DEXA

DEXA — dual-energy X-ray absorptiometry — is a low-radiation imaging technique that measures body composition by passing two distinct X-ray energies through the body. Each tissue type (fat, lean soft tissue, bone mineral) has a characteristic absorption profile at each energy level; comparing the absorption pattern yields a quantitative body-composition breakdown.

What a DEXA scan reports

• Total fat mass (grams)
• Total lean soft tissue mass (essentially muscle + water + organ tissue)
• Bone mineral content and density
• Regional breakdowns — arms, legs, trunk separately
• Visceral adipose tissue estimate (on newer units with VAT software)
• Android/gynoid fat distribution ratio
• Computed body-fat percentage from fat mass / total mass

Accuracy

Against the four-compartment reference model:

• ±1–2% body fat in well-run protocols.
• ±1–2% lean mass.
• Regional accuracy slightly lower than whole-body.
• Visceral-fat quantification varies by manufacturer and software.

Validation studies (Shepherd et al., Kiebzak et al.) consistently show DEXA as the most accurate widely-available method for consumer body-composition assessment — substantially better than BIA, handheld devices, or skinfold calipers.

What affects scan-to-scan precision

For longitudinal tracking, reproducibility matters more than absolute accuracy:

• Hydration state. A dehydrated scan under-reports lean soft tissue (since muscle is ~70% water). Scan fasted and at consistent hydration.
• Time of day. Morning scans (pre-food, post-bathroom) are most consistent.
• Same machine, same technician. Between-scanner variance is 1–2% even on well-calibrated machines.
• Glycogen state. Muscle glycogen and its bound water contribute to lean-tissue mass; a post-carbohydrate-loading scan shows higher lean mass than a fasted, depleted scan.

Radiation exposure

Typical consumer DEXA body-composition scan: 0.4–1.0 µSv of radiation, roughly equivalent to 10–30 minutes of natural background exposure. Much lower than a chest X-ray (~100 µSv) or a CT scan (~10,000 µSv). For routine body-composition tracking, radiation exposure is not a practical concern.

Practical usage for tracking

• Baseline scan before starting a structured cut, bulk, or recomp phase.
• Follow-up scans every 12–16 weeks during active body-composition change.
• Annual scan at stable weight for trend tracking.
• Pre- and post-intervention scans for research-grade self-experimentation.

For most trackers, DEXA is overkill between major body-composition phases. For serious trackers doing deliberate recomposition or contest-prep-style cuts, periodic DEXA provides the ground-truth signal that BIA scales and tape measures can't match.

Limitations

DEXA is a 2D projection, not a 3D measurement — it cannot directly distinguish subcutaneous from visceral fat the way a CT or MRI scan can. Manufacturer-specific VAT estimates exist but carry larger errors than DEXA's headline fat-mass measurement. For users specifically concerned with visceral-fat trajectory, MRI or CT remains the reference — at substantially higher cost and radiation exposure.