How to Read a DEXA Scan as a GLP-1 User

To read a DEXA scan as a GLP-1 user, find two numbers first: total fat mass and total lean mass. On a follow-up scan, the question that matters is how each one changed: you want most of your weight loss to have come from fat, with lean mass roughly held. Keep in mind that "lean mass" includes water, organs, and connective tissue, not just muscle, so small changes are normal and a single scan is only a baseline.

This guide shows you exactly which figures to read on the printout, what a "good" versus "concerning" change looks like, and how to avoid the common ways people misread their own results.

What a DEXA scan actually measures

A DEXA scan (dual-energy X-ray absorptiometry) passes two low-dose X-ray beams through your body and separates everything into three compartments: fat mass, lean mass, and bone mineral content. It is the gold-standard body-composition method, with body-fat error research puts around 1 to 2 percentage points, which is why it is used in the major GLP-1 trials.

The printout can look intimidating, with a body image, color overlays, and a wall of numbers. You do not need most of it. For tracking what a GLP-1 like Ozempic or Wegovy (semaglutide) or Zepbound or Mounjaro (tirzepatide) is doing to your body, a handful of figures carry almost all the signal.

It helps to know why DEXA is the method the trials chose. Because it separates fat, lean, and bone directly rather than estimating from electrical impedance, it is not thrown off by hydration and meals the way a smart scale or InBody can be. That stability is what makes it trustworthy enough to compare two scans months apart and believe the difference is real change, not measurement drift. The trade-off is that you have to go to a facility and pay per scan, so you use it sparingly and read it carefully.

The numbers that actually matter

Read these, roughly in order:

1. Total fat mass. The absolute weight of fat in your body, usually in pounds or kilograms. This is the number you want to see falling on a GLP-1.
2. Total lean mass (sometimes "lean tissue mass" or "fat-free mass"). Everything that is not fat or bone. This is your proxy for muscle, with the important caveat below.
3. Body-fat percentage. Fat mass divided by total mass. Useful, but on its own it can move just because fat or lean changed, so read it alongside the absolute numbers.
4. Appendicular lean mass (arms plus legs). This is the figure closest to actual skeletal muscle and is used in sarcopenia assessment, so it is the best single number for tracking muscle specifically.
5. Regional breakdown. Trunk, arms, and legs separately. Helpful for spotting whether loss is even or concentrated.

The single most important move is comparing two scans. One scan tells you where you stand; two scans 8 to 12 weeks apart tell you whether your weight loss is mostly fat (good) or carrying too much lean mass with it.

"Lean mass" is not the same as "muscle"

This is the most common misread, and it matters enough to repeat. Lean mass on a DEXA includes skeletal muscle, but also body water, organs, and connective tissue. So your lean-mass number is always larger than your actual muscle, and it can swing a pound or two between scans purely from hydration, a salty meal, or where you are in a training or menstrual cycle.

The practical implication: do not panic over a small lean-mass dip. Some of it may be water, not muscle. This is also why the widely cited figure that "up to about 40 percent of GLP-1 weight loss is lean mass" does not mean 40 percent muscle. Research suggests roughly 25 to 40 percent of weight lost on these drugs is lean mass (tirzepatide nearer 25 percent in the SURMOUNT-1 DEXA substudy, some semaglutide trials near 40 percent in SUSTAIN 8 and STEP 1), and a real but smaller share of that is skeletal muscle. The complete guide to GLP-1 muscle loss unpacks that range in full.

A sample before-and-after readout

Here is an illustrative example of what two scans 12 weeks apart might show for someone on a GLP-1, and how to interpret the change. These numbers are made up to demonstrate the reading method, not a target or a prediction; your results will differ.

Metric	Baseline scan	12-week scan	Change	Reading
Total weight	200 lb	188 lb	−12 lb	Scale alone can't tell you the split
Fat mass	70 lb	60 lb	−10 lb	Most of the loss is fat: good
Lean mass	124 lb	122 lb	−2 lb	Roughly held; some may be water
Body-fat %	35.0%	31.9%	−3.1 pt	Getting leaner overall: good
Appendicular lean mass	52 lb	51 lb	−1 lb	Limb muscle largely preserved: good

In this "good" example, 10 of the 12 pounds lost came from fat, and the small lean-mass dip could be partly water. Now compare a "concerning" pattern for the same starting point:

Metric	Baseline scan	12-week scan	Change	Reading
Total weight	200 lb	188 lb	−12 lb	Same 12 lb down
Fat mass	70 lb	64 lb	−6 lb	Only half the loss is fat
Lean mass	124 lb	118 lb	−6 lb	Large drop relative to fat: flag this
Body-fat %	35.0%	34.0%	−1.0 pt	Barely leaner despite 12 lb lost
Appendicular lean mass	52 lb	48 lb	−4 lb	Notable limb muscle loss: flag this

Same 12 pounds on the scale, completely different story underneath. The second person lost half their weight as lean mass and barely moved their body-fat percentage, which is exactly the pattern protein and resistance training are meant to prevent. This is the difference the scale can never show you, and why fat loss vs muscle loss is the real question on a GLP-1.

What "good" versus "concerning" actually looks like

You will not find one universal cutoff, and individual results vary, but the directional patterns are clear:

• Good: most of your weight loss is fat, lean mass is roughly held or down only slightly, body-fat percentage is clearly dropping, and appendicular lean mass is largely preserved.
• Worth watching: lean mass falling at a similar pace to fat, or body-fat percentage barely moving despite real weight loss.
• Concerning: lean mass dropping faster than fat, or a large appendicular lean-mass loss. This is the cue to revisit protein intake (a common evidence-based target during weight loss is about 1.2 to 1.6 g/kg/day, per the 2025 OMA/TOS joint advisory) and resistance training, and to raise it with your provider.

Encouragingly, this is modifiable. In the real-world SEMALEAN cohort, lean mass dropped about 3 kg early then stabilized, the share of people with sarcopenic obesity fell from 49 percent to 33 percent over a year, and handgrip strength actually improved. Losing weight does not have to mean losing function.

How to use scans over time

A DEXA is only as useful as your ability to compare it. A few rules:

1. Standardize conditions. Same facility, ideally same machine, similar hydration and time of day. Different machines and prep can shift results enough to muddy a real trend.
2. Space scans far enough apart. Every 8 to 12 weeks during active weight loss gives change time to exceed normal variation. Scanning every two weeks mostly measures noise.
3. Read the change, not the dot. Your baseline number means little until you have a second scan to compare it to.
4. Keep the PDFs together. Two scans in two folders are hard to compare; a trend is easy.

That last point is where a tracking app helps. Log each DEXA into Myo and it charts your lean-mass change scan over scan against your protein intake and training, so two separate PDFs become one trendline that tells the real story. Myo does not perform the scan or change the numbers; it organizes the data you bring in so the comparison is one tap instead of a spreadsheet. If you are choosing where to keep this data, the body composition tracking guide compares the options.

The bottom line on reading your scan

Find fat mass and lean mass, then judge the change between two scans, not the absolute numbers on one. You want fat to be most of what you lost and lean mass to hold. Remember that lean mass is not pure muscle, so small dips are normal, and that appendicular lean mass is your best window into actual muscle. Read it that way and a DEXA becomes the clearest proof you have that your GLP-1 weight loss is the kind worth keeping.

References

• DEXA accuracy and body-composition method comparisons: ScienceDirect S2667268526000409; PMC8122302; Highland Longevity DEXA vs InBody.
• Lean-mass loss range on GLP-1s: Tirzepatide SURMOUNT-1 DEXA substudy, Diabetes, Obesity and Metabolism, 2025 (doi:10.1111/dom.16275); Semaglutide SUSTAIN 8, 2020 (PMC6997246); STEP 1 DXA analysis, Journal of the Endocrine Society, 2021.
• Protein target during weight loss: OMA/TOS/ACLM/ASN 2025 joint advisory (PMC12264624).
• SEMALEAN real-world cohort, 2025 (PMC12673431), sarcopenic obesity and handgrip findings.