication-supported weight management, nutrition and resistance training. Not an actual Mounjaro product photograph. Credit: Original AI-generated editorial illustration for FM Coaching.

Mounjaro and muscle loss: the key facts

Most measured weight loss with tirzepatide is fat. In a 160-person DXA substudy of SURMOUNT-1, roughly 74% of the weight lost in the tirzepatide group was fat and 26% was lean tissue at 72 weeks.
Lean tissue is not identical to skeletal muscle. Depending on the scan and definition, it may include water and other non-fat tissue.
Some lean-mass reduction also occurs with non-drug weight loss. The SURMOUNT-1 placebo group showed a similar fat-versus-lean proportion, despite losing much less weight in absolute terms.
Physical-function outcomes remain an evidence gap. Losing measured lean mass does not automatically mean a matching decline in strength.
Exercise, nutritional adequacy and clinical follow-up matter. Treatment decisions belong with the prescriber; fitness coaching can complement, but not replace, clinical care.

What the 2026 studies actually found

It is easy to find striking percentages in headlines. They make far more sense alongside the study designs, the number of people who actually had body-composition measurements, and the outcomes the researchers did not measure.

Research	What it found	Important limitation
SURMOUNT-1 DXA substudy (2025)	Among 160 participants with DXA measurements, about 74% of tirzepatide-associated weight loss was fat and 26% was lean mass over 72 weeks.	This was a substudy, not a direct measurement of kilograms of skeletal muscle lost in every participant.
Batsis and colleagues (2026; 35 studies)	Median 28.3% of weight lost was represented by reductions in various muscle-related body-composition indices within incretin-treated groups.	DXA/BIA and MRI/CT metrics differ; none of the studies reported objective physical-function outcomes.
2026 randomised-trial meta-analysis (20 trials)	Lean mass represented about 25–39% of weight loss across incretin-treatment groups; lifestyle-plus-resistance-training groups had a more favourable pooled proportion.	Comparisons across heterogeneous trials are not proof of an individual treatment effect, nor a direct Mounjaro-plus-training trial.
Beaudart and colleagues (September 2026; 60 publications)	Across the review, lean/fat-free mass was lower with GLP-1 treatments in the pooled analysis, while adjusted bone and joint outcomes did not show clear adverse effects.	Evidence certainty for the lean-mass outcome was low, heterogeneity high, and effects on strength or performance uncertain.
Tirzepatide narrative review (28 September 2026)	Across ten included studies, loss was predominantly fat; reported lean-mass proportions generally ranged from about 10% to 30% of total loss.	Body-composition methods and study populations differed considerably; functional outcomes were lacking.

A vital reading note: the 60-publication review covered more than 1.2 million people across many outcomes, but that does not mean 1.2 million people underwent a body-composition scan. The lean-mass analysis drew on a smaller subset of studies. Presenting that giant participant total as a muscle-scan sample would be misleading.

Does Mounjaro itself destroy muscle?

The evidence does not justify describing tirzepatide as a drug proven to directly destroy skeletal muscle. Tirzepatide is a prescription medicine that acts at GIP and GLP-1 receptors, helping reduce appetite and food intake. Significant weight reduction commonly involves some loss of non-fat tissue, whether the weight loss comes from medication, diet or other interventions.

That does not mean the reduction is irrelevant. Someone beginning with low muscle reserves, losing weight rapidly, eating too little or becoming less active may need additional attention. The unanswered questions are how much actual skeletal muscle is affected in different people, whether strength changes, and which combinations of exercise and nutrition best protect long-term function.

Why 26% lean mass is not 26% skeletal muscle

Lean mass is a body-composition category. On a DXA scan, it is not synonymous with the contractile skeletal muscle that enables you to lift, walk and get out of a chair. Changes in hydration, glycogen-associated water and other non-fat tissues can influence the result; different scanning technologies estimate different compartments.

Consider a hypothetical person who loses 10 kg and has the same 74%/26% DXA partition seen in the tirzepatide substudy:

Component	Illustrative change	Interpretation
Fat mass	7.4 kg	Measured fat reduction
Lean mass	2.6 kg	Non-fat tissue measured by DXA—not necessarily 2.6 kg of skeletal muscle
Total	10 kg	Only an example using a study average; not a forecast for any individual

The specific numbers can vary widely with age, starting composition, diabetes status, training history and the method used. Neither this example nor the trial average tells one person how much muscle they will lose.

What the SURMOUNT-1 trial means—and what it cannot establish

The published SURMOUNT-1 DXA substudy followed 160 people with baseline and week-72 scans. The tirzepatide group lost an average 21.3% of starting body weight, against 5.3% in the placebo group. Fat mass fell 33.9% and lean mass 10.9% from their respective starting values in the tirzepatide group.

These percentages have different denominators: the 21.3% describes total body-weight change, while 33.9% and 10.9% describe changes within the separate fat and lean compartments. They must not be added together.

Researchers calculated that about 74% of tirzepatide-associated weight loss came from fat mass and 26% from lean mass. The placebo group had a near-identical proportion—about 75% fat and 25% lean—despite losing substantially less weight overall. This supports the interpretation that substantial weight loss itself is an important part of the body-composition change; it does not settle every question about direct medication effects or long-term physical function.

The study also involved a small imaging subset, and several authors disclosed financial links to the manufacturer. Readers should consider those limitations when interpreting the result.

Mounjaro versus Wegovy: which causes more muscle loss?

Mounjaro contains tirzepatide (a dual GIP/GLP-1 receptor agonist); Wegovy contains semaglutide (a GLP-1 receptor agonist). Both can produce substantial weight reduction and some lean-mass change. However, percentages quoted from separate studies are not automatically comparable: populations, weight-loss magnitude, scan methods and follow-up periods differ.

A 2026 meta-analysis of randomised trials estimated the proportion of weight lost as lean mass at approximately 25.4% for tirzepatide and 35.2% for semaglutide groups. Those pooled proportions should not be read as proof that one specific medicine is definitively “safer for muscle” for any individual. Comparing medicine effects properly requires matched populations and suitable head-to-head data.

Medication choice and dosing must be discussed with an appropriately qualified prescriber—not determined from muscle-loss headlines or an online fitness article.

What the research suggests about resistance training

Illustrative resistance-training photograph; not a participant in a clinical trial. Credit: Original AI-generated editorial illustration for FM Coaching.

Resistance training puts muscles under a repeated load and can support strength, function and retention of muscle through weight loss. A supportive-care expert consensus emphasises individualised exercise, nutritional adequacy and the importance of strength training alongside weight-management treatment. A 2026 consensus in The Lancet Diabetes & Endocrinology similarly highlights nutritional and functional needs during incretin therapy.

For healthy adults, the UK Chief Medical Officers' guidelines recommend muscle-strengthening activity on at least two days per week, alongside regular aerobic movement. These are general public-health targets, not a personalised prescription for someone who is unwell, frail or newly starting treatment.

A gentle two-day strength-training example

This is an illustrative beginner framework for adults cleared to exercise, not a treatment plan or a requirement to train through medication side effects. Choose loads and exercises appropriate to your ability and ask a qualified clinician or exercise professional for modifications when needed.

Session A	Session B	Start-small approach
Chair squat or supported squat	Step-up or sit-to-stand	1–2 comfortable sets of 6–10 repetitions
Wall or incline push-up	Light dumbbell chest press or incline push-up	1–2 comfortable sets of 6–10
Seated resistance-band row	Supported one-arm dumbbell row	1–2 comfortable sets of 8–12
Hip bridge	Supported hip hinge or glute bridge	1–2 comfortable sets of 8–12
Optional easy walking	Optional easy walking	Build up from a manageable baseline

Leave recovery time between sessions. If you become dizzy, unusually weak, persistently nauseated or unable to maintain fluids, stop exercising and seek medical advice rather than trying to push through. Over time, progression might mean better movement control, slightly more repetitions or a modest resistance increase—not pushing every set to exhaustion.

What to eat when appetite is lower

Illustrative balanced-meal image. Individual nutritional needs vary. Credit: Original AI-generated editorial illustration for FM Coaching.

Reduced appetite can make it easier to eat less, but it can also make regular meals, protein and fluids difficult to manage. The aim is not to eat as little as possible. Adequate nutrition supports everyday function, training, recovery and medical treatment.

An international expert advisory on GLP-1 nutrition notes that protein targets during active weight loss must be individualised, especially because using total body weight to calculate needs can overestimate requirements in some people with obesity. Higher protein intakes are sometimes proposed, but no single grams-per-kilogram rule is appropriate for everyone—particularly those with kidney disease or other medical restrictions.

A practical balanced-meal framework

Include a protein-containing food at meals: eggs, fish, lean poultry, tofu, lentils, yoghurt or alternatives appropriate to your dietary needs.
Keep carbohydrates where useful: oats, potatoes, rice, wholegrains and fruit can help supply energy, particularly if you are training.
Choose fibre-rich foods: vegetables, fruit, pulses and suitable wholegrains, adapting choices if gastrointestinal symptoms make them difficult to tolerate.
Drink regularly: follow advice from your prescribing team, especially when experiencing nausea, vomiting or diarrhoea.
Seek help early if food intake falls sharply: persistent nausea, prolonged very low intake or unexpected weakness deserve clinical review.

Simple options might include Greek-style yoghurt with oats and berries, beans or tofu on toast, and chicken or fish with rice and vegetables. These are food ideas, not fixed calorie or protein prescriptions.

Who needs extra attention to strength and nutrition?

The stakes can be higher for older adults; people with already low muscle mass or mobility problems; those recovering from illness; people with very restricted intake or repeated vomiting; and those with medical conditions that affect exercise or nutrition. People with type 2 diabetes taking other glucose-lowering medicines may also need a prescriber to review their treatment and monitoring.

The right response is not to stop a prescribed medicine on the basis of a headline. It is to raise symptoms, functional changes and nutrition concerns with the clinician overseeing treatment, and to ask about dietetic, rehabilitation or exercise support when appropriate.

How to monitor progress beyond the scales

What to follow	What it may help show	Caution
Body-weight trend	Overall change over time	It cannot distinguish fat and muscle.
Waist measurement and clothing fit	Context for body-size changes	These are not direct muscle measurements.
Training log	Whether comparable movements feel stronger or weaker	Performance also changes with fatigue, hydration and technique.
Daily function	Ability to walk, climb stairs and complete normal tasks	New or persistent decline warrants medical assessment.
Clinical body-composition measures, where indicated	Changes in fat and estimated lean compartments	DXA, BIA and imaging use different assumptions; BIA is hydration-sensitive.

No single number reliably captures health. A clinically meaningful picture considers weight, eating adequacy, symptoms, medical conditions and everyday function together.

When to contact your prescriber

Contact your prescribing service if you have ongoing difficulty eating enough, repeated vomiting or diarrhoea, severe weakness, dizziness, concerning dehydration or a marked decline in function. Seek urgent medical assessment for severe, persistent abdominal pain or other serious symptoms. Do not change your dose, combine treatments or discontinue prescription medication solely because of advice in this article.

For UK readers, the NHS tirzepatide information, NICE guidance and MHRA guidance provide current clinical and medicine-safety context.

What about new drugs designed to protect muscle?

Researchers are investigating additional medicines and combinations that might help retain more lean mass during pharmacological weight loss. Early company announcements and phase 2 trial results can be scientifically interesting, but these should not be confused with approved routine care. Long-term safety, meaningful strength outcomes and clinical value still require further evidence. For most readers today, the immediate discussion is about their prescribed treatment, nutritional adequacy and achievable activity.

What this means for FM Coaching readers

There are two distinct jobs here. A medical team assesses whether weight-loss medication is appropriate, monitors treatment and manages adverse effects. A fitness coach may help a medically cleared person develop a sustainable strength-training routine, improve movement confidence and build consistent habits. Neither role should be misrepresented as the other.

FM Coaching provides online fat-loss coaching and strength and muscle-building support built around individual training experience and schedule. This is non-clinical fitness coaching: it does not prescribe Mounjaro or Wegovy, provide medical nutrition therapy, or replace the prescribing clinician. The service offers a seven-day coaching trial for people considering that separate type of support.

Editorial conclusion: the question is not only how much weight is lost

It is accurate to say that studies of Mounjaro and similar medicines often show some decline in lean tissue alongside considerable fat loss. It is not accurate to equate every kilogram of lean-mass change with a kilogram of skeletal muscle loss, or to assume every measured change causes weakness. The 2026 reviews underscore just how much research still needs to focus on muscle quality, physical performance and long-term function.

Until then, a useful goal is weight management that is medically supervised where medication is used, nutritionally adequate and compatible with maintaining the strength needed for ordinary life.

Research sources and editorial method

This article prioritises published primary trials, peer-reviewed systematic reviews, professional consensus and official UK health guidance. It does not imply that FM Coaching conducted the studies, interviewed the authors or provided independent medical peer review. Sources were checked on 27 October 2026; readers should consult updated prescribing information when making treatment decisions.

Frequently asked questions about Mounjaro and muscle loss

Does Mounjaro cause muscle loss?

Studies show that lean mass can decline during substantial weight loss with Mounjaro. Because lean mass includes more than skeletal muscle, the amount of actual muscle lost cannot be assumed from a lean-mass percentage alone.

How much muscle do you lose on Mounjaro?

There is no reliable single percentage for every person. A 2025 tirzepatide substudy found about 26% of lost weight was lean mass, not necessarily muscle. Starting body composition, health, intake and measurement method matter.

Is losing 25% lean mass the same as losing 25% muscle?

No. Lean-mass estimates include non-fat tissues and fluid components. A DXA figure should not be relabelled as kilograms of skeletal muscle without an appropriate muscle-specific measurement.

Does Wegovy cause more muscle loss than Mounjaro?

Separate studies have produced different proportions, but they do not establish which medicine is better for preserving muscle in a particular person. Discuss medicine selection with your prescriber.

Can you prevent all muscle loss on Mounjaro?

No method is proven to prevent every lean-tissue change. Appropriate resistance training, adequate nutrition and medical review can support muscle and function, but outcomes vary.

Should I lift weights while taking Mounjaro?

Many people can benefit from suitable strengthening activity, but exercise needs to fit health status, symptoms and experience. If you are unwell, frail or have relevant medical conditions, ask your clinician about a safe plan.

How many days a week should I strength train?

General UK guidance suggests strengthening activity on at least two days each week. This is a public-health benchmark, not a personalised prescription.

How much protein should I eat on Mounjaro?

Protein needs vary with body size, age, activity, medical history and kidney health. Seek a personalised recommendation from a registered dietitian or clinician rather than following a single universal number.

Does Mounjaro cause muscle weakness?

A reduction in measured lean mass does not necessarily mean weakness. Researchers still lack enough direct strength and functional evidence; new or worsening weakness should be discussed with a clinician.

Why do I feel tired during weight loss treatment?

Possible contributors include low food or fluid intake, gastrointestinal side effects, sleep, other medicines and underlying health conditions. Persistent or significant fatigue deserves medical assessment.

Can you build muscle while taking Mounjaro?

Some people may make strength or muscle gains when training and nutrition are suitable, but it is not guaranteed and depends on circumstances. Clinical priorities and adequate intake come first.

Can I use a body-fat scale to check muscle loss?

Consumer impedance scales estimate body composition and are sensitive to hydration. They can offer a rough trend but should not be treated as a clinical diagnosis of muscle loss.

What does the 2026 research say about muscle loss?

Reviews consistently report some reduction in lean/fat-free mass during GLP-1-associated weight loss, but highlight differences in measurement methods and uncertainty about the impact on strength and function.

Is rapid weight loss more likely to affect muscle?

Larger or faster reductions in body weight can increase concern about the amount of lean tissue lost, especially when nutrition or resistance activity is inadequate. Ask your treatment team about your rate of loss and symptoms.

Should I stop Mounjaro if I think I am losing muscle?

Do not stop or change a prescription because of a headline or online article. Contact the prescribing clinician to discuss symptoms, goals and appropriate assessment.

Can a fitness coach manage Mounjaro treatment?

No. Only an appropriately qualified prescribing healthcare professional manages medication. A fitness coach can support general exercise habits for clients who are medically cleared.

What should I do if I cannot eat or drink enough?

Contact your prescribing service promptly if side effects make adequate intake difficult. Seek urgent assessment for severe persistent abdominal pain, serious dehydration or other severe symptoms.