GLP-1 Muscle Loss: What to Know About Lean Mass

Important note: This article is for general education only. It is not medical advice. Do not start, stop, switch, or change any medication, supplement, exercise plan, or nutrition plan based on this article. If you have questions about GLP-1 medications, muscle loss, protein, strength training, frailty, or body composition, talk with your doctor, pharmacist, registered dietitian, or another qualified healthcare professional.

For many people taking a GLP-1 or GLP-1-related medication, the scale may move faster than expected.

That can feel encouraging, but it can also raise a practical question: what kind of weight is being lost?

Weight loss is not only fat loss. Some amount of lean mass can be lost too. Lean mass includes muscle, water, organs, bone, and other non-fat tissue. Muscle is only one part of lean mass, but it is the part many people are most concerned about because it supports strength, balance, metabolism, mobility, and daily function.

That is why muscle preservation is becoming a bigger part of the GLP-1 conversation.

Why lean mass matters on a GLP-1

GLP-1 medications can reduce appetite and help people lose weight. But when weight comes off, the body may lose both fat mass and some lean mass.

Clinical trial and body composition research has found that lean mass can account for a meaningful share of weight lost with GLP-1 medications, although the exact amount varies by medication, dose, study design, baseline health, age, diet, activity, and how body composition is measured. One 2025 review noted that in STEP 1, approximately 30% of weight lost with semaglutide was attributed to lean tissue, while fat loss still made up most of the reduction.

That does not mean GLP-1 medications are “burning muscle” in a simple or direct way. It means that significant weight loss, from any method, can include some loss of lean tissue.

The goal is not to stop weight loss. The goal is to make sure the weight-loss process supports health, strength, and function as much as possible.

What new research says about protecting lean mass

A 2026 phase 2 trial published in Nature Medicine tested whether an investigational medication called apitegromab could help preserve lean mass during tirzepatide-associated weight loss. Tirzepatide is the active ingredient in Mounjaro and Zepbound.

The study enrolled 102 adults with obesity and randomly assigned participants to receive tirzepatide plus either apitegromab or placebo for 24 weeks. Apitegromab is a myostatin inhibitor, which means it targets a pathway involved in limiting muscle growth.

The results were notable. Participants receiving placebo lost an average of 3.5 kg of lean mass, while those receiving apitegromab lost an average of 1.6 kg of lean mass. That means apitegromab was associated with about 54.9% relative retention of lean mass compared with placebo, while total weight loss was similar between groups.

In plain English: the experimental drug did not mainly make people lose more weight. It appeared to change the composition of weight loss by helping preserve more lean mass.

That is scientifically interesting, but it is not a reason for patients to seek it out. Apitegromab is investigational for this use. It is not a standard part of GLP-1 care for weight management, and more research is needed to understand who may benefit, how durable the effect is, and whether preserving lean mass this way improves strength, mobility, or long-term health.

Lean mass is not the same as strength

One of the most important findings from the apitegromab study is what it did not show.

Although apitegromab helped preserve lean mass, the study did not show clear improvements in physical function or grip strength during the 24-week trial.

That matters because lean mass and strength are related, but they are not identical.

A person can preserve more lean tissue and still need movement, resistance training, balance work, enough protein, and daily activity to support real-world function. Muscle is not only something the body has. It is something the body uses.

That is why the next phase of GLP-1 care is not only about the number on the scale. It is about how people feel, move, eat, recover, and function over time.

Why “Ozempic butt” is not the right frame

Social media often talks about body changes from GLP-1 medications with phrases like “Ozempic face” or “Ozempic butt.” Those terms may describe visible changes some people notice after weight loss, but they can also make a real health topic sound like a cosmetic joke.

The more useful question is not whether a body part looks different. It is whether someone is maintaining strength, balance, energy, and mobility while losing weight.

For some people, body composition changes may be subtle. For others, they may feel weaker, lose stamina, notice looser skin, or feel less steady during daily activities. Those changes are worth discussing with a care team, especially for older adults, people with a history of falls, people with low protein intake, people with chronic illness, or anyone who feels weaker during treatment.

Movement may not automatically increase with weight loss

Many people assume that losing weight will naturally lead to more physical activity. That may happen for some people, but it is not guaranteed.

Research presented at ENDO 2026 found that people taking GLP-1 medications had lower physical activity after starting treatment, based on Fitbit data. Average daily steps decreased from 5,047 to 4,487, and moderate-to-vigorous physical activity fell from 28 minutes to 22 minutes per day.

This was observational research, not proof that GLP-1 medications directly cause people to move less. Side effects, lower appetite, fatigue, nausea, joint pain, life circumstances, or other factors may play a role.

But the finding is still useful. It suggests that movement may need to be supported intentionally, not assumed.

Protein and strength support are part of the conversation

For people using GLP-1 medications, muscle preservation is usually discussed in terms of a few core supports:

enough protein,

regular movement,

resistance or strength training,

hydration,

sleep,

and medical follow-up when symptoms or weakness appear.

That does not mean everyone needs the same protein target or the same workout plan. Needs can vary by age, kidney function, medications, health conditions, activity level, and treatment goals. A registered dietitian or healthcare provider can help tailor guidance.

The broader point is that appetite reduction can make it easier to unintentionally under-eat. If someone is eating much less overall, protein, fiber, fluids, and micronutrients may need more attention.

For many people, the practical goal is not perfection. It is noticing patterns early: Am I eating enough? Am I getting protein regularly? Am I losing strength? Am I too tired to move? Am I skipping meals because I am not hungry? Am I feeling dizzy, weak, or unsteady?

Those are the kinds of details worth bringing to a healthcare professional.

The GLP-1 pipeline is making this issue more important

The muscle conversation is becoming more important because newer GLP-1 and GLP-1-related medications are producing larger weight-loss results in clinical trials.

Eli Lilly has reported phase 3 results for retatrutide, an investigational triple agonist that targets GLP-1, GIP, and glucagon receptors. In the TRIUMPH-1 trial, Lilly reported average weight loss of 28.3% at the highest dose over 80 weeks, and 30.3% at 104 weeks in a study extension group with baseline BMI of 35 or higher. Retatrutide is investigational and is not FDA-approved as of this article’s publication.

Novo Nordisk has also studied CagriSema, an investigational combination of semaglutide and cagrilintide. In the REDEFINE 4 trial, CagriSema produced substantial weight loss but did not meet the primary endpoint of noninferiority compared with tirzepatide.

As medications become more effective at producing weight loss, body composition may become an even bigger part of patient care. The question is not only “How much weight was lost?” It is also “How much strength, function, and quality of life were preserved?”

What patients can ask their care team

If you are using a doctor-prescribed GLP-1 or GLP-1-related medication and are concerned about muscle loss, consider asking your care team questions like:

How should I think about muscle and lean mass during weight loss?

How much protein is appropriate for me?

Are there any reasons I should be careful with high-protein eating?

What kind of strength or resistance training is safe for me?

Should I work with a registered dietitian or physical therapist?

Should we track body composition, strength, labs, or symptoms?

What should I do if I feel weaker, dizzy, unsteady, or unusually fatigued?

The right plan depends on your health history. That is especially true for older adults, people with kidney disease, people with diabetes, people with heart conditions, people with a history of eating disorders, and anyone recovering from illness or injury.

Where Glo fits in

Glo is designed to support the everyday space between appointments.

For people using doctor-prescribed GLP-1 and GLP-1-related medications, Glo can help track meals, hydration, movement, side effects, symptoms, habits, and routines by text. That kind of tracking can make it easier to notice patterns and bring clearer information to a doctor, pharmacist, registered dietitian, or other healthcare professional.

For example, someone might use Glo to notice that they are skipping protein at breakfast, drinking less water than usual, moving less on injection days, or feeling weaker during certain parts of the week.

Glo does not diagnose muscle loss. It does not prescribe protein targets or exercise plans. It does not replace medical care. But it can help people pay attention to the daily details that often matter during GLP-1 treatment.

Bottom line

GLP-1 medications can support significant weight loss, but weight loss is not only about the number on the scale. Some lean mass may be lost too, and that has made muscle preservation an important part of the conversation.

New research on apitegromab suggests that future treatments may be able to help preserve lean mass during GLP-1-associated weight loss. But preserving lean mass is not the same as preserving strength, and investigational drugs are not a substitute for personalized care.

For patients, the practical takeaway is simple: pay attention to strength, protein, movement, hydration, energy, and function. If something feels off, bring it to your care team.

The goal is not just to lose weight. It is to support the body underneath the weight loss.