Most of the weight lost is fat.
A real share is lean mass.
GLP-1 medicines bring real cardiometabolic changes, and the scale counts every pound the same way. DXA scans in two large trials separated the two.
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STEP 1, semaglutide 2.4 mg (Wegovy)
In the DXA substudy, participants lost an average of about 15% of body weight. Fat mass fell by 19.3% and lean mass by 9.7%, so roughly 39% of the weight lost was lean tissue.1
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SURMOUNT-1, tirzepatide (Zepbound)
Weight loss reached 20.9% at the 15 mg dose. In the DXA substudy, fat mass fell by 33.9% and lean mass by 10.9%, so about a quarter of the weight lost was lean tissue.2 The trial did not include structured resistance training.
Muscle does work
long after the weight comes off.
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Resting energy use
Skeletal muscle is a major part of the energy your body uses at rest. Losing lean mass lowers daily energy needs, which may make weight harder to maintain if a medication is adjusted or stopped.
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Blood sugar
Muscle is the main place the body takes up glucose in response to insulin.
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Strength and mobility
Losing lean tissue faster than usual can add to age-related loss of strength and mobility over the long term.
Muscle is measured by scan and by strength,
not by blood.
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DXA scan
Dual-energy X-ray absorptiometry measures lean mass, fat mass and bone density, by body region. It is the standard way to see what share of weight lost was lean tissue.
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Strength measures
Grip strength and the loads you lift over time show how muscle is working, which a scan can’t.
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Blood tests
A blood sample holds immune cells, not muscle. Creatinine and BUN reflect kidney function and protein metabolism, not lean mass. The Inflammation Compass reads gene activity in immune cells, and it doesn’t measure muscle mass or breakdown either.
A hard workout raises inflammation on purpose.
The goal was never zero.
Strength training damages muscle a little, and your inflammation response is part of how it repairs and grows. Immune cells arrive, clear what is damaged, and then a second set of signals ends the response so repair can finish.
The concern in the research is a response that doesn’t end. Persistent inflammation is associated with slower muscle protein building, a pattern called anabolic resistance, seen especially with aging and long-term illness. The aim is a response that fits the situation and ends when its work is done, not simply low inflammation.
The Inflammation Compass reads both sets of signals as six readings: three for the signals that start a response and three for the signals that end it. With samples before and during a change in your training or therapy, you can see which readings moved and by how much, beside your DXA results and training log.
What the 2025 joint advisory recommends
for lean mass on GLP-1 therapy.
Four obesity and nutrition societies published joint guidance in 2025.3 Your clinician can tell you what fits you.
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Resistance training
Progressive resistance training with weights, machines or bands at least three times a week, alongside about 150 minutes of moderate aerobic activity.
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Protein
GLP-1 medicines reduce appetite, so protein intake can fall. The advisory cites about 1.2–2.0 grams of protein per kilogram of adjusted body weight a day during major weight loss, paired with strength training, and notes that protein alone is unlikely to preserve muscle.
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Body composition
A DXA scan at the start and repeated at intervals your clinician suggests shows what share of the weight you lose is lean tissue.
What it
can't tell you.
The Inflammation Compass doesn’t measure muscle mass, muscle breakdown or strength, and it can’t show whether your training, protein intake or medication is working. It shows what changed in your readings and by how much, not why.
It doesn’t diagnose, rule out or predict any disease. What a change means is for you and your clinician to weigh.
Questions about GLP-1 therapy and muscle
How much muscle do people lose on GLP-1 medications?
In the trial DXA substudies, about 39% of weight lost on semaglutide (STEP 1) and about 25% on tirzepatide (SURMOUNT-1) was lean mass rather than fat. Lean mass includes muscle as well as organs, water and other non-fat tissue. The share is thought to depend on resistance exercise, protein intake, the size of the calorie deficit and starting body composition.
Can a blood test measure muscle loss?
No blood test reliably measures muscle mass or muscle breakdown. Markers such as creatinine and BUN reflect kidney function and protein metabolism, not lean mass. Clinicians use DXA scans and strength measures to track muscle.
Does the Inflammation Compass measure muscle loss?
No. It reads which genes your immune cells are using and reports six readings across the signals that start a response and the signals that end it, plus an estimate of your immune cell mix. It doesn’t measure muscle mass, muscle breakdown or strength.
What does the research recommend for protecting muscle on GLP-1 therapy?
A 2025 joint advisory from four obesity and nutrition societies cites progressive resistance training at least three times a week, about 150 minutes of moderate aerobic activity, and about 1.2–2.0 grams of protein per kilogram of adjusted body weight a day during major weight loss. It notes that protein alone is unlikely to preserve muscle without strength training. Your clinician can tell you what fits you.
Read more
about GLP-1 therapy.
- GLP-1 biomarkers
What hs-CRP, blood counts and other routine markers measure, and what the six readings add.
- What RNA adds
How reading which genes your immune cells are using differs from a routine blood test.
- GLP-1 benefits and trade-offs
The benefits and risks of GLP-1 therapy, from the clinical trials.
Read your inflammation response beside your scans and training log.
Your first sample starts your personal baseline. Each later sample shows which readings moved and by how much.