By SmashMeals Editorial · Published · Updated
The scale tells you how much weight you lost. It does not tell you what you lost. On a GLP-1 medicine, that difference matters, because some of the weight that comes off is lean mass, including muscle. Here is what the trials show, how body composition is measured, and how to track it without getting lost in numbers.
This guide is general information, not medical advice. Ask your clinician which measurements make sense for you.
How much of the weight is lean mass?
Any weight loss, with or without medicine, includes some lean mass. The best data come from trials that scanned participants with DEXA, an X-ray body-composition test.
In the STEP 1 trial's DEXA substudy of semaglutide 2.4 mg (abstract), 95 participants on semaglutide lost 19.3% of their fat mass and 9.7% of their lean mass from baseline. Because fat fell faster, the share of their body that was lean actually rose by 3.0 percentage points. The authors found that greater weight loss was linked to a better lean-to-fat ratio.
That is reassuring, and it still means real lean tissue is lost. For older adults, people who are already less active, and people losing 20% or more of their weight, that matters more.
New data on support between visits
At the World Obesity and Weight Management Congress in October 2026, Omada Health presented a 245-person, 12-week study. GLP-1 users in its care track lost 6.0% of body weight and cut body fat percentage by 3.3%, versus 3.3% and 1.6% with usual care. It was company-run and not randomized, but it supports the idea that a plan around the medicine helps. Read our news coverage.
Ways to measure body composition
| Method | What it is | Good for | Watch out for |
|---|---|---|---|
| DEXA scan | Low-dose X-ray of fat, lean and bone | The most detailed common option; used in trials | Cost, access; hydration and timing affect results |
| Bioelectrical impedance (smart scales, handheld devices) | Estimates from a small electrical current | Cheap, convenient, good for trends | Swings with water, meals and exercise; less accurate per reading |
| Waist circumference | Tape measure at the navel | Free; tracks belly fat | Measure the same way each time |
| Strength tests | Grip strength, chair stands, how much you lift | Shows function, which is the point of muscle | Needs a consistent routine |
| Progress photos and fit of clothes | Same light, same spot, monthly | Motivation; catches changes the scale misses | Not a number |
For most people, a simple combination works: weight weekly, waist monthly, a strength check monthly, and a DEXA scan at the start and after several months if your clinician thinks it is worth it.
How to read the numbers
- Look at trends, not single readings. Smart-scale body-fat numbers can move several points in a day.
- Measure under the same conditions: same time of day, before eating, after using the bathroom, before exercise.
- Celebrate strength. If you can lift the same or more while the scale goes down, you are likely keeping muscle.
- Watch the rate. Very fast loss is linked to more side effects, including hair shedding. Our GLP-1 hair and nail guide explains.
What helps protect lean mass
The two tools with the most support are enough protein and resistance exercise. Spread protein across meals, especially when appetite is small, and do some form of strength training, even body-weight moves, a few times a week. Our guide to protein and strength training on a GLP-1 goes deeper, and if you are over 65, see GLP-1 after 65.
Ask your clinician or a registered dietitian for a protein target. It depends on your size, age, kidney health and activity.
Make the protein part automatic
The hardest part of a protein target is hitting it on a day you are not hungry. SmashMeals makes portioned, protein-forward meals in a dedicated gluten-free kitchen and posts the nutrition for each one, so you know what you are getting. See our GLP-1 meal planning page and high-protein meals, then order this week's meals.
Sources
Frequently asked questions
- How much muscle do you lose on a GLP-1?
- In the STEP 1 DEXA substudy, lean mass fell 9.7% and fat mass 19.3% from baseline on semaglutide 2.4 mg. Lean mass includes muscle, water and organs.
- Are smart scales accurate for body fat?
- They are useful for trends but can swing several points in a day with hydration and meals. Measure under the same conditions each time.
- What is the best way to measure body composition?
- DEXA is the most detailed common option and is used in trials. Waist measurements and strength tests are free and useful between scans.