Preserving Muscle Mass on GLP-1 Treatment

Concern about muscle loss on Mounjaro (tirzepatide) and Wegovy (semaglutide) is common, and it has a real basis. Whenever people lose a lot of weight, some of it is lean tissue, including muscle, not just fat. Trial data suggest that on tirzepatide roughly a quarter of the weight lost was lean mass, and on semaglutide a somewhat larger share. That is broadly in line with other ways of losing weight. The good news is that the two most strongly evidenced countermeasures, regular strength training and enough protein, are within most people's reach. This guide explains the evidence, who is most at risk, and what you can do.

What the evidence shows

How body composition is measured

Most data on body composition in GLP-1 trials come from DXA (dual-energy X-ray absorptiometry) scans. DXA estimates fat mass, bone mineral content and "lean mass". It is important to understand that lean mass is not the same as muscle. It includes skeletal muscle, but also organs, water, skin and connective tissue. Some of the lean mass lost during weight loss reflects a smaller body needing less supporting tissue and carrying less water.

Tirzepatide: SURMOUNT-1

A substudy of the SURMOUNT-1 trial, published in Diabetes, Obesity and Metabolism in 2025, scanned 160 participants at the start and after 72 weeks. Tirzepatide reduced both fat mass and lean mass compared with placebo. Roughly three-quarters of the weight lost was fat and around a quarter was lean mass, meaning fat loss exceeded lean loss by about three to one. The authors noted that this ratio is consistent with what is generally seen with calorie restriction, and that it was fairly consistent across subgroups.

Semaglutide: STEP 1

In a DXA substudy of the STEP 1 trial of semaglutide 2.4 mg, lean mass made up a larger share of the weight lost, close to two-fifths, over 68 weeks. Total fat mass and visceral (deep abdominal) fat both fell substantially. Despite the loss of lean mass in absolute terms, the proportion of lean tissue relative to total body mass increased by around three percentage points, because fat was lost faster.

What this means

These findings tell us that:

• GLP-1 medicines lead to loss of both fat and lean tissue, with fat loss predominating
• the overall pattern is not dramatically different from other forms of significant weight loss
• body composition, in terms of the ratio of lean to fat tissue, generally improves
• the absolute amount of lean tissue lost can still matter, especially for people who start with low muscle reserves

Muscle quality, not just quantity

Some researchers have pointed out that losing fat stored within and around muscle could improve muscle quality, meaning how well muscle works, even if total muscle size falls slightly. Function, such as strength, mobility and the ability to carry out daily activities, is arguably more important than the number on a scan. This is an active area of research, and the long-term functional effects of GLP-1-related weight loss are not yet fully understood.

Who is most at risk?

Muscle loss during weight loss is a greater concern for some people than others. Higher-risk groups include:

• older adults, because muscle mass and strength naturally decline with age (sarcopenia)
• people with "sarcopenic obesity", meaning high body fat combined with low muscle mass or strength
• people losing weight very rapidly with very low food and protein intake
• people who are inactive, or who have conditions that limit movement
• people with long-term illnesses such as heart failure, chronic kidney disease or COPD
• people with low protein intake, including some on restrictive diets

If you are in one of these groups, talk to your prescriber about how your treatment is monitored. Our guide to GLP-1 medicines over 65 covers age-related considerations.

Why muscle matters

Muscle is far more than appearance. It:

• enables strength, mobility and independence, particularly in later life
• helps regulate blood sugar, as muscle is a major site of glucose uptake
• contributes to resting energy use, which may matter for long-term weight management
• protects joints and supports balance, reducing the risk of falls
• supports recovery from illness and surgery

Losing muscle is not only a short-term concern. If weight is later regained, as often happens after stopping GLP-1 medicines, it tends to be regained mainly as fat. Repeated cycles of loss and regain could gradually shift body composition in the wrong direction. Our guide to weight regain after stopping GLP-1 medication explores this.

Strategies to protect muscle

1. Resistance (strength) training

Strength training is the most powerful signal for the body to keep muscle. The UK Chief Medical Officers recommend muscle-strengthening activities on at least two days a week for all adults. The 2025 joint advisory on nutritional priorities for GLP-1 therapy went further, recommending strength training at least three times a week alongside at least 150 minutes of moderate aerobic activity.

Key principles:

• Work all major muscle groups: legs, hips, back, chest, shoulders, arms and core.
• Use enough resistance that the last few repetitions of each set feel challenging.
• Progress gradually: add repetitions, sets or resistance over weeks.
• Be consistent: regular sessions matter more than occasional hard ones.

You do not need a gym. Bodyweight exercises, resistance bands and household objects can all work, particularly at first. Our step-by-step guide to strength training for beginners on weight loss medication includes example routines.

2. Adequate protein

Muscle needs protein to maintain and rebuild itself, particularly after strength exercise. GLP-1 medicines reduce appetite, and protein intake often falls along with overall food intake.

The 2025 joint advisory suggested that during weight reduction, higher intakes of around 1.2 to 1.6 g of protein per kg of body weight per day have been proposed, often around 80 to 120 g a day in practice, without routinely exceeding 2 g per kg. It also acknowledged uncertainty about whether targets should be based on actual or adjusted body weight in people with a high BMI.

Practical tips:

• include a source of protein at every meal and snack
• eat the protein part of your meal first if you fill up quickly
• spread protein across the day rather than having most of it in one meal
• include protein in the meal or snack after strength exercise

People with kidney disease should not increase protein without medical advice. For more, see our guide to protein intake on weight loss injections.

3. Avoid extreme calorie restriction

GLP-1 medicines can reduce appetite so much that some people eat very little. Very low intakes make it harder to get enough protein and other nutrients and may increase the share of weight lost as lean tissue. The joint advisory noted that micronutrient shortfalls become more likely below roughly 1,200 kcal a day for women and 1,800 kcal a day for men. If you are struggling to eat much at all, tell your prescriber, who may consider whether your dose or pace of dose escalation is right for you.

4. Stay generally active

Beyond structured exercise, daily movement, such as walking, taking the stairs, and breaking up long periods of sitting, helps maintain muscle function. Aerobic activity also has its own benefits for heart health and fitness. See our guide to exercise on GLP-1 medication.

5. Sleep and recovery

Muscle repair and adaptation happen during rest. Poor sleep is associated with poorer recovery and may affect appetite regulation. Aim for regular, sufficient sleep and allow rest days between hard strength sessions for the same muscle groups.

6. Vitamin D and overall nutrition

Vitamin D supports muscle and bone health. The NHS advises everyone in the UK to consider a 10 microgram daily supplement in autumn and winter. A varied diet with fruit, vegetables, wholegrains and dairy or alternatives provides other nutrients that support muscle function. See our guide to vitamins and supplements on GLP-1 treatment.

Summary table: muscle-protecting strategies

Putting it together: a sample week

This example shows how strength training, protein and general activity might fit into an ordinary week. It is a general illustration, not a personalised plan; adapt it to your fitness, health and schedule.

Common myths about muscle and GLP-1 medicines

"GLP-1 medicines destroy muscle"

This overstates the evidence. As with any significant weight loss, some lean tissue is lost, but fat loss predominates and the ratio of lean to fat tissue generally improves. Practical steps can help limit lean loss.

"All lean mass lost is muscle"

DXA lean mass includes water, organs and connective tissue as well as muscle. Some loss of lean mass reflects a smaller body needing less supporting tissue.

"You can't build muscle while losing weight"

Building significant muscle is harder in a calorie deficit, but beginners in particular can often maintain, and sometimes gain, some strength and muscle while losing fat, especially with progressive strength training and adequate protein.

"Protein shakes alone will protect your muscles"

Protein provides the building blocks, but without the stimulus of strength training it does far less to preserve muscle. The combination matters.

How to monitor muscle and strength

What scales can and cannot tell you

Standard bathroom scales measure total weight only. Body composition scales that use bioelectrical impedance give estimates of fat and lean mass, but they are affected by hydration, recent meals and exercise, and are not very accurate for individuals. They may be useful for spotting broad trends over months if used consistently, but should not be relied on.

Practical functional checks

Strength and function are arguably more meaningful than scan results. Simple ways to track them include:

• Sit-to-stand: how many times can you stand up from a chair and sit down again in 30 seconds without using your arms?
• Stairs: are stairs getting easier or harder?
• Carrying: can you carry your shopping as easily as before?
• Strength training log: are you able to gradually lift more or do more repetitions?
• Grip strength: some clinics measure this with a handheld device, as it is a recognised marker of overall muscle strength.

If you notice increasing weakness or reduced function, raise this with your prescriber. Our guide to measuring progress beyond the scales has more ideas.

Clinical monitoring

Routine DXA scans are not part of standard GLP-1 treatment in the UK. Your prescriber may monitor your weight, rate of weight loss, side effects and general wellbeing. If you are in a higher-risk group, it is reasonable to ask how muscle and strength will be considered in your care.

What about medicines or supplements to preserve muscle?

At the time of writing, no medicine is licensed in the UK specifically to prevent muscle loss during GLP-1 treatment. Several investigational medicines designed to preserve muscle while people lose weight are being tested in clinical trials. Until they are properly evaluated and licensed, they are not an option outside research.

Be cautious about products sold online claiming to protect or build muscle during weight loss, particularly unlicensed peptides, hormones or anabolic steroids. These can carry serious risks, and products bought outside the regulated supply chain may not contain what they claim. Our guide to getting weight loss injections online safely explains how to stay within regulated services. Creatine is a widely used sports supplement with evidence in combination with strength training in some populations, but it is not specifically studied in people taking GLP-1 medicines, and anyone with kidney problems should avoid it unless advised by a doctor.

The bigger picture

It is worth keeping the muscle question in perspective. For many people living with obesity, the health benefits of substantial fat loss, including improvements in blood pressure, blood sugar, joint pain, sleep apnoea and, in some groups, cardiovascular risk, are significant. Losing some lean tissue is part of the process. The aim is not to avoid it entirely, which is not realistic, but to limit it as much as possible and maintain strength and function.

For an overview of the medicines themselves, visit our weight loss injections hub. If you are comparing treatment services and want to know which offer exercise or nutrition support, see our provider comparison.

When to speak to a professional

Speak to your GP or prescriber if:

• you are over 65, frail, or have a condition that affects your muscles or mobility
• you notice increasing weakness, such as difficulty getting up from a chair, climbing stairs or gripping objects
• you are eating very little for several days or are losing weight much faster than expected
• you have had a fall, or feel unsteady on your feet
• you have kidney disease and want to increase your protein intake
• you are considering any supplement or product claiming to build or protect muscle

Seek urgent help (call NHS 111, or 999 in an emergency) if you have:

• severe abdominal pain that does not go away, with or without vomiting
• signs of severe dehydration, such as passing little or no urine, confusion or feeling faint
• dark (cola-coloured) urine with severe muscle pain or weakness, especially after intense exercise, which can be a sign of muscle breakdown

Decisions about your medication, including dose changes, belong to your prescriber. A physiotherapist, registered dietitian or qualified exercise professional can provide individual guidance on training and nutrition.