What is a DEXA scan?

How it works

DEXA was originally developed to measure bone mineral density. The scanner sends two X-ray beams at different energies through the body. Bone, fat and lean soft tissue each absorb the two energies differently, and software turns those differences into a three-part estimate:

• bone mineral content
• fat mass
• lean soft-tissue mass

You get it for the whole body and by region (arms, legs, trunk). You lie still on a table for a few minutes, and the radiation dose is low.

Why it’s used as the reference

DEXA gives a regional breakdown, not just one number. It’s also less sensitive to hydration and recent meals than bioelectrical impedance. That’s why research on cheaper methods (smart scales, skinfold equations, photo-based apps) usually compares them against DEXA.

In one 2021 study, three consumer smart scales matched DEXA well for body weight: median errors of 0 to 0.3 kg. They underestimated fat mass by a median of 2.2 to 4.4 kg.

What it still gets wrong

• It’s an estimate. DEXA infers composition from X-ray attenuation using built-in assumptions. It doesn’t weigh fat.
• Machines disagree. Different manufacturers, models and software versions can produce different results for the same person.
• Hydration still matters, less than for impedance, but water sits in the lean-tissue estimate.
• Cost and access make frequent scans impractical, so it’s a periodic snapshot, not a weekly tracking tool.

How to use it well

Treat DEXA as an occasional anchor. Scan on the same machine, at the same time of day, in a similar hydration and fed state, and compare like with like. Between scans, a cheaper method you actually repeat the same way gives you the week-to-week trend. See how often to measure body composition and how the four common methods compare.