Losing weight on Ozempic or Mounjaro: how much of it is muscle?

It's a question we often hear during consultations at our studio. Someone comes in feeling relieved because, after months or even years of struggling with their weight, the number on the scale is finally going down. But alongside that excitement, there's usually one concern: "Am I losing muscle along with the fat?"

The short answer is yes, some of the weight you lose while taking medications like Ozempic or Mounjaro can come from lean tissue. Large clinical trials suggest that around 15% to 40% of total weight loss may come from lean mass, with the majority coming from fat. That might sound worrying, but lean mass isn't the same as muscle mass. While some muscle loss can occur, strength training can help you preserve muscle as you lose weight.

What does the research actually show?

In the STEP 1 DXA substudy, 140 participants were followed for 68 weeks: 95 received semaglutide 2.4 mg (the active ingredient in Ozempic and Wegovy) and 45 received a placebo. Among those taking semaglutide, fat mass decreased by 19.3% and lean mass by 9.7%.

The SURMOUNT-1 DXA analysis examined 160 participants over 72 weeks taking part in a tirzepatide trial (tirzepatide is the active ingredient in Mounjaro). Approximately 75% of the weight lost was fat and 25% was lean mass. The placebo group lost fat and lean mass in similar proportions, but considerably less weight overall.

A 2025 review found lean mass accounted for approximately 15% to 40% of total weight lost across studies. This is why protecting muscle during weight loss deserves attention.

Lean mass isn't the same as muscle

A DXA scan separates body composition into fat, bone and lean mass. Lean mass includes muscle, but also water, organs and connective tissue. When you lose substantial weight, fluid levels change and organs such as the liver may become smaller. These changes contribute to decreases in lean mass.

So a drop in lean mass on a scan doesn't mean the same amount of muscle was lost. DXA alone cannot determine exactly how much of that loss was muscle. In the STEP 1 semaglutide group, the ratio of lean mass to fat mass actually improved. Participants finished with a better overall body composition than when they started.

Muscle loss is worth paying attention to, but there's no reason to panic.

Why muscle matters, especially after 35

As we get older, particularly from around our mid-thirties, maintaining muscle becomes increasingly important. Muscle supports movement and joint function, helps regulate blood sugar, and makes daily activities easier.

A retrospective study presented at ENDO 2026 found average daily steps among people taking GLP-1 medication fell from 5,047 to 4,487. This is an early observational signal, not proof the medication caused the change. Lower energy or reduced appetite may contribute to people moving less without noticing. Combined with significant weight loss, less activity may make it harder to maintain muscle.

What can you do to protect your muscles?

A 2025 joint advisory from the American College of Lifestyle Medicine, American Society for Nutrition, Obesity Medicine Association and The Obesity Society emphasised two priorities during GLP-1 therapy: adequate protein and strength training.

1. Eat enough protein

The advisory suggests approximately 1.2 to 1.6 grams of protein per kilogram of body weight daily during weight loss. At 90 kilograms, that is approximately 110 to 145 grams per day using actual body weight. Which weight to use for this calculation remains debated, so personal targets may be lower. Discuss your needs with your doctor or dietitian.

A lower appetite can make meeting protein needs difficult. Some of our members find a protein shake easier than eating another full meal. Our coaches do not prescribe meal plans.

2. Start strength training

Muscle responds to the demands placed on it. Resistance training gives your body a reason to maintain muscle tissue. Research directly testing resistance training during semaglutide or tirzepatide treatment remains limited. A small exploratory semaglutide study reported maintained muscle measures during resistance training but could not establish causation. LEAN-PREP is designed to examine resistance exercise and protein supplementation more rigorously.

Decades of broader exercise research nevertheless support strength training to help preserve muscle. It cannot guarantee that every gram of muscle loss is prevented.

Should you wait until you've lost weight before training?

No. The months when you're losing weight fastest are exactly when your muscles need a reason to stay.

Some people feel energetic during GLP-1 treatment; others experience fatigue, lightheadedness or weakness. At NXT LVL, we adapt training volume and load to how someone feels while maintaining good technique. The goal is steady progress, not exhaustion. If you feel lightheaded during a session, stop and tell your coach straight away. Discuss persistent lightheadedness or weakness with your doctor.

A simple strength training plan to start this week

If you have gym access and are cleared to exercise, begin with two full-body sessions per week, at least 48 hours apart. Each session can include:

• Squat: Squat or leg press.
• Hip hinge: Romanian deadlift or hip thrust.
• Push: Chest press.
• Pull: Seated row.

Complete two sets of 8 to 12 repetitions per movement, stopping with roughly two or three good repetitions still possible. Increase the load gradually as sets become controlled. Prioritise protein with meals and track your steps for a week to see if activity is declining.

For more detail, read A Good Training Week on a GLP-1.

Why professional guidance can help

A personalised plan should fit your schedule, fitness, energy, injuries and goals. A coach can adjust exercises, improve technique and adapt sessions on low-energy days. Consult your doctor before starting, particularly if you also take medication for diabetes or blood pressure, and let your coach know what medications you use.

How we monitor progress at NXT LVL

The scale cannot tell you what your weight loss is made of. At our studio in Amstelveen, near Amsterdam, our Longevity programmes include a Visbody 3D scan every four weeks. It estimates changes in fat and muscle mass. Because hydration and other factors affect results, we look for trends over several scans rather than treating a single scan as definitive. If estimates suggest muscle is declining faster than we'd like, we review training and protein intake.

Our Longevity programmes start from €709 and include coached strength training through one-to-one sessions or small groups of up to five people, regular scans and WhatsApp coaching support. Training frequency depends on the chosen programme. Medication decisions remain between you and your doctor.

Your next step

If you're taking Ozempic, Mounjaro or similar medication, don't look only at the scale. Pay attention to your strength, protein intake, daily activity and body composition trends.

The goal isn't simply to lose weight. It's to lose fat while preserving as much muscle and strength as possible.

Start with two manageable strength sessions this week. If you'd prefer guidance, NXT LVL can help you build a personalised plan and monitor your progress.

Ready to get started? Explore our Longevity programmes or book an introductory session at NXT LVL Personal Training in Amstelveen.

Written by

Team NXT LVL

The coaching team at NXT LVL Personal Training in Amstelveen.