GLP-1 medications cause large, fast weight loss, and a meaningful share of it can be lean mass. A DEXA scan before starting and every 8–12 weeks shows exactly what is being lost; bioimpedance between scans catches a muscle-loss trend early enough to act on it.
What the trials found
In the STEP 1 trial of semaglutide (Wegovy), a DEXA substudy found that roughly 39% of the tissue participants lost was lean mass. In the SURMOUNT-1 trial of tirzepatide (Zepbound), about a quarter of the weight lost was lean mass. Some lean loss is expected with any large weight loss, but the amount varies a lot between people, and it depends on protein intake, resistance training and dose.
The scale cannot tell the difference. Body composition testing can.
A DEXA + BIA monitoring plan for GLP-1 patients
| When | Test | Why |
|---|---|---|
| Before or at start | DEXA scan + BIA reading | Accurate baseline for fat, lean mass, visceral fat and bone |
| Weekly | BIA (home scale or clinic) | Catch a lean-mass downtrend early |
| Every 8–12 weeks | DEXA scan | Confirm fat vs. lean loss and bone density |
| At maintenance or taper | DEXA scan | Set the long-term baseline |
What to do if lean mass is trending down
- Review protein intake with the prescriber or a dietitian.
- Add or increase supervised resistance training.
- Discuss the rate of loss and dosing with the prescriber.
- Confirm with a DEXA scan before making big changes.
This article is educational and not medical advice. Patients should make medication decisions with their healthcare provider.
For GLP-1 clinics and telehealth programs
Body composition data turns a weight-loss prescription into a muscle-preservation program. Clinics can offer an in-office DEXA+ clinical bioimpedance analyzer for quick check-ins at each visit, send patients home with the DEXA+ Body Fat Scale, and refer to a DEXA location for baseline and quarterly scans. Patients see their progress in the DEXASCAN app, which keeps them engaged between appointments.
Frequently asked questions
How much muscle do people lose on GLP-1 medications?
In the STEP 1 DEXA substudy, roughly 39% of the tissue lost on semaglutide was lean mass. In SURMOUNT-1, about a quarter of weight lost on tirzepatide was lean mass. Individual results vary widely.
How often should GLP-1 patients get a DEXA scan?
A baseline before or soon after starting, then every 8–12 weeks while the dose is changing, and again at maintenance.
Can bioimpedance detect muscle loss on a GLP-1?
BIA is not as precise as DEXA, but weekly readings on the same device can show whether lean mass is trending down so the care team can adjust protein, training or dosing sooner.
Sources
- Wilding JPH, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med. 2021;384:989-1002
- Look M, et al. Body composition changes during weight reduction with tirzepatide in SURMOUNT-1. Diabetes Obes Metab. 2025;27:2720-2729
- Balampanos D, et al. Agreement Between Standing Eight-Point Multifrequency BIA and DXA in Apparently Healthy Greek Adults. Healthcare. 2026;14(12):1807