Key facts
- Lean mass loss: Approximately 30–40% of total weight lost on GLP-1 RAs is lean mass (muscle, water, organ tissue)
- Comparable to dieting: This ratio is similar to calorie-restriction diets producing equivalent weight loss
- The difference: GLP-1 medications often produce larger and faster total weight loss, so the absolute amount of lean mass lost can be greater
- Prevention: High protein intake (1.2–1.6 g/kg/day) + resistance training (2×/week minimum) significantly reduces muscle loss
- Higher risk groups: Over 65s, those with low baseline muscle mass, sedentary patients, those on very low calorie intake
Why weight loss includes muscle
When the body is in a calorie deficit — whether from reduced food intake, increased exercise or medication-induced appetite suppression — it draws on both fat stores and lean tissue for energy. This is a fundamental aspect of human physiology, not a unique effect of GLP-1 medications.
Lean mass is not solely muscle. It includes skeletal muscle, water, bone mineral content, connective tissue and organ mass. When researchers measure lean mass loss using DEXA scans or bioelectrical impedance, they capture all of these components. Some of the initial lean mass loss on any weight loss programme is water (particularly glycogen-bound water), not actual muscle protein.
Nevertheless, a significant proportion of lean mass loss during prolonged weight loss is genuine skeletal muscle. Losing too much muscle can reduce metabolic rate, impair physical function, affect bone health and increase the risk of falls — particularly in older adults.
What the clinical trials show
STEP trials (semaglutide 2.4 mg / Wegovy)
The STEP programme is the largest body of evidence for semaglutide at the weight management dose. STEP 1 included body composition sub-studies using DEXA scans:
- Participants on semaglutide 2.4 mg lost a mean of 15.3 kg over 68 weeks
- Of this, approximately 39% was lean mass and 61% was fat mass
- In the placebo group (who also received lifestyle intervention), the lean-to-fat ratio of weight lost was similar
This finding is important: semaglutide did not appear to cause a disproportionately higher percentage of lean mass loss compared with standard calorie restriction. The concern arises because the total weight loss is substantially larger, so the absolute kilograms of lean mass lost are greater.
SURPASS trials (tirzepatide / Mounjaro)
Body composition data from the SURPASS trials and the dedicated SURMOUNT weight management programme for tirzepatide show a broadly similar pattern. Tirzepatide at the highest dose (15 mg) produced greater total weight loss than semaglutide, with DEXA data suggesting approximately 33% lean mass loss and 67% fat mass loss. This slightly more favourable ratio may relate to the dual GIP/GLP-1 mechanism, though head-to-head body composition comparisons are limited.
Context matters: A lean mass fraction of 25–40% during weight loss is well-established in obesity research. Bariatric surgery, very low calorie diets and moderate calorie restriction all produce similar ratios. GLP-1 medications are not uniquely harmful to muscle; the concern is proportional to the magnitude of weight loss they achieve.
Who is most at risk?
Not all patients face the same degree of concern about muscle loss. The following groups need particular attention:
Adults aged 65 and over
Older adults have lower baseline muscle mass and strength due to age-related sarcopenia (estimated to affect 10–16% of the UK population over 65). Additional muscle loss from GLP-1-induced weight loss can push some individuals below functional thresholds, affecting their ability to climb stairs, rise from chairs, maintain balance and live independently. Read our detailed guide on GLP-1 for over 65s.
Patients with low baseline muscle mass
Individuals who are sedentary, have chronic illness or have previously lost significant weight may already have depleted muscle stores. Further loss compounds the deficit.
Patients on very low calorie intakes
Some patients on GLP-1 medications experience such profound appetite suppression that their daily calorie intake drops below 800–1,000 kcal. At very low intakes, the body preferentially catabolises muscle for gluconeogenesis (creating glucose from amino acids). Adequate calorie and protein intake is essential.
Patients not exercising
Without a mechanical stimulus (resistance training), the body has no physiological signal to preserve muscle during a calorie deficit. Sedentary patients consistently lose more lean mass than those who exercise.
How to preserve muscle: evidence-based strategies
1. Adequate protein intake
Protein is the most critical dietary factor for muscle preservation during weight loss. Current evidence supports:
| Population | Recommended intake | Source |
|---|---|---|
| General adults losing weight | 1.2–1.6 g protein per kg body weight per day | BDA, ESPEN |
| Older adults (≥ 65) losing weight | 1.2–1.5 g/kg/day (minimum) | ESPEN, NICE |
| Adults doing resistance training | 1.6–2.2 g/kg/day | ISSN position stand |
Practical tips for hitting protein targets on a reduced appetite:
- Prioritise protein at every meal: Start meals with the protein portion before vegetables and carbohydrates — this ensures protein is consumed even if you cannot finish the full plate
- Spread intake across the day: Aim for 25–40 g of protein per meal across three to four meals rather than one large protein-heavy meal
- Choose protein-dense foods: Chicken breast, Greek yoghurt, eggs, fish, cottage cheese, tofu, lentils and lean mince offer high protein per calorie
- Consider protein supplements: Whey protein, casein or plant-based protein powders can help when appetite is very low. A 30 g scoop of whey provides approximately 24 g of protein in under 130 kcal
- Leucine is key: The amino acid leucine is the primary trigger for muscle protein synthesis. Animal proteins and whey are naturally rich in leucine; plant-based eaters may benefit from leucine-enriched supplements
For comprehensive meal planning advice, see our diet on GLP-1 medication guide.
2. Resistance training
Resistance (strength) training is the single most effective intervention for preserving muscle during weight loss. The mechanical loading of muscles stimulates muscle protein synthesis and signals the body to retain lean tissue even in a calorie deficit.
Minimum effective dose: Two sessions per week targeting all major muscle groups (legs, back, chest, shoulders, arms, core). Each session should include compound exercises (squats, deadlifts, rows, presses) and can be completed in 30–45 minutes.
Progressive overload: Gradually increase weight, repetitions or sets over time. The muscle needs a progressively increasing stimulus to maintain or grow.
If you are new to resistance training, consider:
- A referral to an NHS exercise programme or local authority gym scheme (many offer subsidised or free sessions for patients with chronic conditions)
- Working with a qualified personal trainer, at least initially, to learn proper form
- Starting with bodyweight exercises (squats, press-ups, lunges) if gym access is limited
- Resistance bands as a low-cost, portable alternative
Our exercise on GLP-1 medication guide provides detailed workout recommendations.
3. Adequate overall calorie intake
While GLP-1 medications are designed to reduce calorie intake, excessively low intake is counterproductive for muscle preservation. As a general guide:
- Women should aim for at least 1,200 kcal per day; men at least 1,500 kcal per day
- If your appetite is so suppressed that you consistently fall below these thresholds, discuss it with your prescriber — dose adjustment may be appropriate
- A dietitian can help plan energy-dense, protein-rich meals that provide adequate nutrition within a reduced appetite
4. Other supportive measures
- Vitamin D: Deficiency is common in the UK (especially in winter) and is associated with muscle weakness. NICE recommends 10 micrograms (400 IU) daily for all UK adults from October to March; higher supplementation may be appropriate for deficient individuals
- Creatine monohydrate: Well-evidenced supplement that supports muscle strength and may help preserve lean mass during calorie restriction. Typical dose: 3–5 g per day
- Sleep: Poor sleep impairs muscle recovery and increases muscle protein breakdown. Aim for 7–9 hours per night
- Omega-3 fatty acids: Some evidence suggests omega-3s may have anti-catabolic effects, though the data is less robust than for protein and resistance training
Measuring body composition
Scales alone cannot distinguish between fat loss and muscle loss. More informative methods include:
| Method | Accuracy | Availability in UK | Cost |
|---|---|---|---|
| DEXA scan | Gold standard for body composition | NHS (limited); private clinics (Bodyscan, DexaFit) | £100–200 private |
| Bioelectrical impedance (BIA) | Moderate; affected by hydration | Many gyms and pharmacies | Free–£30 |
| Waist circumference | Simple proxy for central adiposity | GP surgery; at home | Free |
| Grip strength | Functional marker of muscle quality | GP; physiotherapy; some gyms | Free–£20 |
Practical recommendation: If you are concerned about muscle loss, consider a baseline DEXA scan before or soon after starting GLP-1 treatment, and a follow-up scan at 6–12 months. This provides objective data to guide your exercise and nutrition strategy. Grip strength testing is a simple, free alternative that correlates well with overall muscle function.
Emerging research: bimagrumab and beyond
The pharmaceutical industry is actively researching ways to preserve or build muscle during GLP-1-induced weight loss. The most advanced candidate is bimagrumab.
What is bimagrumab?
Bimagrumab is a human monoclonal antibody that blocks activin type II receptors (ActRII). By inhibiting myostatin and activin signalling, it promotes muscle growth and simultaneously reduces fat mass. It was originally developed for sarcopenia and inclusion body myositis.
Bimagrumab + semaglutide trials
A phase II trial combining bimagrumab with semaglutide in adults with obesity showed promising results:
- Total weight loss was similar to semaglutide alone
- The combination group lost significantly more fat mass
- Lean mass was largely preserved in the combination group, whereas the semaglutide-only group lost approximately 5–6 kg of lean mass
- Body composition shifted dramatically in favour of muscle retention
As of April 2026, bimagrumab is in phase III clinical trials. It is not yet licensed or available in the UK, but if results are confirmed, it could become a significant adjunct to GLP-1 therapy, particularly for older adults and those at high sarcopenia risk.
Other research directions
- Myostatin inhibitors: Several other anti-myostatin compounds are in development
- GLP-1/amylin combinations: Cagrilintide combined with semaglutide (CagriSema) may produce a more favourable body composition profile
- Exercise-mimetic drugs: Early-stage research into compounds that replicate some benefits of exercise at a molecular level
When to speak to your doctor
Contact your GP, diabetes team or prescriber if you notice:
- Significant weakness or difficulty with everyday activities (climbing stairs, getting up from a chair, carrying shopping)
- Calorie intake consistently below 800–1,000 kcal per day despite trying to eat more
- Unintentional loss of more than 1 kg per week over several consecutive weeks
- Falls or balance problems
- Muscle cramps, fatigue or exercise intolerance that worsens over time
For patients over 65: Muscle loss can have serious consequences including falls, fractures, loss of independence and increased hospital admission risk. If you are aged 65 or over on a GLP-1 medication, ensure you have regular reviews with your clinician and ideally input from a dietitian and physiotherapist. See our GLP-1 for over 65s guide.
Frequently asked questions
How much muscle do you lose on Ozempic or Wegovy?
DEXA data from the STEP trials shows approximately 30–40% of total weight lost is lean mass. For a patient losing 15 kg, that equates to roughly 4.5–6 kg of lean mass. This ratio is broadly comparable to calorie-restriction diets of similar magnitude.
Is muscle loss on GLP-1 medication worse than normal dieting?
The proportion of lean mass lost is similar (25–40%). The concern is that GLP-1 medications often produce larger and faster total weight loss, so the absolute amount of lean mass lost can be greater. Combining medication with protein and resistance training significantly improves outcomes.
How can I prevent muscle loss while on a GLP-1 medication?
Two key strategies: adequate protein intake (1.2–1.6 g per kg body weight per day, spread across meals) and regular resistance training (at least two sessions per week targeting all major muscle groups). These interventions preserve significantly more lean mass.
What is bimagrumab and could it help with muscle loss?
Bimagrumab blocks myostatin signalling to promote muscle growth and reduce fat. Early trials combining bimagrumab with semaglutide showed preserved lean mass with similar total weight loss. It is in phase III trials but not yet available in the UK.
Should I worry about sarcopenia on GLP-1 medication?
Sarcopenia is a particular concern for adults over 65 who already have reduced baseline muscle. NICE guidelines emphasise supervised exercise and nutritional support for older adults on weight management programmes. Discuss muscle preservation strategies with your clinical team.
Related guides
Sources
- Wilding JPH et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med 2021; 384:989-1002
- Jastreboff AM et al. Tirzepatide for Obesity (SURMOUNT-1). N Engl J Med 2022; 387:205-216
- Heymsfield SB et al. Mechanisms, Pathophysiology, and Management of Obesity. N Engl J Med 2017; 376:254-266
- Bimagrumab + semaglutide phase II trial data — presented at ObesityWeek 2024 and published in peer review
- ESPEN — Protein Requirements in Older Adults (espen.org)
- BDA — Protein fact sheet (bda.uk.com)
- NICE NG196 — Managing overweight and obesity in adults (nice.org.uk/guidance/ng196)
- Cruz-Jentoft AJ et al. Sarcopenia: revised European consensus on definition and diagnosis. Age Ageing 2019; 48(1):16-31