GLP-1 and muscle: what the research actually says

If you are on semaglutide or tirzepatide, the scale is telling you a story that is only half true. Some of what you are losing is muscle, and unlike the fat, you do not get it back for free.

This is the question I get most often right now, from clients and from people who find me through the app. So here is what the research actually says, and what it means for the week in front of you.

What the trials found

The body composition substudy of STEP 1, the main semaglutide trial, found that roughly 45 percent of the total weight lost came from lean tissue. The equivalent substudy from SURMOUNT-1, the tirzepatide trial, put the figure closer to 25 percent.

Those numbers look alarming out of context, so here is the context: that ratio is normal for rapid weight loss. It is not something the drugs do to you specifically. Lose weight quickly by any method and a meaningful share of it comes from muscle.

What makes GLP-1 different is the scale. The weight loss is larger, so the absolute amount of muscle lost is larger too.

The part almost nobody talks about

At the Endocrine Society meeting in June, a study reported something I find more concerning than the substudy numbers: adults losing weight on GLP-1 medication significantly decreased their physical activity.

That makes sense when you think about it. You are eating far less. Energy is lower. The weight is coming off without effort, so the motivation to train drops. And the appetite suppression that makes the drug work also makes it hard to eat enough protein to hold onto muscle.

So the drug creates the deficit, and the behaviour it encourages removes the one thing that protects your muscle inside that deficit.

What protects lean mass

Two things, and they are not complicated.

Resistance training. A 2018 pooled analysis of six trials in older adults with obesity found that resistance training preserved nearly all lean mass during caloric restriction. Not some of it. Nearly all of it.

Protein. The target during active weight loss is roughly 1.2 to 1.6 grams per kilogram of bodyweight per day. On a GLP-1 this is genuinely hard, because you are not hungry. The practical answer is to anchor every meal around a protein source and eat that part first.

A trial protocol published this year, LEAN-PREP, is testing exactly this combination during semaglutide and tirzepatide therapy. What interests me is the exercise dose they chose: home-based resistance training, three sessions a week, progressing from one to three sets across the major muscle groups.

That is not a gym program. That is not two hours a day. That is three sessions a week you can do at home, which is precisely the dose most people on a GLP-1 need and precisely the dose most people can actually keep.

What I would tell you if you were in front of me

Start training before you think you need to. Muscle is much easier to keep than to rebuild, and the first months on the medication are when the loss is steepest.

Train the whole body, three times a week. You do not need to chase soreness or novelty. Squat, hinge, push, pull, and carry something heavy. Add weight when your form allows it, not before.

Eat protein first at every meal. If your appetite is gone by the third bite, at least the first three bites did the work.

And be patient with the strength numbers. In a deficit this large, holding your strength steady is a win. That is the whole goal for now.

The bigger picture

These medications are a real tool and I am not interested in arguing about whether people should use them. That is between you and your physician.

What I care about is the shape of the body you end up with, and whether you can still carry your own shopping at seventy. The scale number is not the outcome. What the weight was made of is the outcome.

Lose it with training and protein, and you come out of this lighter and stronger. Lose it without them, and you come out lighter and weaker, at an age where weaker matters.

If you are further along, carrying an injury, or you want the whole thing built around your own bloodwork and schedule, that is online coaching. And if you are in Greater Boston and would rather have me in the room, start here.

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Sources: STEP 1 body composition substudy; SURMOUNT-1 body composition substudy; Sardeli et al. (2018), resistance training and lean mass during caloric restriction; Maharjan et al., ENDO 2026, physical activity during GLP-1 treatment; LEAN-PREP trial protocol (2026).

Nothing here is medical advice. I am a strength coach, not a physician. If you are taking or considering a GLP-1 medication, those decisions belong with your doctor. What I can help with is what happens in the gym and on your plate while you are on it.