What can a DEXA or body composition scan tell me?

DXA machines do two different jobs. Bone density scanning measures the spine and hip to diagnose osteoporosis. Whole body composition scanning divides your body into fat mass, lean soft tissue and bone mineral, and reports it by region. Both use the same low dose x-ray technology, but they answer different questions and are ordered separately. The composition scan is the one worth understanding if you are losing weight, since what you lose matters as much as how much.

What a body composition scan measures

A whole body DXA reports total fat mass, total lean mass, and the split between trunk and limbs, and most systems estimate visceral adipose tissue in the abdomen. That regional detail is the part a bathroom scale cannot give you. Someone who loses 10 kg can lose 8 kg of fat and 2 kg of lean tissue, or 5 kg of each, and the scale reports the same number in both cases. Appendicular lean mass, the muscle in the arms and legs, is the measurement used to assess low muscle mass.

Who the guidance says it is for

The International Society for Clinical Densitometry lists total body composition analysis as indicated in specific situations, including people with HIV assessing fat distribution on antiretroviral therapy, people with obesity undergoing bariatric surgery where weight loss above 10 percent is anticipated, and people with muscle weakness or poor physical function where fat and lean mass need to be assessed. It also states that pregnancy is a contraindication to DXA body composition.

How it compares with other methods

Bioelectrical impedance scales are inexpensive and repeatable but sensitive to hydration, recent food and recent exercise. Three-dimensional optical scanners capture shape and circumferences accurately and estimate composition from that shape. DXA measures tissue directly and is the practical reference standard in clinical settings. Whichever method you use, the rule is the same: repeat on the same device under the same conditions, because the change over time is the useful number, not the absolute value.

If you are on a weight loss medication or in a hard calorie deficit, a baseline scan and a repeat after three to six months answers a question worth asking: is the weight coming off fat, or is muscle going with it. Take the scan under standard conditions, fasted and hydrated the same way each time, on the same machine.