Dual-Energy X-ray Absorptiometry (DEXA) is widely recognized as the gold standard for measuring body composition. Unlike estimation methods such as Body Mass Index (BMI), the DEXA scan uses a low-dose X-ray to directly measure and differentiate three components: bone mineral content, fat mass, and lean soft tissue mass. This precise, three-compartment model provides a detailed map of how weight is distributed. The resulting report is an invaluable resource for understanding one’s health, and this guide simplifies how to interpret the data.
Decoding the Core Body Composition Metrics
The DEXA report details the three fundamental components of total body mass. Fat Mass is presented in absolute kilograms and as a percentage of total body weight. This percentage indicates the proportion of the body composed of fat tissue, which serves as stored energy, insulation, and organ protection.
Healthy ranges for total body fat percentage differ significantly based on sex and age. For example, a healthy average for a non-athlete male (18-49) is typically below 25%, while a female in the same age group generally maintains a healthy range below 37%. Females naturally carry a higher percentage of essential fat for hormonal and reproductive functions. Comparing the absolute mass of fat between scans reveals the precise amount gained or lost.
The next major component is Lean Soft Tissue Mass (LST), which includes muscle, organs, blood, and water—everything that is not fat or bone. LST is often used as a proxy for muscle mass and is tracked by those focused on building strength. Finally, Bone Mineral Content (BMC) measures the density and mass of bone tissue. Analyzing the absolute mass of these components, rather than just total weight, provides a clearer picture of how body composition is changing.
Understanding Body Fat Distribution and Health Ratios
Beyond the total body fat percentage, the DEXA scan provides regional data on fat storage location, which has greater implications for metabolic health. The report highlights this through the Android-to-Gynoid (A/G) ratio, comparing fat percentage in the Android region (trunk and abdomen) to the Gynoid region (hips and thighs).
A higher A/G ratio indicates a greater concentration of fat stored around the midsection, often described as an “apple shape.” This distribution is associated with increased risk for conditions like type 2 diabetes and cardiovascular disease. Conversely, a lower ratio suggests more fat is stored in the hips and thighs (“pear shape”), which is a metabolically healthier distribution. For both sexes, an A/G ratio below 1.0 is generally considered ideal for minimizing health risks.
The scan also estimates Visceral Adipose Tissue (VAT), which is fat stored deep inside the abdominal cavity, surrounding internal organs. VAT is distinct from subcutaneous fat and is highly correlated with inflammation and cardiometabolic risk. DEXA technology estimates the mass and volume of this internal fat layer.
Interpreting Lean Mass and Muscular Symmetry
The focus shifts to Lean Soft Tissue (LST) mass to assess muscle development and retention. Muscle is a metabolically active tissue, crucial for long-term health and mobility. The report breaks down LST by region, providing mass measurements for the arms, legs, and trunk. This regional analysis helps identify how training programs affect specific muscle groups or where muscle loss may be occurring.
Assessing Muscle Mass and Sarcopenia
Analyzing the lean mass of the limbs relative to height, known as the Appendicular Lean Mass Index (ALMI), is a standard metric used to screen for sarcopenia, the age-related loss of muscle mass. A low ALMI strongly indicates low muscle mass, particularly in non-overweight individuals.
Evaluating Muscular Symmetry
The DEXA scan is also effective for evaluating muscular symmetry by directly comparing the LST of the left limb against the right limb. While a slight difference between dominant and non-dominant limbs is normal, a significant imbalance can indicate a need for targeted rehabilitation or an asymmetry in training. For athletes, quantifying these regional differences provides actionable data for optimizing fitness and recovery programs.
Tracking Results and Recognizing Limitations
To get the most value from a DEXA scan, establish a consistent baseline and track results over time. Health and fitness objectives are best monitored by repeating the scan every three to six months to capture meaningful changes in body composition. Maintaining consistent pre-scan conditions is paramount for accurate tracking, including arriving in a similar state of fasting and avoiding intense exercise immediately beforehand.
A primary limitation is the influence of hydration levels on the LST measurement. Since LST includes all water in the body, acute changes in fluid intake or severe dehydration can slightly skew the lean mass readings. For example, excessive water consumption before a scan can temporarily inflate the LST mass, while dehydration can cause it to be undercounted.
DEXA measures bone mineral content, but it is not a direct measure of functional strength or bone quality. The report provides objective data on body components, but it does not assess how well the muscles or bones perform. Due to the complexity and health implications of the ratios and indices, all DEXA results should be interpreted by a qualified professional, such as a physician, registered dietitian, or certified trainer, to form an effective health strategy.