How much of GLP-1 weight loss is muscle?

Across the trials that measured it directly, somewhere between a quarter and about 40 percent of the weight lost on a GLP-1 came off as lean tissue rather than fat. That is the answer, and the uncomfortable part of it is the second sentence: the trials can tell you the average, and nothing on your bathroom scale can tell you which share is yours.

People search for this under semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro, Zepbound). The body-composition answer below is the same whichever one you are on, and it is the same again if you are losing weight without any of them.

What the trials found

Two body-composition substudies do most of the work here. In the SURMOUNT-1 substudy of tirzepatide, roughly 75 percent of the weight lost was fat and roughly 25 percent was lean mass. The 2025 joint advisory from four medical societies reports the STEP 1 numbers on semaglutide differently and more starkly: of an average 13.6 kilograms lost, 8.3 kilograms were fat and 5.3 kilograms were lean body mass, which works out at 39 percent.

Those are two ways of reading the same kind of measurement, and both are honest. The gap between them is why a single number is the wrong thing to carry around. A quarter and two fifths are both large.

The counterpoint, which is also true

As a proportion of your whole body, lean mass often holds steady or improves on these medicines, because fat loss dominates the arithmetic. Some of what a scan records as lost lean is water, and some of it is the fat stored inside muscle, which leaving makes the muscle better rather than worse. Both of those things are real.

Neither of them gives back the muscle that did leave. A ratio that looks fine is cold comfort at seventy, when the question is whether you can carry shopping up a flight of stairs. So the useful position is neither alarm nor dismissal: the split is worth watching, and it is watchable.

Why the scale cannot answer this

A scale reports the sum of everything in your body at one moment. Two people can lose the same six kilograms, one of them almost entirely fat and the other with two kilograms of muscle gone, and the scale gives both of them the same congratulation. The difference between those two outcomes shows up years later, in how you stand up from a chair.

The reference measure is a whole-body composition scan, which reads fat, lean and bone separately. The problem with scans is not accuracy, it is spacing: most people get one, if any, and a single scan is a snapshot with nothing to compare it against.

What you can check this week

• Pick three lifts and write down the numbers. A goblet squat, a row and a press are enough. If those numbers hold or rise while the scale falls, muscle is coming along for the ride. If they slide over a month, you have an early signal months before any scan would give you one.
• Take a waist measurement. Same place, same time of day, once a week. Waist shrinking while strength holds is the shape of good weight loss.
• Count one ordinary day of protein. Not a good day. An ordinary one. Many people on these medicines fall short of the advisory's protein target, and knowing your real number is the whole first step.
• Fix your week zero. If you have not started losing yet, this is the week to get a baseline body-composition scan, because every later scan is only as useful as the one it is compared against.

Where ProForta comes in

ProForta reads the data your phone already collects, on a private node that belongs to you, and turns it into one continuous read on whether your muscle is holding. It never shows raw values and never sends them to a cloud model. You see a score, a band and a sentence about what to change this week. Identity and consent run through LifeCare.ID®, so every access is recorded where you can read it and revoke it.

ProForta is a wellness tool, not medical advice. Nothing here speaks to your medicine or your care. Your prescriber stays the decision-maker on both.

Corrected 8 October 2026: an earlier version of this page said most people on these medicines are far below the range the 2025 joint advisory sets. The study behind that line reports average intake of about 77 grams a day, just under the advisory's 80 to 120 gram target.