SIDE EFFECTS & MANAGING THEM

Preserving Muscle on GLP-1 Medications — Protein, Training, and What the Trials Show

By G1 Peptides · Medically reviewed by the G1 Peptides clinical team · Updated September 24, 2026 · 3 min read

THE SHORT ANSWER

Roughly a quarter to a third of the weight lost on GLP-1 medications is lean mass, similar to any large weight loss. That is not unique to the drugs, but the speed and appetite suppression make it easy to under-eat protein and stop training. Targeting 1.2–1.6 g of protein per kg, resistance training two to three times a week, and not escalating faster than you can eat well are the levers that matter.

What the trials measured

Body-composition substudies in the semaglutide and tirzepatide trials used DEXA scans on a subset of participants. Lean mass fell alongside fat mass — in STEP 1's substudy, roughly 40% of total weight lost was lean tissue by DEXA, though the proportion of lean mass in the body improved because fat fell faster (Wilding 2021). SURMOUNT-1's substudy reported around a quarter of the loss as lean mass (Jastreboff 2022).

Two caveats. "Lean mass" on DEXA includes water, organ tissue, and glycogen, not just muscle, and some of it is expected to fall with fat. And the trial participants were not given protein targets or training programs. Those are the variables you control.

Why it happens on these drugs

The deficit is large. Fifteen to twenty-five percent weight loss is a big, sustained calorie shortfall, and the body draws on muscle when protein intake is low.
Appetite suppression hits protein. Meat and dense protein are often the first things people stop wanting; carbohydrates and soft foods are easier when nauseous.
Training falls off. Fatigue in the early months and a general "I'm losing weight anyway" effect reduce gym attendance.

Protein: the first lever

Target 1.2–1.6 grams per kilogram of body weight per day, spread across three or four servings of 25–40 g. On a 90 kg body that is 110–145 g a day — a real effort when you are not hungry. Practical tactics:

Eat protein first at every meal, before anything else fills you up.
Use liquid protein (whey or casein shakes, Greek yogurt, cottage cheese) when solid food is unappealing.
Keep cold, ready protein on hand — eggs, deli turkey, tinned fish, tofu.
Track for a couple of weeks; most people are surprised how far below target they are.

Resistance training: the second lever

Muscle is retained when it is used. Two to three full-body sessions a week of compound movements — squats or leg press, rows, presses, hinges — at a challenging effort is enough. Progressive overload matters more than volume: add weight or reps over time. Walking is good for you but does not preserve muscle; lifting does.

If you are new to training, starting during GLP-1 therapy is a good time — bodyweight and machine work are fine — and a few sessions with a coach to learn the movements pays for itself.

Pace the escalation

Muscle loss is worst when weight falls fastest. If you cannot eat to your protein target on a given step, that is a reason to hold the step, not push through. Slower loss with maintained strength beats faster loss with lost strength every time.

Other supports

Creatine monohydrate (3–5 g daily) has solid evidence for strength and lean mass retention and is inexpensive.
Sleep — muscle repair happens there; seven-plus hours.
Vitamin D and adequate calories on training days — a deficit is fine, but training on empty is counterproductive.
GH secretagogues (sermorelin, CJC-1295/ipamorelin) are sometimes added for body composition; evidence for the combination is limited and it is a provider decision, not a substitute for protein and training.

Back to the GLP-1 guide, or see how GLP-1 medications work.

FREQUENTLY ASKED

Do GLP-1 drugs cause muscle loss directly?

No mechanism has been shown that targets muscle specifically. The loss reflects a large calorie deficit plus low protein intake — the same thing that happens with bariatric surgery or aggressive dieting. Managing the diet and training manages the loss.

Should I take a growth-hormone peptide to protect muscle?

Some providers pair GLP-1 therapy with a GH secretagogue for body-composition reasons; the evidence for that specific combination is limited. Protein and training have far stronger evidence and should come first. Discuss any stacking at your consultation.

Is losing some lean mass a problem if I am carrying a lot of fat?

Some lean-mass loss accompanies any large fat loss and includes water and organ mass, not only muscle. The concern is strength and function, especially with age. If you are getting stronger in the gym, you are managing it.

SOURCES

This article is educational and is not medical advice. Peptides are prescription medications; whether any of them is appropriate for you is a decision for a licensed provider who knows your history.