A DEXA body composition report breaks your body into three main compartments: fat mass, lean mass (which includes muscle, organs, and water), and bone mineral content. The report typically shows each of these as a total value and a percentage, then slices them further by body region. The numbers are straightforward once you know what each line means, but a few details, like visceral fat estimates, regional ratios, and the quirks of scan accuracy, deserve a closer look before you draw conclusions about your health.
The Three Compartments on Your Report
Every DEXA body composition scan works by passing X-rays at two different energy levels through your body and measuring how much each tissue type absorbs. Bone, lean tissue, and fat each attenuate the beam differently, and the scanner’s software uses those differences to separate the three compartments pixel by pixel across the scan image.1Nutrition. Dual-energy X-ray absorptiometry and body composition When you look at your printout, you will generally see a whole-body summary table listing total mass, total fat mass, total lean mass, and bone mineral content, along with body fat percentage. Below that, most reports break the same numbers into regions: arms, legs, trunk, and sometimes the head.
The International Society for Clinical Densitometry recommends that all body composition reports include at minimum your BMI, bone mineral density, bone mineral content, total mass, total lean mass, total fat mass, and percent fat mass.2PubMed. The Official Positions of the International Society for Clinical Densitometry: body composition analysis reporting Some clinics and newer software packages add visceral fat estimates, android-to-gynoid ratios, and appendicular lean mass indices. If your report has those, they are worth understanding.
Body Fat Percentage and Fat Mass
Body fat percentage is the number most people look at first. It tells you how much of your total weight is fat tissue. There is no single universally agreed-upon “healthy” range, but population-based reference data from the NHANES survey, which scanned thousands of Americans, provides age-, sex-, and ethnicity-specific percentile curves that many clinicians use as a benchmark.3PLoS ONE. Dual Energy X-Ray Absorptiometry Body Composition Reference Values from NHANES If your report includes a percentile ranking, it is likely drawn from this dataset or something similar. Being at the 50th percentile means half the population of your same age, sex, and ethnicity group has more body fat than you, and half has less.
Keep in mind that DEXA tends to slightly underestimate body fat in leaner people when compared with four-compartment models, which are considered the gold standard in body composition research. One study found that the average difference was about two percentage points, but the gap widens in very lean individuals and narrows in those with more fat.4PubMed. Percent body fat via DEXA: comparison with a four-compartment model If you are an athlete with low body fat, your true percentage may be slightly higher than the number on the report. For most people, the DEXA estimate is close enough to be clinically useful, especially for tracking changes over time.
Lean Mass and What It Actually Includes
The “lean mass” number on a DEXA report is not pure muscle. It includes skeletal muscle, organ tissue, blood, and water. DEXA cannot distinguish between water sitting in your gut and water inside muscle cells, so anything that is not fat and not bone gets lumped together as lean soft tissue.5PubMed Central. Assessment of body composition by whole-body densitometry: what radiologists should know This is one of the biggest misunderstandings people have when reading their results. If your lean mass goes up by a kilogram between two scans, it could mean you gained muscle, but it could also mean you were better hydrated on the second scan day.
A more useful metric for tracking muscle specifically is appendicular lean mass, which is the combined lean tissue of your arms and legs. Because organs and large fluid shifts are concentrated in the trunk, appendicular lean mass is a better proxy for skeletal muscle. Many reports list this already, or you can add up the arm and leg lean mass columns yourself.
Appendicular Lean Mass and Sarcopenia Screening
If you divide your appendicular lean mass by your height in meters squared, you get the appendicular lean mass index, or ALMI. This is the metric used by the Foundation for the National Institutes of Health Sarcopenia Project to flag low muscle mass.6PubMed Central. DXA-Derived Indices in the Characterisation of Sarcopenia Sarcopenia is the age-related loss of muscle mass and strength, and catching it early matters because it is linked to falls, fractures, and loss of independence. In an Italian population study, the cutoffs for low muscle mass based on a young adult reference group were roughly 7.6 kg/m² for men and 5.5 kg/m² for women.7PubMed. Prevalence of sarcopenia based on different diagnostic criteria using DEXA and appendicular skeletal muscle mass reference values in an Italian population aged 20 to 80 Different research groups use slightly different thresholds, so your clinician should interpret this number in context rather than relying on a single cutoff.
For younger adults or athletes, ALMI is less about diagnosing sarcopenia and more about having a baseline. If you are tracking the effects of strength training, watching ALMI over months or years gives you a cleaner signal than watching total lean mass, which fluctuates with hydration and food timing.
Where Your Fat Sits Matters More Than You Might Think
Total body fat percentage gets the attention, but the regional breakdown on your report can be more telling. Most modern DEXA reports divide fat into at least two zones: the android region (roughly your midsection, from the ribs to the top of the pelvis) and the gynoid region (your hips and upper thighs). The ratio between them, the android-to-gynoid ratio or A/G ratio, gives a snapshot of how centrally your fat is distributed.
A higher A/G ratio means more of your fat is packed around your midsection, a pattern consistently linked to worse cardiovascular markers. In a study of early postmenopausal women, the A/G ratio correlated with higher systolic blood pressure, higher triglycerides, and lower HDL cholesterol.8PubMed Central. DXA Android-to-Gynoid Ratio and Cardiovascular Risk Assessment in Age and BMI Propensity-Matched Early Postmenopausal Women This pattern shows up in younger populations too: in children and adolescents, an A/G mass ratio above 1 was associated with higher BMI, insulin resistance, and markers of inflammation and liver stress, and it was moderately useful for identifying metabolic syndrome.9Journal of Clinical Densitometry. The Abnormal Ratio of Android-Gynoid Fat, Measured by Dual-Energy X-Ray Absorptiometry (DEXA) Scans, is Moderately Useful for Diagnosing Metabolic Syndrome in Children and Adolescents
If your report shows an A/G ratio, a value below 1.0 generally means you carry more fat in your hips and thighs than your midsection. Values above 1.0 mean the opposite. This ratio can be more clinically meaningful than body fat percentage alone, especially for people whose total fat is in a “normal” range but who carry it centrally.
Visceral Fat Estimates
Some DEXA machines now estimate visceral adipose tissue, the fat packed around your internal organs inside the abdominal cavity. This is the fat people are usually worried about when they talk about “belly fat,” and for good reason. In a large population study, each standard-deviation increase in DEXA-measured visceral fat roughly doubled the odds of type 2 diabetes in both men and women, even after accounting for BMI and waist circumference.10PubMed. Abdominal visceral fat measurement using dual-energy X-ray: association with cardiometabolic risk factors A longitudinal study found that increases in visceral fat over time tracked tightly with worsening metabolic syndrome severity scores, independent of overall weight change.11Nutrition, Metabolism and Cardiovascular Diseases. Longitudinal associations of DXA-measured visceral adipose tissue and cardiometabolic risk in middle-to-older aged adults Even in lean, healthy young women with normal weight, visceral fat correlated with insulin resistance and lower HDL cholesterol.12PubMed Central. The associations between cardiometabolic risk factors and visceral fat measured by a new dual-energy X-ray absorptiometry-derived method in lean healthy Caucasian women
If your report includes a visceral fat number, typically given in grams or as a volume, pay attention to changes over time more than the absolute value. There is no universally adopted clinical threshold yet, though some software packages display risk categories. The practical takeaway is that visceral fat carries outsized metabolic risk compared to the fat under your skin, and even people with a healthy total body fat percentage can have concerning amounts of it.
Bone Mineral Content and Density
Body composition scans produce bone data as a byproduct of the same measurement. Your report may list total body bone mineral content (BMC), measured in grams, and bone mineral density (BMD), measured in grams per square centimeter. These are not the same as a diagnostic bone density scan of the hip and spine, which uses a different scanning protocol, but they give a general picture.5PubMed Central. Assessment of body composition by whole-body densitometry: what radiologists should know If your whole-body bone numbers look unusually low, it may be worth discussing a dedicated bone density study with your doctor.
Segmental Asymmetry
Because DEXA scans measure each side of the body separately, you can compare your left and right limbs for lean mass differences. In most healthy people, mild asymmetry is normal. Among university soccer players, lower limb lean mass asymmetry averaged under 4%, which is well within expected ranges. Upper limb lean mass asymmetry was higher, with males averaging about 7.3% and females about 4.4%, likely reflecting hand dominance and sport-specific demands.13PubMed Central. DEXA Body Composition Asymmetry Analysis and Association to Injury Risk and Low Back Pain in University Soccer Players For athletes and people in physical rehabilitation, watching limb asymmetry across scans can flag whether a weaker side is catching up after injury or surgery. For most other people, small differences between left and right are nothing to worry about.
Things That Throw Off Your Numbers
DEXA is highly repeatable when conditions are controlled, but a few factors can meaningfully shift your results from one scan to the next. The most common culprit is hydration. Drinking a large amount of water shortly before a scan inflates your lean mass reading, because the scanner counts water as lean tissue. In one study, drinking a liter of water before a scan significantly increased whole-body lean soft tissue, with roughly half or more of the increase concentrated in the trunk region.14PubMed. Acute water ingestion influences lean soft tissue estimation from dual-energy x-ray absorptiometry: Implications for clinical diagnosis Going the other direction, losing body water through exercise-induced sweating before a scan reduced lean mass by over a kilogram while artificially increasing body fat percentage.15International Journal of Sport Nutrition and Exercise Metabolism. Errors in Dual Energy X-Ray Absorptiometry Estimation of Body Composition Induced by Hypohydration
Metal implants and breast implants also affect results, though the effect on fat and lean mass is smaller than you might expect. Joint replacements and spinal hardware significantly inflate bone mineral readings in the affected region, but fat and lean mass estimates stay within about 1% of their true values. The standard workaround for bone measurements is to substitute values from the unaffected opposite side. In participants with unilateral hip replacements, this adjustment decreased whole-body bone mineral density by about 14%, revealing how much the metal artifact had inflated the original reading.16PubMed Central. Accounting for Surgical Confounding Factors Affecting Dual-Energy X-ray Absorptiometry in a Large Clinical Trial Breast implants similarly inflate bone mineral content in the chest region but do not substantially affect fat or lean estimates.17PubMed Central. Does a spinal implant alter dual energy X-ray absorptiometry body composition measurements? Effects of spinal implant on DXA
For women, hormone fluctuations across the menstrual cycle or oral contraceptive cycle can subtly shift body fat readings on DEXA, likely due to fluid retention changes. Researchers have recommended that repeat testing be done during the same phase of the menstrual or pill cycle to avoid this noise.18PubMed. The acute effect of the menstrual cycle and oral contraceptive cycle on measures of body composition
Tracking Changes and Least Significant Change
If you are getting repeat scans to track progress, the concept you need to know is least significant change, or LSC. Every measurement has a small amount of built-in variability, and the LSC tells you how much of a shift you need to see before you can confidently say something actually changed, rather than the machine just measuring slightly differently. Your scan facility should have its own LSC values, calculated from repeat scans on the same machine.19PubMed. Effect of Hand Positioning on DXA Total and Regional Bone and Body Composition Parameters, Precision Error, and Least Significant Change
LSC varies by body region. Whole-body fat and lean mass measurements tend to have smaller precision errors than measurements of individual limbs or the trunk, simply because larger regions average out more of the noise.20PubMed. Dual-energy X-ray absorptiometry measured regional body composition least significant change: effect of region of interest and gender in athletes As a practical rule, if the change you see between two scans is smaller than the LSC for that measure, treat it as noise. Ask your scan provider for the specific LSC values for your machine. If they cannot tell you, the numbers are still useful directionally but should not be over-interpreted.
To get the most reliable comparisons over time, scan on the same machine, at the same facility, at the same time of day, in a normal state of hydration, and ideally with the same technician positioning you. Switching facilities or machine brands between scans is one of the fastest ways to introduce meaningless variation.
Why You Should Not Compare Numbers Across Machines
The two dominant DEXA manufacturers, GE Healthcare Lunar and Hologic, use different X-ray energies, detector designs, and software algorithms. Their raw numbers are not interchangeable. Researchers have developed cross-calibration equations to translate between the two, but these equations are population-level corrections, not precise individual conversions.21Journal of Bone and Mineral Research. A multinational study to develop universal standardization of whole‐body bone density and composition using GE Healthcare Lunar and Hologic DXA systems Even with calibration, systematic differences remain between manufacturers due to hardware and software design choices.22JBMR Plus. Standardization of body composition parameters between GE Lunar iDXA and Hologic Horizon A and their clinical impact Direct comparisons of the same individuals on both systems show high correlations but consistent offsets that require correction equations to reconcile.23PubMed. Quantitative Comparison of 2 Dual-Energy X-ray Absorptiometry Systems in Assessing Body Composition and Bone Mineral Measurements
The practical implication: if you switch from a Hologic machine to a GE machine, or vice versa, your body fat percentage and lean mass numbers will shift in ways that have nothing to do with your actual body. Stick with one machine, or at least one manufacturer, for all your follow-up scans. If you do switch, do not compare the new numbers to the old ones without cross-calibration.
How DEXA Compares to Other Body Composition Methods
DEXA is often described as the gold standard for body composition, but that label is slightly misleading. The true reference method in research is the four-compartment model, which combines DEXA data with measurements of body water and body volume to separate the body into fat, water, mineral, and protein fractions. When DEXA-derived body volume was plugged into a four-compartment model in one validation study, there were no significant differences in body fat percentage, fat mass, or lean mass compared with the traditional four-compartment approach.24PubMed Central. Validity and reliability of a 4-compartment body composition model using dual energy x-ray absorptiometry-derived body volume In another comparison, DEXA and other common lab methods all overestimated body fat percentage and underestimated fat-free mass relative to a four-compartment reference, with DEXA being among the methods that differed significantly.25PubMed. Reliability and validity of various laboratory methods of body composition assessment in young adults
None of this means DEXA is unreliable. It means it is a very good two-dimensional estimate of a three-dimensional, biologically complex body. Its real strength is precision across repeated measurements: it gives you consistent numbers scan to scan, which makes it excellent for tracking trends. The absolute number on any single scan might be off by a percentage point or two compared to the theoretical “true” value, but the direction and magnitude of change over time are trustworthy when conditions are controlled.
Reference Ranges and What “Normal” Means
Your report may include Z-scores or percentile rankings alongside the raw numbers. These compare you to a reference population of the same age, sex, and sometimes ethnicity. The most commonly used reference dataset in the United States comes from the NHANES 1999-2004 survey, which provides percentile curves for body fat percentage, fat mass adjusted for height, lean mass adjusted for height, appendicular lean mass, trunk-to-limb fat ratios, and bone measures for people aged 8 to 85.3PLoS ONE. Dual Energy X-Ray Absorptiometry Body Composition Reference Values from NHANES Pediatric body fat percentile curves derived from NHANES skinfold data have also been developed for children and adolescents aged 5 to 18.26PubMed. Body fat percentile curves for U.S. children and adolescents
A word of caution: percentile rankings describe where you fall in the population, but the population itself may not be a picture of health. If the average American carries more fat than is metabolically ideal, being at the 50th percentile does not necessarily mean you are in a healthy range. Percentiles are best used alongside clinical markers like blood lipids, fasting glucose, and blood pressure. A person at the 70th percentile for body fat with perfect metabolic bloodwork is in a different situation than someone at the 40th percentile who already shows insulin resistance.
Practical Tips for Getting the Most From Your Scan
To reduce the noise in your results and make your scan as interpretable as possible, a few simple steps help:
- Stay normally hydrated: Do not chug water or avoid fluids before your scan. Drink as you usually would throughout the day.
- Avoid exercising beforehand: Heavy sweating before a scan can artificially lower lean mass and inflate body fat percentage.
- Wear lightweight clothing: Remove anything metal, including belt buckles, zippers, and underwire bras. Most facilities provide a gown.
- Mention implants: Tell the technician about any joint replacements, spinal hardware, or breast implants so they can note these on the report or adjust the analysis.
- Same machine, same time: For follow-up scans, book at the same facility, on the same machine, and ideally at a similar time of day.
If you are a woman tracking body composition closely, scheduling repeat scans during the same phase of your menstrual cycle removes another source of variability. These steps will not make your scan perfect, but they will make the numbers comparable from one visit to the next, which is where the real value of repeat scanning lies.