GLP-1s and muscle loss: how to keep the muscle while the weight comes off

GLP-1s are very good at taking weight off. What they do not do is choose which weight goes. Left alone, a meaningful share of it is muscle. The good news is that this is the part you can control.

How much of the weight is muscle?

In the large STEP 1 trial, people using a weekly GLP-1 for 68 weeks lost about 15 percent of their body weight. Body composition scans on 140 of them showed lean mass down by almost 10 percent, which works out to roughly two-fifths of everything they lost.

In SURMOUNT-1, a trial of a newer dual-action GLP-1, scans on 160 participants put the figure at about a quarter. For every four kilograms lost, roughly three were fat and one was lean tissue.

So a fair range is that somewhere between a quarter and two-fifths of the weight lost on a GLP-1 is lean mass, if nothing is done to protect it.

The honest caveat

Lean mass on a scan is not the same thing as muscle. It also includes water, organs and connective tissue. Researchers writing in JAMA in 2024 argued the alarm is overstated: fat falls proportionally more than lean tissue, so muscle as a share of body weight actually rises, and the split is similar to weight loss from dieting alone.

That is a fair point and worth keeping in proportion. But it answers a different question. For a man in his forties, the question is not whether the loss is dangerous today. It is whether he wants to spend strength he will need at sixty. Muscle is slow to build and gets slower with age. Losing it is a real cost, even when it is not an emergency.

Why it happens

Any large calorie deficit puts muscle at risk, and a GLP-1 creates a large one by turning appetite down. Less appetite usually means less protein, less energy to train, and often less training. The body keeps the tissue it has a reason to keep. Take the reason away and some muscle goes with the fat.

1. Lift two to three times a week

Resistance training is the signal that tells your body the muscle is still needed. In a 2017 trial of older adults losing weight by dieting, lean mass fell by about 5 percent in the group that only did aerobic exercise, against about 2 percent with resistance training and 3 percent with both. The combined group also improved physical function the most.

The same pattern holds alongside a GLP-1. In a 2021 trial, people who paired a GLP-1 with a structured exercise programme for a year cut their body fat percentage by roughly twice as much as people who only exercised.

In practice that means two or three full-body sessions a week built on squats, hinges, presses and pulls, with the load nudged up over time. On low-energy days, keep the weights and cut the number of sets. Doing less is fine. Stopping is the problem.

2. Protein first, every meal

Protein is the raw material, and it is the first thing to fall when appetite drops. A 2025 joint advisory from four US nutrition and obesity organisations suggests roughly 1.2 to 1.6 grams per kilogram a day while losing weight, based on goal weight rather than current weight. For a man aiming for 85 kilograms, that is about 100 to 135 grams a day.

More can help when training is hard. In a well-known 2016 trial, young men in a steep calorie deficit who trained intensely and ate 2.4 grams per kilogram gained about a kilogram of lean mass and lost more fat than a group on half the protein. It was a short, extreme setting, but the direction is clear.

With a small appetite, tactics matter more than the target. Eat the protein on your plate first. Lean on soft, easy options like Greek yoghurt, skyr, eggs, cottage cheese and shakes. Split intake into smaller, more frequent meals so it does not all have to fit into two.

3. Measure what matters

The scale tells you weight has gone, not what kind. Track strength on your main lifts, your waist, and if you can, a body composition scan every few months. If the scale is falling and your strength is falling with it, that is the signal to raise protein or training, not to celebrate.

4. Build what lasts

In the STEP 1 follow-up, people regained around two-thirds of the weight they had lost within a year of stopping. Whatever happens next with the GLP-1, the habits are what stay: the training routine, the protein-first plate and the muscle you kept. They are easier to build now, while appetite is under control, than later.

Where coaching fits

We coach the training, nutrition and habits around a GLP-1, not the GLP-1 itself. How and when to use it stays with whoever provides it. Our job is making sure the weight you lose is fat, and that you come out stronger than you went in. See how we coach men using a GLP-1.

This article is general training and nutrition information. It does not cover how to use a GLP-1, or when to start or stop one.