BSc (Hons), RNMH, NMP

Advanced nurse practitioner

With any significant weight loss, the body can lose some lean mass as well as fat, especially if the loss is rapid, protein intake is low, or resistance training isn’t part of the plan.

This article explains why muscle loss can happen during weight loss, what this means if you’re using GLP-1 medicines, such as semaglutide or tirzepatide, and how to preserve muscle through protein intake, resistance training, appropriate monitoring, and clinical support.

What are GLP-1 weight loss injections?
Can weight loss injections cause muscle loss?
Lean mass is not the same as muscle
How much lean mass is lost during GLP-1 treatment?
Why muscle preservation matters in men
Why muscle loss can happen during GLP-1 treatment
How to preserve muscle while losing weight
When should you seek advice?

What are GLP-1 weight loss injections?

GLP-1 weight loss injections are medicines used to support weight management in people who meet clinical criteria. They work by acting on hormone pathways involved in appetite, satiety, and glucose regulation.

Semaglutide is a GLP-1 receptor agonist. It mimics the action of GLP-1, a naturally occuring gut hormone involved in appetite regulation. Tirzepatide acts on two hormone pathways – GLP-1 and GIP. Both medicines are used alongside diet, physical activity, and wider clinical support, rather than as stand-alone treatments [1,2].

Can weight loss injections cause muscle loss?

Weight loss injections can contribute to some lean mass loss, but this isn’t unique to GLP-1 medications.

Whenever you lose weight, your body typically loses a combination of fat mass and lean mass. In many people, around 20-40% of the weight loss may come from lean mass, although this varies depending on factors like protein intake, resistance training, age, starting body composition, and the speed of weight loss [3]. Lean mas includes muscle, but also water, organs, connective tissue, and other non-fat tissues.

The aim is for most weight loss to come from fat. However, if the calorie deficit is large, protein intake is inadequate, or muscles aren’t challenged through regular resistance exercise, a great proportion of lean tissue, including skeletal muscle, may be lost.

Lean mass is not the same as muscle

Lean mass includes skeletal muscle, but it also includes water, bone, organs, blood, connective tissue, and other non-fat tissue. A reduction in lean mass doesn’t automatically mean the same amount of skeletal muscle has been lost.

Some of the reported reduction in lean mass during weight loss may reflect changes in body water, organ size, liver fat, or other tissues, not muscle alone.

Even so, preserving skeletal muscle is important. Skeletal muscle supports mobility, strength, glucose metabolism, and healthy ageing.

How much lean mass is lost during GLP-1 treatment?

The amount of lean mass lost during weight loss varier between individuals and depends on several factors, including:

The medicine used
Starting weight
Age
Baseline muscle mass
Activity level, particularly resistance training
The rate of weight loss
How body composition is measured

Concerns about semaglutide muscle loss and tirzepatide muscle loss usually relate to lean mass changes observed during significant weight loss. Clinical trials show that semaglutide and tirzepatide can produce substantial weight loss, and body composition studies suggest that most of the weight loss is fat mass. However, some lean mass may also be lost [4,5].

The aim isn’t to avoid weight loss because lean mass may decrease. It’s to support weight loss in a way that helps preserve skeletal muscle through adequate protein intake, resistance training, and ongoing clinical monitoring.

Why muscle preservation matters in men

Preserving muscle during weight loss is not just a gym-related concern. It can be a medical issue.

Muscle helps support:

Strength
Mobility
Balance
Glucose metabolism
Physical independence
Recovery from illness or injury
Long-term weight maintenance

Loss of muscle becomes more relevant with:

Age
Type 2 diabetes
Low baseline activity
Previous rapid weight loss
Symptoms of testosterone deficiency

Why muscle loss can happen during GLP-1 treatment

GLP-1 medicines reduce appetite, but they can also reduce total food intake to the point where protein, calories, and micronutrient intake become insufficient.

Common issues include:

Eating too little overall
Skipping meals because the appetite is very low
Prioritising small, low-protein foods
Stopping exercise because of nausea, reflux, constipation, or fatigue
Losing weight faster than planned
Not adjusting the plan as the dose increases

How to preserve muscle while losing weight

Prioritise protein

Protein intake is central to preserving lean tissue during weight loss. During a calorie deficit, your body needs enough amino acids to support muscle maintenance and repair.

A practical approach is to include protein at each meal.

Suitable options may include:

Lean meat
Fish
Eggs
Greek yoghurt
Cottage cheese
Tofu
Tempeh
Beans
Lentils

Men with kidney disease or other relevant conditions should get individual advice before increasing protein intake.

Use resistance training

Resistance training can help maintain muscle strength during weight loss. Exercises don’t need to be extreme or bodybuilding focused.

They may include:

Weights
Resistance bands
Gym machines
Bodyweight exercises
Squats to a chair
Step-ups
Press-ups against a wall or bench
Supervised strength work if mobility is limited

Avoid excessive calorie restriction

GLP-1 treatment can make it easier to cut back on food, but if you’re eating too little, you may feel more tired, find exercise harder, and struggle to get enough protein.

If your appetite is very low, nausea is persistent, or meals are regularly missed, your dose or eating pattern may need to be reviewed.

Keep daily activity in the plan

Regular movement, such as walking, cycling, swimming, gardening, or simply staying active day-to-day, can help preserve physical function as your weight comes down.

Monitor function, not just weight

Scale weight is useful, but it’s not everything. You may hear about NSVs (non-scale victories), which are helpful when you feel a difference, but the number on the scale is staying the same.

It may also be worth tracking:

Waist measurement
Strength
Walking tolerance
Energy
Training performance
Blood pressure
Blood glucose markers
Clothes fit
Ability to manage daily tasks

When should you seek advice?

Seek advice if you’re using weight loss injections and notice:

Reduced strength
Worsening fatigue
Poor exercise tolerance
Dizziness
Persistent nausea
Difficulty eating enough

It’s also worth seeking advice if you’re worried about muscle loss or losing weight faster than expected.

Not sure where to start? Use our simple Get Started tool.

References

European Medicines Agency (2023) Wegovy: EPAR – Medicine overview. Available at:https://www.ema.europa.eu/en/medicines/human/EPAR/wegovy (Accessed: 29 June 2026).
European Medicines Agency (2026) Mounjaro: EPAR – Medicine overview. Available at: https://www.ema.europa.eu/en/medicines/human/EPAR/mounjaro (Accessed: 29 June 2026).
McCarthy D, Berg A. Weight Loss Strategies and the Risk of Skeletal Muscle Mass Loss. Nutrients. 2021 Jul 20;13(7):2473. doi: 10.3390/nu13072473. PMID: 34371981; PMCID: PMC8308821.
Wilding, J.P.H., Batterham, R.L., Calanna, S., Davies, M., Van Gaal, L.F., Lingvay, I., McGowan, B.M., Rosenstock, J., Tran, M.T.D., Wadden, T.A., Wharton, S., Yokote, K., Zeuthen, N. and Kushner, R.F. (2021) ‘Once-weekly semaglutide in adults with overweight or obesity’, The New England Journal of Medicine, 384(11), pp. 989–1002. doi: 10.1056/NEJMoa2032183.
Jastreboff, A.M., Aronne, L.J., Ahmad, N.N., Wharton, S., Connery, L., Alves, B., Kiyosue, A., Zhang, S., Liu, B., Bunck, M.C. and Stefanski, A. (2022) ‘Tirzepatide once weekly for the treatment of obesity’, The New England Journal of Medicine, 387(3), pp. 205–216. doi: 10.1056/NEJMoa2206038.