DXA is among the more accurate ways to estimate changes in fat mass, lean mass, and bone density. DXA measurements of cadavers, stillborn infants, and animal carcasses are within 1% of scale measurements of isolated parts of the dead body, such as the bones. However, DXA scans can underestimate fat mass by up to 5 kg (11 lb), depending on hydration,^[1] and they’re subject to technical and interpretation errors.^[2][3] Getting a DXA scan involves laying down on a bed while a robotic arm moves up and down the length of the body, emitting very low-level X-rays at two different frequencies. The DXA machine measures how many photons of each frequency are absorbed. This is a quick process, usually taking 3–10 minutes.

The original application of DXA scanning was to measure bone mineral density in order to help detect or track the development of osteoporosis. DXA was approved for this use by the FDA in 1988, and it remains the gold standard for osteoporosis diagnosis and prediction of fracture risk.^[4] However, the use of DXA has broadened as the technology has matured, and modern machines can now capture fat mass and lean muscle mass, as well as bone mineral density, in a single scan.^[5]

DXA is not the only way to measure body composition. More accurate methods include computerized tomography (CT) scans and magnetic resonance imaging (MRI). Less accurate methods of measuring body composition include water displacement, air displacement, and bioelectrical impedance analysis (BIA).^[6] An autopsy is also a very accurate method for measuring body composition because different components of the body are cut out and weighed during the process. However, this is not a practical method for living people for obvious reasons.