What to Do Before a DEXA Scan for Accurate Results

Arriving at your DEXA scan in a fasted, normally hydrated state, having skipped your morning workout and any calcium supplements, gives you the most reliable results. DEXA (dual-energy X-ray absorptiometry) is considered the gold standard for measuring bone density and body composition, but the numbers it produces are sensitive to what you eat, drink, and do in the hours before you lie on the table. Most of the preparation is straightforward, yet overlooking even one step can quietly shift your readings enough to matter, especially if you are tracking changes over time.

Why Preparation Matters for a DEXA Scan

A DEXA scanner works by passing two low-energy X-ray beams through your body and measuring how much each beam is absorbed. The difference in absorption lets the machine distinguish bone mineral from soft tissue, and within soft tissue, fat from lean mass. The math behind this separation assumes a relatively stable internal environment. Anything that changes the amount or distribution of fluid, food, or dense material in your body between one scan and the next can show up as a change in composition that is not actually a change in you. The preparation steps below are all about keeping those variables as steady as possible.

Stay Normally Hydrated but Do Not Chug Water

Drinking a normal amount of water in the hours before your scan is fine, but gulping down a large volume right before you lie down will skew your lean mass reading upward. In a study of 100 adults, drinking roughly 500 milliliters of water (about 17 ounces, a standard water bottle) immediately before a DEXA scan inflated lean mass by an average of about half a kilogram and lowered body-fat percentage by a small but statistically meaningful amount, while fat mass itself did not change.¹ The scanner cannot tell the difference between water sitting in your stomach and actual muscle tissue; it all registers as lean mass.

The flip side is also true. Showing up dehydrated, whether from a hard workout, a sauna session, or simply not drinking enough, will deflate your lean mass reading. One study deliberately dehydrated participants through exercise and then super-hydrated them afterward. After exercise-induced dehydration, body mass dropped by about 1.9 kilograms and lean tissue mass dropped by about 1.7 kilograms. After rehydration and fluid loading, body mass climbed by roughly 2.5 kilograms and lean tissue mass rose by about 2.4 kilograms. Fat mass and bone mineral content did not change at any point.² The takeaway: drink your usual amount of water throughout the morning. Avoid anything that would put you significantly above or below your normal hydration level.

Fast for at Least Two Hours, Ideally Overnight

Eating before a DEXA scan introduces undigested food into your torso, and the scanner will count that mass as part of your body composition. Research on active adults found that consuming breakfast and going about daily activities produced a meaningful increase in the measurement error for both total and regional lean mass estimates. The study concluded that having subjects fasted is the most practical way to minimize those errors.³ Most clinics will tell you to fast for two to four hours before the scan. If you can schedule your scan first thing in the morning and skip breakfast until afterward, that is the simplest approach.

If your scan is later in the day and a full overnight fast is not realistic, aim for at least two hours without food and stick to small, low-residue meals beforehand. The goal is to have as little undigested material in your gut as possible when the X-rays pass through. A large lunch right before an afternoon scan will inflate your trunk lean mass in ways that have nothing to do with muscle.

Skip Calcium Supplements Before a Spine Scan

If you take calcium tablets and your scan includes a lumbar spine measurement, this one matters more than most people realize. An undissolved calcium pill sitting in your gastrointestinal tract can overlap with the spine image and artificially raise the bone mineral density reading for the vertebrae it covers. One study found that a single 500-milligram calcium tablet placed over the scan area could increase the measured density of an individual vertebral body by as much as roughly 13 percent.⁴ The effect on the full L1-L4 region was smaller (around 2 percent or less), but at the level of a single vertebra, the distortion could be enough to mask real bone loss or falsely suggest improvement.

A separate study confirmed that calcium tablet intake can push bone mineral density readings outside the margin of measurement error within as little as 30 minutes of ingestion, with visible tablets appearing in over half of scanned subjects after they swallowed a calcium supplement.⁵ Newer scanner software can sometimes identify and exclude tablets that sit clearly in the soft-tissue field, but when a tablet overlaps bone it often requires manual correction by the technologist, and that is not always caught.⁴

The fix is simple: hold off on calcium supplements for at least 24 hours before a spine DEXA, or at minimum the morning of the scan. Other mineral supplements that come in dense tablet form (iron, for example) may pose a similar theoretical risk, though the published data focuses on calcium. When in doubt, take your supplements after the scan rather than before.

Avoid Exercise the Day of Your Scan

Working out before a DEXA scan introduces several overlapping problems. You lose fluid through sweat (which lowers lean mass readings), you may drink extra water to compensate (which raises them), and you often eat something around your workout. Research found that an exercise session and its associated food and fluid intake shift both the average estimates and the measurement precision of DEXA body composition values, increasing the typical error of measurement by about 10 percent.³ That extra noise makes it harder to detect real changes between scans.

The safest approach is to skip your workout on scan day entirely, or at least schedule the scan before you train. If you exercised hard the day before the scan, making sure you rehydrate fully before bed goes a long way toward normalizing your fluid status by morning.

Menstrual Cycle Timing Is Unlikely to Matter

If you have been wondering whether to time your DEXA around your period, the evidence suggests you do not need to worry about this one. A study comparing body composition readings taken during menstruation versus pre-ovulation found no significant differences in body fat percentage or total body water, regardless of whether participants used hormonal birth control.⁶

A second study did detect a small increase in fat mass during the early follicular phase compared to later in the cycle, but the difference (under a third of a kilogram) fell within the measurement error of the DEXA device itself.⁷ In other words, while hormonal fluctuations do cause real changes in fluid retention across the cycle, those changes are too small for DEXA to reliably distinguish from scanner noise. Schedule your scan whenever it is convenient.

What to Wear and What to Remove

Metal interferes with X-ray imaging. Zippers, underwire bras, belt buckles, and even some athletic compression garments with metallic threads will show up on the scan and may distort readings in the region they cover. Most clinics will ask you to change into a hospital gown, but if you would rather not, wearing lightweight clothing with no metal is the way to go: a plain cotton t-shirt, elastic-waist shorts or sweatpants, and a sports bra without underwire. Remove all jewelry, piercings you can take out, and anything in your pockets.

Transdermal patches (nicotine, hormone, pain) and dense topical creams applied over the scan area can also register as artifacts. If you wear a patch, let the technologist know so they can note its location or ask you to remove it temporarily.

Use the Same Scanner Every Time

This is not a preparation step in the traditional sense, but it affects accuracy as much as anything you eat or drink. Different DEXA scanner models from different manufacturers produce systematically different numbers. A study comparing two widely used systems (one from Hologic and one from GE Lunar) found that the GE system reported significantly higher appendicular lean mass values in both men and women. Body fat percentage readings also diverged, and the gap widened at extreme ends of the BMI spectrum.⁸ Mathematical conversion equations exist to translate between the two, but they add uncertainty. The cleanest solution is to go to the same clinic, on the same machine, every time you scan.

If your clinic upgrades its scanner or you move and switch facilities, ask whether they use the same manufacturer and model. If they do not, treat the first scan on the new machine as your new baseline rather than comparing it directly to old numbers.

Positioning on the Table

You might assume that lying flat on a table leaves little room for variability, but small differences in how your hands and arms are placed change the results. A study testing the effect of hand positioning found that placing hands in a mid-prone position (palms partly facing down) versus palms up altered arm bone mineral density, total body bone density, and even shifted fat and lean mass estimates for the arms and trunk.⁹ The absolute differences were small, but the study recommended that clinics maintain consistency in their positioning protocol. From your side, the practical advice is: follow the technologist’s instructions exactly, and try to remember (or ask) how you were positioned so you can replicate it next time.

Your feet should typically be turned inward and held in place with a strap or positioning block, which standardizes hip geometry for the femur scan. Legs should be straight, arms at your sides or in whatever position the technologist specifies, and you should stay completely still during the scan, which usually takes 6 to 20 minutes depending on what regions are being measured.

If You Are Very Tall or Broad

Most DEXA tables have a scanning area that accommodates the average adult, but taller or broader people, particularly large athletes, may not fit within the active scan region. When this happens, technologists sometimes scan the body in two or three sections and add the results together. This approach works well for people who are too wide for the table: summing left and right halves produces results that closely match a full-body scan. For very tall individuals, the picture is less clean. Summing partial scans that include a separate head scan has been shown to overestimate whole-body lean mass by about 3 kilograms and fat mass by roughly 1 kilogram, with a meaningful increase in measurement error.¹⁰

If you know you are close to the size limits of the table, mention it when scheduling. Some facilities have larger scanning beds, and knowing in advance lets the technologist plan the most accurate approach rather than improvising on the spot.

What About Recent Medical Imaging

If you have had a barium swallow, a CT scan with contrast dye, or a nuclear medicine study (like a bone scan or PET scan) in the days before your DEXA, the residual contrast material or radioactive tracer in your body can interfere with the X-ray attenuation measurements. Most guidelines recommend waiting at least 48 to 72 hours after any contrast-based imaging procedure before a DEXA scan. If you have had a barium study, the wait may need to be longer because barium clears the GI tract slowly. Let your DEXA provider know about any recent imaging so they can advise on timing.

A Practical Pre-Scan Checklist

Pulling together the evidence above, a good routine for the day before and morning of your scan looks like this:

The day before: Skip calcium supplements (and any dense mineral tablets). Avoid extremely heavy exercise that might leave you deeply dehydrated the next morning.
Morning of: Do not eat breakfast. Drink a normal amount of water but do not chug a large bottle right before walking in. Skip your workout.
At the clinic: Wear lightweight, metal-free clothing. Remove jewelry and transdermal patches. Tell the technologist about any recent contrast imaging, implants, or surgical hardware. Ask to be positioned the same way as your previous scan if you have had one.

None of these steps involve anything painful or complicated. The challenge is simply remembering to do all of them, especially the ones that feel trivial (like the hand position or the water bottle). Over a single scan, any one deviation might shift your numbers by a fraction of a percent. But if you are comparing scans months or years apart to track bone density trends or body composition changes, those fractions stack. The whole point of DEXA’s precision advantage over other methods is lost if the conditions are not held steady from one measurement to the next.

How Much Radiation Are You Getting

A common concern, especially for people who plan to scan regularly for fitness tracking, is radiation exposure. The dose from a standard spine-and-hip DEXA is extremely low, on the order of 0.001 millisieverts. For context, a single chest X-ray delivers roughly 100 times more radiation, and you absorb a comparable dose from normal background radiation in a few hours of ordinary life. Whole-body scans for body composition deliver a slightly higher dose, roughly 0.004 to 0.01 millisieverts depending on the device, but still negligible by any clinical standard. Even scanning several times a year adds up to a tiny fraction of the annual background radiation most people absorb from natural sources, cosmic rays, and household radon combined. Pregnancy is the one situation where even this small dose is generally avoided as a precaution, so let your provider know if there is any chance you are pregnant.