DEXA Scan (Body Composition + Bone Density)

What it is

A low-radiation X-ray (~0.001 mSv) that uses two energy levels to discriminate between fat, lean (muscle + organ + water), and bone. A typical DEXA scan reports:

• Total and regional lean mass (arms, legs, trunk).
• Total and regional fat mass.
• Visceral adipose tissue (VAT) — an estimate, not direct measurement.
• Bone mineral density (BMD) at hip, spine, and total body.
• T-score — standard deviations from young-adult mean BMD.

Why it matters

• Lean mass / appendicular lean mass index (ALMI) — sarcopenia diagnosis and tracking.
• Visceral fat — the metabolically harmful fat depot; weakly related to BMI.
• Bone density — osteoporosis screening and treatment monitoring.

T-score interpretation

• > −1.0: normal.
• −1.0 to −2.5: osteopenia.
• < −2.5: osteoporosis.

Frequency

For body composition and longevity tracking, annual scans are typical. For osteoporosis monitoring, every 1–2 years.

Limitations

• Hydration status modestly affects lean-mass measurement.
• VAT estimation is less accurate than CT/MRI but adequate for tracking.
• Cannot distinguish muscle from non-fat lean (water, organ tissue).

Related entries

Sarcopenia, Osteoporosis, Grip strength, VO2max.