DEXA Body Composition Scan: What It Measures & How It Works

What does a DEXA body composition scan measure?

A DEXA body composition scan measures your total and regional fat mass, lean (muscle) mass and bone mineral content — and, on scanners with the right software, an estimate of visceral adipose tissue (the fat wrapped around your abdominal organs). It divides the body into a three-compartment model rather than the single bone-density number most people associate with the letters DXA.

DXA (dual-energy X-ray absorptiometry) evaluates body composition as three tissues: bone mineral content (BMC), fat mass and lean mass — the lipid-free soft tissue that includes body water, protein and soft-tissue minerals Messina et al., Quantitative Imaging in Medicine and Surgery, 2020. Because the scan maps the whole body, it also reports regional results — arms, legs, trunk, android (around the waist) and gynoid (around the hips) — so you see where fat and muscle sit, not just a single total.

The International Society for Clinical Densitometry (ISCD) sets out what a body composition report should contain. For adults, total-body reports should include BMI, bone mineral density, BMC, total mass, total lean mass, total fat mass and percent fat mass. Further measures — visceral adipose tissue, appendicular lean mass index, the android/gynoid fat ratio, and fat/lean mass indices — are listed as optional, and ISCD notes their clinical utility is still uncertain International Society for Clinical Densitometry, Official Adult Positions, 2023. If your interest is specifically the fat-percentage number, our companion guide on the DEXA body fat percentage breaks down how that figure is derived and read.

How does a DEXA body composition scan work?

It passes two X-ray beams of different energies (a higher and a lower peak) through the body and measures how much of each is absorbed by different tissues; because fat, lean tissue and bone attenuate the two energies differently, the software can separate them. You lie still on an open table for several minutes while a scanning arm passes over you — there is no tunnel and nothing to climb into.

The radiation involved is very small. A whole-body DXA scan on a modern densitometer exposes you to about 4–5 microsieverts — lower than the roughly 6.7 microsieverts of natural background radiation you receive in a single day Messina et al., Quantitative Imaging in Medicine and Surgery, 2020. It is still ionising radiation, though, so it is not dose-free: pregnancy is a stated contraindication to DXA body composition scanning International Society for Clinical Densitometry, Official Adult Positions, 2023. For a fuller look at DEXA and radiation, see is a DEXA scan safe?

Why is DEXA considered a reference method for body composition?

Because it is fast, precise and validated against gold-standard imaging, DXA has become the technique most researchers benchmark other methods against. There is a general consensus that DXA should be considered the reference technique for assessing body composition in clinical practice, and its whole-body fat and lean estimates show very good accuracy compared with CT and MRI Messina et al., Quantitative Imaging in Medicine and Surgery, 2020.

That reputation is strongest for total and regional fat and lean mass. For visceral fat specifically, DXA-derived VAT correlates highly with CT — reported around r=0.93 Messina et al., Quantitative Imaging in Medicine and Surgery, 2020. Even so, VAT is a derived estimate rather than a direct measurement, and ISCD lists it among the optional body-composition measures whose clinical utility is still uncertain International Society for Clinical Densitometry, Official Adult Positions, 2023. This is why DXA is often the yardstick used to check cheaper tools such as bioelectrical impedance scales and skinfold callipers.

Who uses DEXA body composition scans?

Athletes and sports scientists, fitness and body-transformation settings, and clinical or research teams all use DEXA body composition scans — most often to track how fat and muscle change over time. In sport, DXA is widely used to monitor body composition in athletic populations, but reliable results depend on strict standardisation — with subjects rested, overnight-fasted and in minimal clothing, and positioned consistently and centrally on the scanning bed Nana et al., International Journal of Sport Nutrition and Exercise Metabolism, 2015.

In clinical and research work, the lean-mass figures matter most: DXA total-body composition with regional analysis is used in people with muscle weakness or poor physical function to assess fat and lean mass, and appendicular lean mass indices feed into the assessment of low muscle mass and sarcopenia International Society for Clinical Densitometry, Official Adult Positions, 2023. If you are exploring muscle loss with age, our guide to sarcopenia and muscle loss after 50 sits alongside this.

How is a body composition DEXA different from a bone-density DEXA scan?

They use the same underlying X-ray technology, but a bone-density DEXA scans only the spine and hip to produce a T-score for osteoporosis, while a body composition DEXA scans the whole body to report fat, lean and bone across regions. A standard bone-density DEXA is the scan the NHS uses to diagnose or assess the risk of osteoporosis, measuring how dense your bones are and comparing the result against a young healthy adult NHS, DEXA scan, 2022.

In other words, the output is what differs: a bone-density DEXA answers "how strong are my bones?" via a T-score, whereas a body composition DEXA answers "how much fat and muscle do I have, and where?" For a plain-English tour of the bone-density version, see what a DEXA scan shows.

How does DEXA body composition differ from a REMS scan?

REMS (Radiofrequency Echographic Multi Spectrometry) is a radiation-free ultrasound scan of bone that estimates bone density and fracture risk — it does not measure body fat, lean mass or visceral fat at all. So REMS is not an alternative to a body composition DEXA; the two answer completely different questions.

Where they can be compared is bone density. Unlike DEXA's X-rays, REMS uses raw radio-frequency ultrasound signals and is described in a 2025 systematic review as non-ionising and radiation-free Diagnostics (Basel), 2025. That makes it well suited to repeat bone-health monitoring. It is worth being honest, though, that UK guidance still treats DEXA as the reference test for diagnosing osteoporosis, and the 2024 UK (NOGG) guideline does not recommend REMS or other ultrasound methods for diagnosis Gregson et al., Archives of Osteoporosis, 2025. Our full comparison lives at REMS vs DEXA.

If your goal is proactive bone health rather than body composition, a radiation-free REMS bone-health assessment checks your bone density and fracture risk without any X-ray dose — which is why it can be repeated more freely for monitoring. You can learn how REMS technology works or book a bone-health screening directly.

Is a DEXA body composition scan worth it?

For tracking fat and muscle over time — in athletes, weight-management, or research — a DEXA body composition scan is one of the most accurate accessible tools, provided it is done to a consistent protocol. Its main limits are that it involves a small radiation dose, is a contraindication in pregnancy, and that VAT and some derived indices carry more uncertainty than the headline fat and lean numbers International Society for Clinical Densitometry, Official Adult Positions, 2023.

Just remember the scan does not assess your fracture risk — a low body-fat reading tells you nothing about how strong your bones are. Bone health is a separate question, and for that a dedicated bone-density check, such as a radiation-free REMS screening, is the right test.