When you take a GLP-1 medication for weight loss, the number on a standard scale tells you how much weight you lost — not what kind. Clinical trials show that part of the weight lost on these medications is lean mass, which includes muscle. Tracking body fat, muscle mass, and visceral fat alongside weight gives you and your doctor a much clearer picture of whether you are losing mostly fat. A body composition Smart Scale such as the RENPHO MorphoScan Nova makes that picture easy to follow at home, week by week.
Why weight alone can be misleading on a GLP-1
GLP-1 medications are now a mainstream part of weight management in the US. A November 2025 KFF poll found that about one in eight US adults say they are currently taking a GLP-1 medication for weight loss, diabetes, or another condition, and 18% have taken one at some point.
The FDA approved semaglutide and tirzepatide for chronic weight management as additions to "a reduced calorie diet and increased physical activity". The diet-and-exercise pairing matters, because rapid weight loss of any kind tends to take some muscle with it.
How much of GLP-1 weight loss is muscle?
Trial data show that most GLP-1 weight loss is fat, but a meaningful share is lean mass — and the exact share varies from study to study.
In the STEP 1 trial of semaglutide 2.4 mg, a substudy of 140 participants used DXA — a clinical body scan — to measure what changed over 68 weeks: total fat mass fell 19.3%, visceral fat mass fell 27.4%, and lean mass fell 9.7%. Another study put the split at roughly 62% fat mass and 38% lean mass. The SURMOUNT-1 tirzepatide substudy of 160 participants found that about 75% of the weight lost over 72 weeks was fat mass and 25% was lean mass. A 2024 review found that across studies, lean mass ranged from about 15% or less up to 40–60% of total weight lost.
Researchers do not all agree on how much the lean-mass loss matters. The STEP 1 investigators noted that although lean mass fell in kilograms https://www.nejm.org/doi/full/10.1056/NEJMoa2032183, lean mass as a proportion of total body mass increased. A 2024 JAMA viewpoint argued that the concern that GLP-1-driven weight loss causes frailty "is not supported by data." Most experts agree that protecting muscle during weight loss is worth the effort — and you can only protect what you measure.
What to track besides weight
A RENPHO body composition scale reports over 50 metrics. On a GLP-1 medication, five of them tell you the most.
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Body fat percentage. It shows whether the weight you are losing is mostly fat. A falling body fat percentage alongside falling weight is the pattern you want.
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Muscle mass and skeletal muscle percentage. Muscle mass is the early-warning metric. Expect a steady or slowly declining muscle mass, and raise a sharp drop with your doctor.
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Segmental muscle. An 8-electrode scale, such as the RENPHO MorphoScan Nova, breaks muscle down by arm, leg, and torso. Segmental readings show whether strength work is protecting your legs and upper body evenly.
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Visceral fat grade. Visceral fat is the fat stored around your organs, and it is linked to cardiovascular and metabolic risk. In the STEP 1 trial, visceral fat fell by a larger share than total fat. Read our guide to what your visceral fat number means.
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Protein and body water. Together, they reflect how well you are eating and hydrating while your appetite is lower. Body water also affects the accuracy of every other reading.
How to protect muscle while on a GLP-1
A 2025 joint advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society sets out practical targets for people taking GLP-1 medications:
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Protein: 1.2–1.6 g per kilogram of body weight per day during active weight loss, or an absolute target of 80–120 g per day.
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Strength training: regular resistance training at least three times a week.
Your doctor or dietitian can tailor those numbers to you. Your RENPHO scale then shows whether the plan is working — muscle holding steady while body fat falls is the sign you are on track.
How to get consistent readings at home
Bioelectrical impedance scales estimate body composition from how a small current moves through your body, so hydration and timing change the result. Read our guide on how to reduce fluctuations in your weight readings, including weighing at the same time each day and avoiding readings right after exercise, caffeine, or alcohol. Reduced appetite on a GLP-1 can also mean you drink less, so keep hydration steady from day to day.
A simple routine that works:
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Weigh in first thing in the morning, after using the bathroom and before eating or drinking.
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Use the same scale, on the same hard floor, in bare feet.
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Hold the handle the same way each time on an 8-electrode scale.
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Compare weekly averages rather than day-to-day swings.
Why an 8-electrode scale suits GLP-1 tracking
A standard 4-electrode Smart Scale only passes current through your legs and estimates your upper body. When the goal is to spot muscle loss early, that blind spot matters. The RENPHO MorphoScan Nova adds four electrodes in a handle and reports fat and muscle for each arm, each leg, and your torso. A 4.3-inch color display shows the results. A 2008 study in Obesity Facts found that hand-and-foot electrode arrangements agreed more closely with MRI and DXA than foot-to-foot devices. Read MorphoScan Nova vs. a DEXA scan for how close a home scale gets to a clinical scan.
The bottom line
A GLP-1 medication can change how much you weigh quickly, but the scale number alone cannot tell you what you are losing. Tracking body fat, muscle mass, and visceral fat alongside weight shows whether the weight coming off is mostly fat — and gives you and your doctor real data to adjust protein and training. A RENPHO MorphoScan Nova puts that data on your bathroom floor, so every weigh-in answers the question that matters most on a GLP-1: am I keeping my muscle?
This article is for general information and is not medical advice. GLP-1 medications are prescription drugs — talk to your doctor before starting, stopping, or changing any medication, diet, or exercise plan. RENPHO body composition scales are not medical devices and should not be used for diagnosis.
Frequently asked questions
Do GLP-1 medications cause muscle loss?
Some of the weight lost on GLP-1 medications is lean mass, which includes muscle. In the STEP 1 semaglutide trial, roughly 38% of the weight lost was lean mass, and in the SURMOUNT-1 tirzepatide trial it was about 25%.
How can I prevent muscle loss on a GLP-1?
A 2025 joint medical-society advisory recommends 1.2–1.6 g of protein per kilogram of body weight per day during active weight loss and resistance training at least three times a week. Ask your doctor to tailor those targets to you.
Can a smart scale tell if I am losing muscle on a GLP-1?
A body composition Smart Scale estimates muscle mass using bioelectrical impedance so that it can show a trend in muscle over time. An 8-electrode scale such as the RENPHO MorphoScan Nova adds arm, leg, and torso readings, which makes changes easier to spot.
How often should I weigh myself on a GLP-1?
Weighing daily at the same time and comparing weekly averages smooths out day-to-day swings in water. Weekly trends in body fat and muscle mass are more useful than any single reading.
Is a body composition scale safe for everyone?
People with a pacemaker or another implanted electronic device should not use a bioelectrical impedance scale. Readings may also be less accurate during pregnancy.
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MorphoScan Nova Body Composition Scale
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