Do GLP-1s and Tirzepatide Cause Muscle Loss? What the Research Shows

by | Oct 20, 2026 | 0 comments

GLP-1 and eating disorder history represented by a person standing on a bathroom scale.If you started tirzepatide or a GLP-1 and the scale is moving, you might also be wondering what exactly is leaving your body.

Body weight is not one thing. It includes fat, muscle, water, organs, and bone. So when a medication helps you lose 15 or 20 percent of your body weight, a fair question is how much came from fat and how much came from lean tissue.

Researchers scanned people’s bodies before and after treatment to find out. Here is what they found, what it does and does not mean, and what you can do with it.

 

What “Lean Mass” Actually Means

Lean mass sounds like muscle, but it covers more. Body scans put muscle, organs, water, and connective tissue in that bucket. Fat goes in the other bucket. Scans track bone separately.

That matters because a drop in lean mass does not equal a drop in muscle. Some of it is muscle. Some of it is water and organ tissue that shrinks as your body gets smaller. The numbers below likely overstate how much muscle you lose.

 

What the Tirzepatide Research Found

The biggest body scan study on tirzepatide comes from a trial called SURMOUNT-1. A group of 160 adults with overweight or obesity had scans at the start and again after 72 weeks.

Here is what changed on average:

    • Total body weight dropped about 21 percent, compared with about 5 percent on placebo
    • Fat mass dropped about 34 percent
    • Lean mass dropped about 11 percent

Put another way, roughly three quarters of the weight lost was fat and about one quarter was lean mass.

The placebo group split the same way, about 75 percent fat and 25 percent lean, even though they lost far less weight overall. Tirzepatide helped people lose more total weight, so they lost more lean mass in pounds. The ratio stayed the same.

Here is what that looks like in real numbers. Someone who starts at 225 pounds and loses 21 percent drops about 47 pounds. At a 75 to 25 split, that is roughly 35 pounds of fat and 12 pounds of lean mass. This is an illustration using the study averages, so your own results will vary.

 

What About Semaglutide and Other GLP-1s?

Semaglutide (Wegovy and Ozempic) tells a similar story with a higher lean share. In its main body scan study, people lost about 15 percent of their body weight over 68 weeks, and roughly 40 percent of the weight they lost showed up as lean mass.

When researchers pooled results across many GLP-1 trials, lean mass made up about a quarter to a third of the weight lost.

These were separate studies with different people, so I would not read the gap between tirzepatide and semaglutide as proof that one protects muscle better than the other. The research does not answer that question yet.

 

How Fast Does It Happen?

The studies scanned people at the start and at the end. That means they can show the average result, but they cannot show how the fat and lean split changes week to week.

The averages: about 21 percent total weight loss over 72 weeks across the tirzepatide doses studied, and about 15 percent over 68 weeks on semaglutide. Individual results vary quite a bit around those averages.

 

Does Losing Lean Mass Matter?

It depends on who you are and where you start. Researchers are still working out how much it matters. A few things help put it in context:

    • Lean mass includes water and organ tissue, not only muscle
    • People in the tirzepatide study reported better physical function scores, even with the drop in lean mass
    • Strength and how you feel in daily life tell you more than a number on a scan

That said, it deserves a plan for some people. Older adults, anyone who already has low muscle, and anyone who wants to protect their strength have more reason to pay attention. When you start with less muscle, losing some has a bigger effect.

 

What Can You Do to Protect Muscle?

Protein

No finished study has tested higher protein on tirzepatide or semaglutide and measured lean mass. Trials are underway. Until they report, dietitians borrow from research on weight loss in general, which points to about 1.2 to 1.6 grams of protein per kilogram of body weight per day. That is roughly half to three quarters of a gram per pound.

For someone who weighs 180 pounds, that works out to about 98 to 131 grams a day.

Appetite on these medications can make that number hard to reach. Spreading protein across the day helps. Aim for around 25 grams at a meal when you can, and use smaller protein snacks to fill gaps.

Your best number depends on your health history, including kidney health, so check it with your care team.

 

Strength Training

Strength training is the other half. The strongest evidence comes from weight loss in general, where lifting a few times a week helps people hold onto more lean mass.

One trial paired liraglutide, an older GLP-1, with exercise. Over a year, the people who did both lost body fat and kept their lean mass. That study looked at keeping weight off after a diet instead of the active weight loss phase, so treat it as a hint, not a promise.

A reasonable starting point is two or three sessions a week using weights, bands, or bodyweight moves that work the big muscle groups.

Woman lifting pink dumbbells during a home workout.

 

Track More Than the Scale

Pay attention to how your clothes fit, how strong you feel, and whether stairs and carrying groceries feel easier or harder. Those signs tell you what the scale cannot.

 

Frequently Asked Questions

Will I lose muscle on tirzepatide?

Probably some. In the main study, about a quarter of the weight lost counted as lean mass, and part of that was not muscle.

 

Is tirzepatide better than semaglutide for keeping muscle?

The research cannot say yet. The two drugs were studied in separate trials with different participants, so a direct comparison is not fair.

 

Can protein shakes fix this on their own?

A shake can help you reach your protein number when food feels like a chore. Studies have not shown that shakes alone protect lean mass on these medications, and they do not replace strength training.

 

The Bottom Line

Most of the weight lost on tirzepatide and semaglutide is fat. Around a quarter to a third is lean mass, and part of that is water and organ tissue instead of muscle. Protein and strength training are reasonable steps while the research catches up.

Daphne is a Real Food Dietitian from South Louisiana who believes food should nourish, not complicate. With a diverse background in clinical and community nutrition, she helps individuals find the foods that make them feel their best—simple, satisfying, and stress-free.

 

This article is general education and does not replace medical advice from your care team.

 

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