What happens to muscle during GLP-1 weight loss
When you lose weight, your body draws energy from both fat stores and muscle tissue. This happens with any form of weight loss, whether through diet, surgery, or medication.
GLP-1 medications reduce appetite, which means you eat less food overall. If your protein intake drops alongside your total food intake, your body has less of the raw material it needs to maintain muscle.
The clinical trials measured this using DXA scans (a type of body composition scan that distinguishes between fat and lean tissue). Without specific dietary or exercise interventions, a meaningful proportion of the weight lost was lean mass1,2.
The clinical trial data
The body composition data comes from DXA substudies within the landmark registration trials for each medication.
Mounjaro (tirzepatide)
The SURMOUNT-1 DXA substudy followed 160 participants over 72 weeks. Those on tirzepatide lost an average of 21.3% of their body weight1.
Of that weight loss, approximately 74% was fat mass and 26% was lean mass. In absolute terms, participants lost 15.9 kg of fat and 5.6 kg of lean tissue.
The fat-to-lean tissue ratio improved from 0.93 at baseline to 0.70 at 72 weeks, indicating that participants’ overall body composition improved even though they lost some muscle1.
Wegovy (semaglutide)
The STEP 1 DXA substudy measured body composition in 140 participants over 68 weeks. Those on semaglutide lost 15% of their body weight2.
Fat mass reduced by 19.3% and lean mass by 9.7%. The proportion of weight lost as lean mass was higher than with tirzepatide, at around 40%2.
Lean mass as a proportion of total body mass actually increased by 3 percentage points, because fat was lost at a faster rate. The lean-to-fat ratio improved overall2.
|
Mounjaro (tirzepatide) |
Wegovy (semaglutide) |
| Trial |
SURMOUNT-1 |
STEP 1 |
| Total weight loss |
21.3% |
15.0% |
| Fat mass change |
-33.9% |
-19.3% |
| Lean mass change |
-10.9% |
-9.7% |
| Lean mass lost (absolute) |
5.6 kg |
~5-6 kg |
| % of weight loss as lean mass |
~26% |
~40% |
| Fat-to-lean ratio improvement |
0.93 to 0.70 |
Improved (lean proportion +3%) |
How protein helps preserve muscle
Protein provides the building blocks your body needs to maintain and repair muscle tissue.
When you’re losing weight, your body breaks down muscle protein at a faster rate, so you need more dietary protein than usual to compensate.
A 2025 joint advisory from four professional societies recommended protein intakes of 1.2-1.6g per kilogram of body weight per day during active weight loss with GLP-1 medications4.
For someone weighing 90 kg, that’s 108-144g of protein daily, spread evenly across meals. This is higher than the standard recommended daily amount of 0.8g/kg/day.
GLP-1 medications reduce appetite, often leading to lower protein intake and less food overall3.
Without paying close attention, protein intake can drop well below what’s needed to maintain muscle.
A separate international consensus panel reached a similar conclusion, recommending protein above 1.2g/kg/day combined with resistance training.
They also found that increased protein intake alone is likely insufficient to preserve muscle without resistance training alongside it3.
In practice, this means prioritising protein-rich foods at each meal. Good sources include chicken, turkey, fish, eggs, Greek yoghurt, beans and lentils, and tofu.
Our planner calculates your personalised protein target based on 1.6g/kg/day and breaks it down into per-meal targets to make it more manageable.
How resistance training helps
Any form of resistance training, whether that’s bodyweight exercises, resistance bands, or weights, signals your body to hold onto muscle tissue.
A 2025 case series followed three patients who combined GLP-1 medications with resistance training and adequate protein5.
One participant on tirzepatide lost 33% of her body weight over 115 weeks.
In the SURMOUNT-1 trial, 26% of the weight lost was lean mass. For this participant, lean mass accounted for just 8.7% of her weight loss, with 91% of her weight loss coming from fat5.
The other two participants actually gained lean mass while losing 26.8% and 13.2% of their starting weight respectively5.
These are individual cases rather than a randomised trial, and the participants were motivated and consistent. But they show that the muscle loss seen in clinical trials isn’t inevitable.
Starting where you are
The planner builds your exercise programme based on where you are right now. If you’re not currently doing any exercise, it starts with something genuinely simple, like two short sessions a week using a chair and a wall for support.
Research on habit formation shows that starting with something small and attaching it to an existing routine is more effective than ambitious plans that are hard to sustain6.
The planner follows this principle, with a 4-week progression that gradually builds as your confidence grows. If you’re already active, it adjusts accordingly with a more structured programme.
Factors that affect your results
Age
A 2025 conference abstract presented at the Endocrine Society’s annual meeting found that older age was independently associated with greater lean mass loss on semaglutide. Higher protein intake at 12 weeks correlated with less muscle loss3.
The planner flags this with specific guidance if you’re in an older age group.
Sex
The same study found that women lost lean mass proportionally more than men during GLP-1 treatment3. This may be partly due to differences in baseline muscle mass and hormonal factors.
Medication type
Tirzepatide appears to preserve slightly more lean mass than semaglutide based on trial data (26% vs 40% of weight lost as lean mass), although the trials weren’t directly comparable1,2.
Both medications benefit from the same approach: adequate protein and resistance training.
Protein intake
The evidence consistently shows that higher protein intake is protective.
Our planner sets your target at 1.6g per kilogram of body weight per day and shows you how to spread it across meals, with practical UK food suggestions.
If you’re looking for a full weekly plan, our free GLP-1 meal plan generator creates personalised 7-day plans with a high-protein option from 687 dietitian-developed recipes.
Current activity level
The planner meets you where you are and builds from there, giving a different programme to someone who isn’t currently active compared to someone who already trains regularly.
What UK guidelines say
NICE guideline NG246 (updated January 2025) recommends physical activity and dietary support alongside weight management medications, but doesn’t include specific protein targets or resistance training recommendations for people on GLP-1s7.
International expert panels now recommend 1.2-1.6g/kg/day protein and structured resistance training for this group3,4.
Our planner applies this evidence in a UK context with practical, accessible recommendations.
Frequently asked questions
How much muscle do you lose on Mounjaro or Wegovy?
Without specific dietary or exercise interventions, approximately 26% of the weight lost with tirzepatide and up to 40% with semaglutide is lean mass1,2. With resistance training and adequate protein, research suggests this can drop to under 10%5.
Can you build muscle while taking a GLP-1 medication?
Yes. A 2025 case series showed that two out of three patients who combined GLP-1 medication with resistance training and adequate protein actually gained lean mass while losing body weight5. It requires consistent training and deliberate protein intake, but it’s possible.
How much protein do I need per day on a GLP-1?
A 2025 joint advisory from four professional societies recommends 1.2-1.6g per kilogram of body weight per day during active GLP-1 weight loss4.
For someone weighing 85 kg, that’s 102-136g daily. Our planner calculates your personalised target based on 1.6g/kg/day.
How often should I do strength training?
The most important thing is doing something regularly, even if it’s short.
Our planner starts with 2 sessions per week for beginners and adjusts based on your current activity level, gradually building as you get more comfortable.
Is the muscle loss from GLP-1 medications permanent?
Not necessarily. Muscle mass can be rebuilt through resistance training and adequate protein intake.
However, rebuilding takes longer than losing it, which is why protecting muscle during weight loss is the better approach.
Is muscle loss worse for older adults?
A 2025 conference abstract presented at ENDO found that older age was independently associated with greater lean mass loss during semaglutide treatment3.
For adults over 50, this is particularly relevant because they have less margin before reaching thresholds associated with frailty.
What is ‘Ozempic face’?
‘Ozempic face’ refers to a gaunt or hollow appearance in the face after weight loss on GLP-1 medications. It’s caused by loss of subcutaneous fat and, in some cases, muscle in the face.
Resistance training and adequate protein intake can help reduce overall lean tissue loss, although facial changes are partly driven by fat loss.
Do I need protein shakes, or is food enough?
Whole food sources are preferable when possible. Good options include chicken, fish, eggs, Greek yoghurt, beans and lentils, and tofu.
On days when your appetite is particularly low, a protein shake blended with milk and fruit can help to provide an easy to digest source of protein. The planner includes practical tips for boosting protein at each meal.
Does the planner account for physical limitations?
Yes. If you have joint pain, back problems, or limited mobility, the planner swaps out exercises that might be unsuitable and replaces them with alternatives that work around your limitations.
Take home message
Muscle loss during GLP-1 weight loss is common but not inevitable.
Clinical trials show that 26-40% of weight loss is lean mass without specific interventions, but with resistance training and adequate protein, this can drop to under 10%1,2,5.
Eating enough protein (at least 1.2g per kilogram of body weight per day, ideally 1.6g/kg/day) and doing some form of resistance training are the two most effective strategies, even if that means starting with two short sessions a week at home.
Second Nature’s programme combines GLP-1 medication with personalised nutrition guidance from a registered health coach, structured around the balanced plate model (half vegetables, a quarter protein, a quarter complex carbohydrates, plus a serving of healthy fat).
In a 12-month study, members on tirzepatide lost an average of 22.1% of their body weight, with 95% achieving at least 10% weight loss8.
Second Nature's Mounjaro and Wegovy programmes
Second Nature provides Mounjaro or Wegovy as part of our Mounjaro and Wegovy weight-loss programmes.
Why choose Second Nature over other medication providers, assuming you're eligible?
Because peace of mind matters.
We've had the privilege of working with the NHS for over eight years, helping people across the UK take meaningful steps toward a healthier, happier life.
Our programmes are designed to meet people where they are, whether that means support with weight loss through compassionate one-to-one health coaching, or access to the latest weight-loss medications (like Mounjaro and Wegovy) delivered alongside expert care from a multidisciplinary team of doctors, psychologists, dietitians, and personal trainers.
At the heart of everything we do is a simple belief: real, lasting change comes from building better habits, not relying on quick fixes. We're here to support that change every step of the way.
With over a decade of experience, thousands of lives changed, and a long-standing record of delivering programmes used by the NHS, we believe we're the UK's most trusted weight-loss programme.
We hope to offer you something invaluable: peace of mind, and the support you need to take that first step.
References
- Look M, Dunn JP, Kushner RF, et al. (2025). Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study. Diabetes, Obesity and Metabolism, 27(5), 2720-2729.
- Wilding JPH, et al. (2022). Impact of semaglutide on body composition in adults with overweight or obesity: exploratory analysis of the STEP 1 study. Journal of the Endocrine Society.
- Neeland IJ, Linge J. (2024). Changes in lean body mass with glucagon-like peptide-1-based therapies and mitigation strategies. Diabetes, Obesity and Metabolism, 26(Suppl 4), 18-31.
- American College of Lifestyle Medicine, American Society for Nutrition, Obesity Medicine Association, The Obesity Society. (2025). Nutritional priorities to support GLP-1 therapy for obesity. American Journal of Clinical Nutrition.
- Tinsley GM, Nadolsky KZ. (2025). Preservation of lean soft tissue during weight loss induced by GLP-1 and GLP-1/GIP receptor agonists: a case series. SAGE Open Medical Case Reports, 13.
- Lally P, van Jaarsveld CHM, Potts HWW, Wardle J. (2010). How are habits formed: modelling habit formation in the real world. European Journal of Social Psychology, 40(6), 998-1009.
- National Institute for Health and Care Excellence. (2025). Overweight and obesity management. NICE guideline [NG246].
- Richards R, Lunt W, Whitman M, Spaltro G, Hall R. (2025). Semaglutide and tirzepatide in a remote weight management program: 12-month retrospective observational study. JMIR Formative Research, 9, e81912.
