A DEXA scan (dual-energy X-ray absorptiometry) is a low-dose X-ray that measures your exact amounts of body fat, lean tissue, and bone, region by region. Originally developed to diagnose osteoporosis, it has become the most accurate widely available method for measuring body composition, with a margin of error of just 1% to 2% for body fat percentage.
How the Scan Works
You lie on a padded table while a scanning arm passes over your body for about 10 to 15 minutes. The machine sends X-rays at two different energy levels through your tissues. Dense materials like bone absorb more energy, while softer tissues like fat and muscle let more pass through. By comparing how much energy each tissue type absorbs at both energy levels, the machine calculates a ratio that distinguishes bone from fat from lean tissue at every point in the image.
Fat produces a consistently different absorption ratio than bone or muscle. A lower ratio at any given point means a higher concentration of fat in that area. This is what allows a DEXA scan to map your body composition in detail, showing not just your total body fat percentage but exactly where fat is stored: your torso, each arm, each leg, and around your organs.
What Your Report Shows
A DEXA body composition report breaks your results into several categories that go well beyond a single body fat number.
- Total body fat percentage is the headline number most people look for. It tells you what proportion of your total weight is fat tissue.
- Regional fat distribution shows fat mass in your trunk, arms, and legs separately, so you can see whether fat is concentrated in your midsection or distributed more evenly.
- Android to Gynoid ratio compares fat stored around your stomach (apple shape) to fat stored around your hips (pear shape). A higher number means more belly-concentrated fat. From a health risk standpoint, ideal values are generally below 1.0 for men and below 0.8 for women.
- Visceral adipose tissue (VAT) measures the fat packed around your internal organs inside the abdominal cavity. This is the fat most strongly linked to insulin resistance, high blood pressure, abnormal cholesterol, and chronic inflammation. Research has found that visceral fat exceeding roughly 600 grams is a meaningful threshold for metabolic risk, and each additional 100 grams above that is independently associated with higher odds of metabolic syndrome.
- Lean mass tells you how much muscle and other non-fat soft tissue you carry. Some reports also calculate appendicular lean mass, which combines the lean tissue in your arms and legs. This measurement is used clinically to screen for sarcopenia, the age-related loss of muscle mass and strength.
- Bone mineral density is included on most scans, giving you a snapshot of skeletal health alongside your soft tissue data.
How It Compares to Other Methods
DEXA’s 1% to 2% error margin makes it substantially more precise than the alternatives most people encounter. Bioelectrical impedance devices, including the scales and handheld analyzers found in gyms and pharmacies, carry error margins of 5% to 15%. That means a person with 25% body fat could get a reading anywhere from 20% to 30% on an impedance device, which makes tracking progress over time unreliable.
Skinfold calipers depend heavily on the skill of the person taking the measurements and only sample a few sites on the body. Hydrostatic (underwater) weighing is accurate but only provides a single whole-body fat percentage. It can’t tell you where your fat is stored or how much visceral fat you carry. DEXA is the only widely accessible method that gives you regional data, visceral fat measurement, lean mass breakdown, and bone density in a single scan.
What Can Throw Off Your Results
DEXA is accurate, but not immune to interference. Hydration is the biggest variable. Drinking 500 ml of water (about two cups) shortly before a scan can significantly inflate your lean mass reading and lower your body fat percentage by a small but measurable amount. The scan can’t distinguish water from muscle tissue, so extra fluid in your system gets counted as lean mass. Food and fluid intake under 500 grams generally keeps the error within acceptable range, but for the most consistent results, scan in a similar state each time.
Exercise and dehydration also shift results. Losing water through a hard workout or heat exposure can increase your body fat reading by roughly 0.3 percentage points, because losing fluid reduces the lean mass side of the equation. Glycogen loading, where muscles store extra carbohydrate and the water that comes with it, can push lean mass readings higher. None of these effects are large enough to invalidate a single scan, but they matter when you’re comparing scans over months to track progress. Scanning at the same time of day, with similar food and water intake, and without exercising beforehand gives you the most reliable comparison.
Radiation Exposure
A DEXA scan delivers about 0.001 millisieverts of radiation, equivalent to roughly 3 hours of the natural background radiation you absorb just going about your day. For comparison, a standard chest X-ray delivers about 100 times more radiation. This makes DEXA one of the lowest-radiation imaging procedures available, and safe to repeat multiple times per year for body composition tracking.
Cost and Insurance Coverage
Body composition scans performed for fitness or wellness purposes are considered elective. Insurance will not cover them, so they’re an out-of-pocket expense. At wellness centers and mobile scan providers, body composition scans typically cost $40 to $200. Hospital imaging departments charge more, generally $150 to $400. Mobile scan providers, which set up at gyms or health events, often fall in the $65 to $150 range and can be a convenient option for routine tracking.
Bone density scans ordered by a doctor to diagnose or monitor osteoporosis are a different story. Most private insurance plans and Medicare cover those. Regardless of whether your scan is for body composition or bone health, DEXA scans qualify as eligible expenses under Health Savings Accounts (HSAs) and Flexible Spending Accounts (FSAs).
How to Prepare
Wear loose, comfortable clothing without metal zippers, buttons, snaps, or belts, as metal interferes with the X-ray imaging. Many facilities provide a gown. Avoid calcium supplements for at least 24 hours before the scan if bone density is being measured. For the most accurate body composition results, avoid eating a large meal or drinking a lot of water in the hour before your appointment, and skip intense exercise that morning. The scan itself requires nothing from you except lying still on the table for 10 to 15 minutes.