More than people expect, and probably less than the internet claims.
Trials of GLP-1 medications that measured body composition properly have generally found that somewhere between a quarter and around 40 percent of total weight lost is lean mass, depending on the study, the population and how it was measured. The wide range is not evasion. It genuinely varies, and it varies partly because of things you can influence.
Here is what that number means and what it does not.
Lean mass is not the same as muscle
The first thing to understand is that DEXA and similar tools report lean soft tissue, which includes muscle but also organ tissue, connective tissue and a large amount of water. When you lose fat mass, some loss of lean tissue follows automatically, because there is less body to supply blood to and less tissue supporting the fat you carried.
So a portion of the lean mass figure is expected, appropriate, and not a problem. This is true of any weight loss, not just weight loss on medication. Someone losing 15 kilos through diet alone will also lose lean tissue.
The concern is not that lean mass falls. It is whether it falls further than it needs to.
Why medication changes the risk profile
Two features of GLP-1 medications make the lean mass question sharper than it is with a standard diet.
Appetite suppression is very effective, which is the point, but it means people often eat far less protein than they intend to without noticing. Protein is the most satiating macronutrient and also the one people drop first when they simply do not feel like eating. We regularly see intakes well under half of what someone needs.
The other is the speed. Faster weight loss is associated with a greater proportion of it coming from lean tissue, and these medications can drive a rate of loss most people would not sustain on their own.
Add reduced appetite to a rate of loss that would normally require deliberate effort, and you get a situation where the muscle question needs actively managing rather than assuming it will sort itself out.
What actually protects lean mass
The evidence points fairly consistently in two directions, and neither is exotic.
Protein intake needs to be high enough and spread across the day. In a meaningful energy deficit that generally means considerably more than the standard population recommendation, and the practical challenge is not knowing the target but hitting it when you are not hungry. That usually means restructuring what you eat rather than eating more of the same, and it is the part a dietitian is genuinely useful for.
Resistance training is the second. Load on the muscle is the signal that tells the body to keep it. Two to three sessions a week covering the major movement patterns does most of the work, and it does not need to be complicated or heavy to start with.
Sleep, total energy intake, and how quickly the dose is escalated all matter around the edges. Those are conversations for your prescriber as much as for us.
Why we suggest measuring rather than guessing
You cannot tell from the scale whether the weight coming off is fat or lean tissue. That is the entire problem. Someone down 12 kilos who has lost 4 of them from lean mass and someone down 12 kilos who has lost 1 look identical on a bathroom scale and have had completely different outcomes.
A DEXA scan separates the two. Taken at the start and again a few months in, it tells you whether the trajectory needs changing while there is still time to change it. We have written about how often that repeat scan is worth booking, because more frequent is not better here either.
If you are weighing up whether the gym InBody would do the same job, it will not, for reasons covered in DEXA against InBody. Fluid shifts on these medications are significant enough to make impedance readings unreliable exactly when you most need them to be reliable.
Working with your prescriber, not around them
We are dietitians. We do not prescribe these medications and we do not tell people whether to take them. What we do is the nutrition side, which is the part most people are left to work out alone after the script is written.
A nutrition consultation with one of our dietitians sits alongside whatever your doctor has you on. For background on how the medications themselves work and what the research says about results and risks, we have covered that in weight loss medications in Australia.
If you want a broader starting point on losing weight without losing the things you want to keep, our dietitian's guide to healthy weight loss covers the fundamentals.
This article is general information and not individual medical advice. Decisions about prescription weight loss medication, including starting, changing or stopping one, are for you and your treating doctor.