DEXA scan

A DEXA (or DXA) scan passes two low-energy X-ray beams through the body. Fat, lean tissue and bone absorb the two energies differently, so the scanner can work out how much of each is in every part of the body. A whole-body scan takes a few minutes lying on a table. The effective radiation dose of a whole-body scan is under 10 microsieverts, which a 2017 review in Bone compared with the natural background radiation received in about one day at sea level.

Two uses

Bone density. The original clinical use. A scan of the hip and spine gives a T-score, which compares bone density with a young adult average. A World Health Organization report defined osteoporosis as a T-score of -2.5 or lower.

Body composition. A whole-body scan reports fat mass, lean mass and bone mineral, by region. It can also estimate visceral fat, the fat around the organs.

Lean mass is not muscle

DEXA lean mass is everything that is not fat or bone: skeletal muscle, but also organs, skin, water and the glycogen stored with water in muscle and liver. When someone starts eating less, glycogen and water fall quickly, and lean mass drops on the scan before much muscle has gone. A lean mass loss on DEXA therefore overstates muscle loss.

Hydration, a recent meal or a hard training session the day before can shift lean mass by enough to blur a real change. Repeat scans are most comparable on the same machine, at the same time of day, under the same conditions.

What GLP-1 DEXA studies found

In both, fat fell proportionally more than lean tissue, so the share of the body that was lean went up. Worked out from the STEP 1 averages, lean tissue was close to 40 percent of the fat plus lean lost; in SURMOUNT-1 about a quarter of the weight lost was lean mass. Neither substudy measured strength. The lean mass studies page has the detail, and preventing muscle loss covers protein and resistance training.

Reading your own report

Look at kilograms, not just percentages. Body fat percentage can fall purely because fat mass fell, with lean mass unchanged or lower. The fat mass, lean mass and bone mineral figures in kilograms, compared scan to scan, show what actually changed.

DEXA and other methods

Bioimpedance scales, tape measures and camera-based estimates are cheaper and easier to repeat. They are less precise than DEXA for any single reading, but a consistent method used often can show a direction of travel. DEXA is the reference most research uses.

If you have DEXA results, they belong on the same timeline as weight, protein intake and training. The body scanner tracks composition with the phone camera between scans, and the training log and nutrition tracking show whether protein and lifting kept pace with the dose.

Related terms: insulin resistance, surrogate endpoint, titration, pharmacodynamics. Back to the glossary.