Posted by Mark Nessim
Reviewed by Dr. Mark, Physician & Formulator
There is a question people ask constantly on a GLP-1: am I losing muscle, or just weight? It is a good question. And there is now a machine in nearly every gym, clinic and strip-mall storefront promising to answer it.
Most of them are answering a different question and presenting it as that one. This is a field guide to what each machine actually measures, and where each one goes wrong.
The smart scale at home
Sends a small current up through the feet and estimates from the resistance. The weight is excellent. The body composition is not: a cross-sectional study of three consumer smart scales against DXA found weight accurate to within about 0.3 kg, while body fat was underestimated by roughly 2 to 4 kg depending on the model, and muscle mass was over-estimated by some models and under-estimated by others. The authors concluded the scales were useful for monitoring a trend but not accurate enough to replace a clinical measurement. (Frija-Masson et al., JMIR mHealth and uHealth, 2021.)
The InBody at the gym or clinic
The eight-electrode column you stand on holding two handles. It does not measure muscle. It measures resistance to a current, predicts total body water, then assumes water makes up a fixed proportion of lean tissue and derives the rest.
A 2024 study compared one against DXA across repeated visits and reported a bias of roughly 4 percentage points low on body fat and about 2.8 kg high on fat-free mass — while showing very high repeatability from visit to visit. (Looney et al., Frontiers in Nutrition, 2024 — a small study, 14 adults across five laboratory visits.)
That repeatability is the catch, and it is worth understanding. A device that returns the same offset answer every time feels more trustworthy than one that fluctuates. Consistency is not accuracy. These devices are reasonable for tracking direction on a single machine over time; the absolute number on the printout is not the thing to take home.
DEXA — increasingly a van or a storefront
Two X-ray energies split the body into bone, fat, and a third compartment called lean soft tissue. That compartment is not muscle. It is organs, blood, connective tissue, skin and water, with skeletal muscle as one component of it.
Which is why the number can move without any change in muscle. In one study, twelve active men were dehydrated by approximately 2.5% of body mass and then carbohydrate-loaded for 48 hours. Total lean tissue mass measured 1.69 kg lower after dehydration and 2.36 kg higher after supercompensation — roughly a 4 kg swing over a few days, with no change in measured fat mass or bone mineral content. (Toomey et al., European Journal of Applied Physiology, 2017.)
The 3D body scanner
Photographs your silhouette and estimates from shape. To be precise about it: this method does not measure muscle, lean mass, or body water at all. It measures your outline.
MRI and CT — the actual reference standards
These image muscle directly, and they are what the other methods are validated against. Nobody is being scanned quarterly at this price, and no clinic offers it as a wellness add-on — but it is where most of what we reliably know about body composition originates.
The method that measures muscle directly
D3-creatine dilution traces a labelled tracer into muscle and estimates skeletal muscle mass directly, rather than inferring it from water or tissue density. It is an area of active research interest and continues to be compared against other body-composition techniques. (Pagano et al., narrative review, The Journals of Gerontology: Series A, 2024.) It is a research method, not something available at a clinic.
The part nobody puts on the printout
After all that, the expected move is a recommendation. Instead, here is the more useful finding: muscle mass is a weaker signal than most people assume.
The Health, Aging and Body Composition study followed 2,292 adults aged 70 to 79. Its conclusion is in the title of the paper: strength, but not muscle mass, was associated with mortality. Low muscle mass did not explain the relationship between strength and mortality — strength did, as a marker of muscle quality rather than quantity. (Newman et al., The Journals of Gerontology: Series A, 2006.) That is an older cohort than most readers, and the direction is still worth knowing: the quantity on the scan is not the outcome anyone actually cares about.
This is reflected in how clinicians are told to assess it. The ESPEN/EASO consensus criteria for sarcopenic obesity assess muscle function using handgrip strength or a five-times sit-to-stand test, alongside — not instead of — measures of fat and muscle mass.
Which means two of the most informative tests available cost nothing and take a minute. Can you rise from a chair five times without pushing off with your hands? Is your grip changing — jars, shopping bags, a suitcase into an overhead bin? Those are not consolation prizes for people who cannot afford a scan. They are closer to the thing that matters.
What nutrition can and cannot do here
To be direct about our own interest: we make a protein powder, and it will not change what any of these machines say about you. What adequate protein does is address one input. Published guidance for adults during active weight loss puts protein intake around 1.2 to 1.5 grams per kilogram of body weight per day, and appetite on a GLP-1 does not cooperate with that. A daily shake such as ASP-1 is one reliable way to cover part of it — and our companion piece on how much protein you actually need on a GLP-1 works through the targets in detail.
Protein is one input, not the whole answer. A 2025 joint advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association and The Obesity Society states that protein intake alone is not adequate without structured resistance training. No supplement substitutes for resistance exercise.
“My appetite is almost nonexistent these days, so getting enough protein has been difficult. This has become the easiest part of my nutrition plan.”
— Garrett L. · Verified buyer
Be skeptical of the printout. Pay attention to what you can actually do. And treat the nutrition side as an input you control, rather than a number you chase.
This article is for general information and is not medical advice. Discuss body composition testing and any changes to your nutrition or exercise with your own clinician.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
