DEXA Body Composition Fact Sheet
DEXA is primarily a tool for measuring bone density but is also a way to test body composition. A person’s body composition describes the amount of fat versus non-fat in their body to give a percentage that indicates health. Being overweight or obese correlates with a shorter life expectancy.
Dual X-ray Absorptiometry (DEXA or DXA) is one way to determine people’s body composition. The DEXA method uses a three-compartment model that divides the body into fat, bone and other mass. Although a useful test, DEXA is only an estimate of body composition. It is superior to two compartment methods (such as fat mass or fat-free mass) that only estimate a person’s fat and lean mass without accounting for bone. Two compartment methods are subject to incorrect results because of bone density variations, particularly amongst those of different ethnicities.
How it is Done
Doctors use a special piece of medical equipment to do DEXA scanning. The machine has a high energy and a low energy ray. As someone lies on the table, a scanner passes over them and beams the rays into the body from an overhead detector. Measurements are taken in 0.5 cm intervals.
The principle behind the DEXA scan for body composition is that fat, bones and soft tissue absorb energy differently, allowing the machine to determine how much of each of the three types of tissue are in a person by accounting for the difference between how the two rays move through the body.
Who Does the Test?
General practitioners and medical specialists.
When and How Often?
Every five years is good on average for frequency of DEXA scans. Women should begin DEXA scans around the age of menopause. Men should get a DEXA scan if they have an increased risk of fracture.
Although DEXA provides a helpful estimate of body composition, it is not 100 percent accurate. Different DEXA machines may give results that vary from one another. Newer machines may use updated software that has algorithms that give different results from older machines with previous versions of the calculation software.
Some machines use a fan beam while others use a pencil beam. The type of X-ray can account for errors. An unavoidable potential source of error is the variation in fat-free mass hydration. People of different races and body types may have variations in hydration. Even a variation as small as 5 percent can lead to an almost 3 percent difference in body fat percentage.
DEXA scans are safe and quick, emitting a low level of radiation that is considerably less than the radiation received from a chest X-ray, in fact a DEXA scan is one of the lowest doses of radiation of any medical procedure. They take only around 10 to 20 minutes, but people must remain perfectly still during the scan, which can cause discomfort for some people.
- Fast Living Slow Ageing; Kate Marie and Christopher Thomas; 2009
- The Pitfalls of Body Fat “Measurement”, Part 6: Dual-Energy X-Ray Absorptiometry (DEXA): Weightology.net
- DEXA (Dual Energy X-ray Absorptiometry): QuickMedical.
Last reviewed 26/Feb/2014