You got the baseline scan, you did the work, and now the second report is in your hands — two columns of numbers that are supposed to tell you whether the last few months went the way you hoped. The trick is knowing which of those numbers actually moved, and which just wobbled.
If you haven't had your first scan yet, start with our companion guide on getting a DXA scan and what it measures. This one picks up afterward: how to read a report against your last one and tell real change from measurement noise.
The numbers that matter, and what "better" looks like
A DXA report hands you more figures than you need. The ones worth watching over time are:
- Total mass — your weight, the same number the scale gives you, and the least informative one here.
- Fat mass — total fat in kilograms or pounds, plus a breakdown by region (arms, legs, trunk). This is the number you most want to see fall.
- Lean mass (sometimes labeled lean soft tissue or fat-free mass) — muscle and everything else that isn't fat or bone. The number you most want to protect.
- Body fat percentage — fat mass divided by total mass. Useful, but it can move just because one of its two inputs moved, so read it alongside the raw kilograms, not instead of them.
- Visceral fat (VAT) — the metabolically active fat around your organs. Often the fastest-improving number early in GLP-1 treatment, but also the least precise one on the report (more on that below).
- Bone mineral density — carried over from DXA's original job as an osteoporosis screen. It changes slowly, so don't expect meaningful movement between scans a few months apart.
On a GLP-1, a good trajectory is straightforward to describe and harder to achieve: fat mass down, lean mass holding as steady as you can keep it. A drop in total weight that is almost entirely fat, with lean mass barely changed, is the outcome you're training and eating for. Falling lean mass alongside the fat is expected to some degree — but it's the signal to double down on protein and resistance work, not to celebrate the smaller number on the scale.
The precision trap: small changes may be noise
Here is the caveat that saves people from over-reacting to a single report. DXA is precise, but it isn't perfect. According to PubMed, a 2026 study measuring people twice in a row put DXA's precision error (coefficient of variation) at roughly 1% for whole-body fat mass and under 1% for fat-free mass — tight, but not zero (DOI (external link)). In practical terms, an earlier study found the repeat-scan error for whole-body fat mass was around 0.5 kg and for lean mass around 0.6–0.7 kg, and it used the International Society for Clinical Densitometry method to calculate a "least significant change" — the smallest difference between two scans that is real rather than machine variability (DOI (external link)).
The upshot: if your lean mass reads half a kilogram lower than last time, that may be noise, not muscle. A change only becomes trustworthy once it clears that least-significant-change threshold — which is why the trend across three or four scans tells you far more than any single scan-to-scan difference. Regional numbers are noisier still, and visceral fat is the noisiest of all: one precision study measured a VAT error near 16%, versus about 1.4% for whole-body fat (DOI (external link)). Treat a big VAT swing as a rough direction, not a decimal-point fact.
Scan the same way every time
Because you're chasing changes that can be smaller than the machine's own error, consistency is everything. Two rules do most of the work:
- Same machine, ideally the same clinic. Different DXA models don't read identically — one study found systematic differences between two densitometers large enough to require conversion equations before their numbers could be compared (DOI (external link)). A "loss" that's really just a change of scanner is a waste of a checkpoint.
- Same hydration and fasting state. Lean-tissue readings partly reflect body water, so a scan after a big meal, a hard workout, or heavy fluids will differ from a fasted, rested one. Researchers have shown that standardizing food, fluid, and activity in the 24 hours before a scan can shrink the measurement error further (DOI (external link)). Book your scans at a similar time of day, fasted, before training, and hold that routine each time.
How much lean loss is normal — and how to shift the ratio
Some lean-mass loss is expected on a GLP-1. According to PubMed, a 2024 review in Diabetes Care reports that lean tissue can make up roughly 25–40% of total weight lost on incretin therapy — a larger share than with slower, diet-only weight loss — amounting to something like 6 kg of lean mass in trials (DOI (external link)). So if your report shows a chunk of the loss coming from lean tissue, you're in the normal range, not failing.
What you can change is how big that share is. The same review found that supervised resistance training lasting more than 10 weeks produced meaningful gains in lean mass and strength even while people lost weight — and it works best paired with adequate protein. Our guides on why strength training matters and protein, fat, and carbohydrates cover the how; your serial DXA reports are how you confirm it's actually working. A trajectory that starts at the high end of that lean-loss range and flattens out as you add training and protein is exactly the story these scans exist to tell.
This is general education and peer information, not medical advice — interpreting your specific results belongs with your prescriber or a clinician who can see the full report. Research findings here are drawn from PubMed; see the linked studies for details.