Dual-Energy X-ray Absorptiometry, or DEXA, is a sophisticated imaging technology used for analyzing the body’s composition. Unlike simple metrics such as Body Mass Index (BMI), which only uses height and weight, the DEXA scan provides a direct measurement of the body’s physical makeup. This level of detail allows for an accurate assessment of health, fitness progress, and metabolic risk that traditional methods like bioelectrical impedance cannot reliably achieve. Initially developed to measure bone density, the technology now serves as a high-precision tool for obtaining data about internal physical makeup.
The Core Mechanism of Dual Energy X-rays
The foundational principle of the DEXA scan relies on the physical property of X-ray attenuation, which is the weakening of an X-ray beam as it passes through matter. The DEXA machine emits two distinct X-ray beams, a high-energy beam and a low-energy beam, which are directed through the body. Different types of tissue absorb or “attenuate” these two energies in unique and predictable ways based on their chemical composition and density.
For instance, highly dense bone mineral content absorbs the beams differently than soft tissue, which is composed of fat and lean mass. The detectors underneath the scanning table measure the amount of energy that successfully passes through the body. A specialized computer algorithm then uses the ratio of absorption between the two energy levels to calculate the density and mass of the underlying tissue.
By comparing the absorption rates from the high and low energy beams, the system mathematically separates the soft tissue into two distinct components: fat mass and lean soft tissue. This dual-energy approach allows the technology to accurately distinguish between the three main compartments of the body: bone mineral content, fat mass, and lean mass. This precision makes the DEXA scan a repeatable and dependable assessment tool.
What the Scan Measures and Reports
The DEXA scan provides a report based on a three-compartment model, detailing the absolute mass of bone mineral, fat tissue, and lean soft tissue in kilograms. The most commonly cited metric is the total body fat percentage, which is the total mass of fat divided by the total body weight. This percentage offers a more meaningful measure of health risk than simply looking at overall body weight.
Beyond total body composition, the technology performs a regional analysis by dividing the body into specific sections, such as the trunk, arms, and legs. This regional breakdown is particularly useful for identifying the distribution of fat, including the android (trunk) and gynoid (hip/thigh) regions. The android-to-gynoid fat ratio can be important, as fat stored around the abdomen is associated with greater metabolic disease risk.
A highly specific and clinically significant output is the quantification of visceral adipose tissue, or VAT, which is the fat stored deep within the abdominal cavity, surrounding the internal organs. This visceral fat is metabolically active and strongly correlated with conditions like type 2 diabetes and cardiovascular disease. The ability to track VAT changes over time provides quantifiable and actionable data for monitoring health interventions.
Preparing for and Undergoing the Scan
To ensure accuracy and repeatability between scans, some preparation is recommended before a DEXA scan for body composition. Providers suggest avoiding strenuous exercise for at least 12 hours prior to the appointment, as intense activity can temporarily alter muscle hydration and glycogen stores, which may affect lean mass readings. Fasting for a few hours is also a common request to minimize the presence of undigested food in the gastrointestinal tract that could influence measurements.
The procedure itself is quick, non-invasive, and painless, typically lasting between seven and twelve minutes. The patient lies flat on their back on a padded table while a mechanical arm slowly passes over the body from head to toe. Before the scan begins, all metal objects, including jewelry, zippers, and underwire bras, must be removed as metal can interfere with the X-ray beams.
Safety is a common question, but the radiation exposure from a full-body DEXA scan is low, often measured at just 4 to 5 microsieverts (μSv). This minimal dose is comparable to the amount of natural background radiation a person receives during a single day or less than the exposure from a short commercial airline flight. The benefit of obtaining precise body composition data is considered to outweigh the negligible risk.