Glossary
Body Composition
Updated February 28, 2026
Body Composition is the ratio of fat mass to fat-free mass, and understanding trend direction matters more than point precision for most programs.
Method precision and use case
| Method | Core measure | Precision profile | Best use |
|---|---|---|---|
| DEXA | Fat mass, lean mass, bone distribution | Highest repeatability when protocol is stable | Baseline mapping and periodic recalibration |
| Hydrostatic methods | Body density and derived partitioning | High precision | Deep analysis when facility access exists |
| BIA scale | Impedance-derived body composition estimate | Highly variable with hydration and electrolyte shifts | Daily trend sampling |
| Skinfolds | Subcutaneous tissue thickness at selected sites | Moderate precision with consistent technique | Practical body change tracking |
| Tape and weight | Waist, hip, and body mass | Lower precision but very repeatable with habit control | Behavior-linked trend monitoring |
Trend tracking versus precision
Precision asks whether one single value is exact. Trend tracking asks whether a direction is stable over time. For most users, trend tracking is the better decision input. Weekly movement on BIA, scale, or circumference values often reflects water, glycogen, and meal timing noise, not only tissue change.
Interpreting method disagreement
| Pattern | Likely explanation |
|---|---|
| Two methods diverge by one cycle | Method noise or timing mismatch |
| Three weeks of divergence plus same direction | Real body composition shift likely |
| Only one metric improves while others worsen | Either metric instability or contextual disturbance |
Use a simple rule: if two independent methods agree for at least three sessions, the direction is likely real. If not, maintain a consistent protocol and wait another cycle before major adjustments.
Practical noise filter
Record waist, weight, strength, and energy together. A true fat change usually aligns at least two of these over a sustained window, while isolated spikes in one metric are usually load, hydration, sleep, or sodium effects. This avoids overreacting to normal week-to-week variation.
Nutrition and training context
Training shifts water, glycogen, and inflammation markers within days. Recovery-heavy phases can preserve performance and strength while body fat trends lag. During this period, avoid aggressive energy cuts based on one snapshot, and instead move based on repeated direction.