Does GLP-1 cause muscle loss? How to protect it

By the DieticiansWorldwide.com editorial team · 18 October 2026 · Weight management & GLP-1

Lean mass, fat-free mass and muscle are not the same thing

Most headlines about muscle loss on GLP-1 drugs are based on scans that do not measure muscle directly. The standard tool in trials is DXA (dual-energy X-ray absorptiometry), which splits the body into fat, bone and "lean" tissue. Lean tissue includes skeletal muscle, but also water, organs, skin and connective tissue. Some papers call this fat-free mass instead.

That matters for interpreting the numbers. When someone loses a large amount of weight, they lose some water and the body has less tissue to support, so a fall in lean mass is expected and not all of it is muscle. The 2025 joint nutrition advisory from four US obesity, nutrition and lifestyle medicine societies estimates that skeletal muscle makes up about half of lean mass, so a lean-mass loss figure overstates the muscle lost (Obesity Pillars 2025). It also does not tell you about muscle strength or function, which are what actually affect daily life.

How much muscle do you lose on semaglutide or tirzepatide?

Three sources give the clearest picture.

Semaglutide: STEP 1

In the STEP 1 body-composition substudy of 140 participants, people on semaglutide lost 15.0% of body weight by week 68, with a 19.3% fall in total fat mass, a 27.4% fall in visceral fat and a 9.7% fall in lean mass. Because fat fell faster than lean tissue, the proportion of the body made up of lean mass rose by 3.0 percentage points (Wilding and colleagues, J Endocr Soc 2021). The joint advisory's reading of the same data is that lean mass accounted for 38% of the weight lost.

Tirzepatide: SURMOUNT-1

The SURMOUNT-1 DXA substudy followed 160 adults for 72 weeks. On tirzepatide, average weight fell by 21.3%, fat mass by 33.9% and lean mass by 10.9%, compared with 5.3%, 8.2% and 2.6% on placebo. In absolute terms, people lost 15.9 kg of fat and 5.6 kg of lean mass, roughly three kilograms of fat for every kilogram of lean tissue (Look and colleagues, Diabetes Obes Metab 2025). The ratio was similar on placebo and across age groups, with fat making up 74% to 76% of the weight lost in under-50s, 50 to 64-year-olds and people aged 65 or over. The study was funded by the manufacturer, most authors were company employees, and the subgroups were small.

The meta-analysis: how drugs compare with diet, exercise and surgery

A 2026 systematic review in Diabetes, Obesity and Metabolism pooled 21 randomised trials or substudies in which participants lost at least 10% of their weight (Busk-Cirera and colleagues, 2026). Fat-free mass accounted for 33.3% of weight lost with incretin-based medicines and 34.2% after bariatric surgery. Diet without exercise came to 22.3%, while diet combined with exercise came to only 7.7%. The authors note high heterogeneity, small drug substudies and that fat-free mass is not the same as muscle, and they did not compare the modalities statistically.

The striking finding is not the drug figure on its own but the gap between diet alone and diet with exercise. It suggests the exercise, not the method of eating less, is what does most of the protecting.

Is it a problem? Both sides of the debate

The case for concern

Losing a third of your weight loss as lean tissue is not trivial if you start with little muscle, are older, or are losing weight fast while eating very little protein. Muscle supports mobility, balance, blood glucose control and recovery from illness. The joint advisory notes that sarcopenia risk rises with age, that older people and women lose more bone with rapid weight loss, and that people aged 65 and over are more likely to stop these medicines. Pharmaceutical companies are also developing drugs intended to preserve muscle during weight loss, which has amplified public worry.

The case for perspective

A 2024 JAMA viewpoint by Caterina Conte, Kevin Hall and Samuel Klein argued that some lean loss happens with any weight loss, that the relative fall in muscle is smaller than the fall in fat, and that the concern that GLP-1 drugs cause frailty or sarcopenia is not supported by data, as reported by TCTMD. The trial data fit part of that argument: in STEP 1, lean mass as a share of body weight went up, not down, and SURMOUNT-1 found similar proportions of fat and lean loss in older and younger adults.

Where that leaves you

Both positions agree on the practical answer. For most people the benefit of losing excess fat is large, and a well-planned programme can limit lean loss. The groups where caution is most justified are older adults, people who already have low muscle mass or strength, people losing a very large share of their weight, and anyone who cannot exercise. For them, muscle protection should be part of the plan from the first appointment.

Why muscle matters more after 60

Muscle mass and strength decline with age even without weight loss. Sarcopenia is the clinical term when this becomes a problem. The European working group on sarcopenia (EWGSOP2) defines it first by low muscle strength, with low muscle quantity or quality confirming the diagnosis and poor physical performance indicating severe sarcopenia (Cruz-Jentoft and colleagues, Age and Ageing 2019). Its cut-offs for low grip strength are below 27 kg for men and below 16 kg for women, and taking longer than 15 seconds to rise from a chair five times signals low strength.

Starting a GLP-1 medicine with probable sarcopenia is not a reason to avoid treatment automatically, but it changes how the plan should look. Our guide to sarcopenia covers screening and treatment in more depth.

How to keep muscle on a GLP-1

Resistance training is the main tool

The joint advisory recommends strength training at least three times a week alongside at least 150 minutes of moderate aerobic activity, tailored to fitness level. It also notes that mixed training preserves lean mass better than aerobic exercise alone. The ESPEN expert group on protein and exercise in ageing recommends daily physical activity, including resistance and aerobic exercise, for all older people for as long as possible.

Direct trial evidence for resistance training during GLP-1 treatment is still thin. The best randomised data come from S-LiTE, in which 195 adults lost 13.1 kg on an eight-week low-calorie diet and were then randomised to a year of exercise, liraglutide, both, or placebo. The combination reduced body-fat percentage by 3.9 percentage points, about twice the reduction seen with either exercise or liraglutide alone (Lundgren and colleagues, NEJM 2021). Trials testing resistance training and protein specifically with semaglutide and tirzepatide, such as the LEAN-PREP study in Kuwait, have published protocols but not yet results (BMJ Open 2026 protocol).

A practical starting programme, to adapt with a physiotherapist or qualified trainer:

1. Two to three sessions a week working the major muscle groups: legs, hips, back, chest, shoulders and arms.
2. Compound movements such as sit-to-stand or squats, step-ups, rows, wall or bench press-ups and carries.
3. Progression. Increase load, sets or repetitions gradually as each becomes manageable.
4. Balance work for older adults, to reduce fall risk.

Protein: enough, spread across the day

Protein supplies the building blocks for muscle, and appetite on these drugs can drop so far that intake falls well below normal. For people actively losing weight, the joint advisory's range is 1.2 to 1.6 g/kg/day, with 80 to 120 g a day offered as an easier fixed target, and warns against going below 0.4 to 0.5 g/kg. For people over 65, the PROT-AGE study group recommends at least 1.0 to 1.2 g/kg/day for healthy older adults, 1.2 g/kg or more for those who exercise, and 1.2 to 1.5 g/kg for those with acute or chronic illness, with an exception for severe kidney disease not on dialysis.

The protein calculator gives a personal estimate, and our article on protein targets on GLP-1 medicines covers how to reach them with a small appetite. Eating protein first at each meal, as the joint advisory suggests, is a simple way to make sure it is not crowded out.

Do not eat too little overall

Very low energy intake speeds lean-tissue loss. The joint advisory reports that calorie intake fell by 16% to 39% in GLP-1 studies, and flags a higher risk of vitamin and mineral shortfalls once intake drops under roughly 1,200 kcal (women) or 1,800 kcal (men) daily. If side effects are stopping you from eating, our guide to easing nausea and constipation on GLP-1 medicines may help, and your prescriber may slow the dose increases.

What about supplements?

Protein powders and nutritionally complete drinks can help people who cannot reach protein targets with food; see our guide to meal replacement shakes for how to compare them. Be wary of products marketed as "GLP-1 support" or "muscle-sparing" with no trial evidence behind them. Creatine is popular with people who strength train, and our creatine supplement guide looks at what it does and does not do, but no trial has established it as a muscle-protecting add-on to GLP-1 treatment.

How to monitor muscle while on treatment

You do not need a DXA scan to keep an eye on muscle. The joint advisory recommends measuring strength, function and, where possible, body composition before treatment, and lists simple tests that work well in older adults:

• Sit-to-stand: how long it takes to stand up from a chair five times without using your arms.
• Stair climb: time to climb a set flight.
• Timed up and go: rising from a chair, walking three metres, turning and sitting again.
• Grip strength, measured with a hand dynamometer at many clinics.

If these get worse while weight falls, that is the signal to review protein, exercise and the pace of weight loss. Improving test results while weight falls is reassuring, whatever the scale says about lean mass.

Book a session with a dietitian

Is muscle loss on GLP-1s reversible?

There is no trial that has followed people specifically to see whether lean tissue lost on these medicines comes back, so be cautious of confident claims either way. What is well established is that muscle responds to resistance training and adequate protein at any age, which is why ESPEN recommends exercise for older people for as long as possible.

There is one important complication. When people stop GLP-1 medicines, most regain weight, and there is little research on what that regained weight is made of. In S-LiTE's follow-up a year after treatment ended, people who had exercised regained less weight than those who had taken liraglutide alone, and the combination group kept a lower body-fat percentage (Jensen and colleagues, eClinicalMedicine 2024). Building strength while on treatment is therefore an investment for whatever happens next. Our guide to what happens when you stop a GLP-1 covers this in detail.

Older adults on the Medicare GLP-1 Bridge

A Medicare demonstration, the GLP-1 Bridge, opened on 1 July 2026: until the end of 2027, eligible people with Part D coverage pay a $50 copay for certain GLP-1 drugs. The CMS page describes drug access only and says nothing about nutrition or exercise support, so more adults over 65 are likely to start treatment without a muscle-protection plan.

Medicare does cover some support. Part B pays for obesity behavioural therapy in primary care for people with a BMI of 30 or more, while its medical nutrition therapy benefit from a registered dietitian is limited to diabetes and kidney disease. Our United States country guide explains how dietitian care is accessed and paid for there.

Putting it together

GLP-1 medicines do cause some loss of lean tissue, in roughly the proportion expected for the amount of weight lost, and fat falls much faster than muscle. That is not a reason to avoid treatment, but it is a reason to plan for it, especially after 60. Strength training two to three times a week, enough protein spread across meals, avoiding very low intakes and simple strength checks are the core of that plan. For more on the treatment itself, see our guide to GLP-1 weight loss and our 7-day GLP-1 diet plan, and for the wider picture our weight management hub.