Glossary
Fat-Free Mass
Updated April 2, 2026
Fat-Free Mass includes everything in the body that is not fat: muscle, organs, bone, water, glycogen, and connective tissue. That is why FFM changes are easy to misread. A shift in hydration, sodium, inflammation, or glycogen can move the number even when actual tissue change is small.
Terminology and interpretation
| Term | Definition |
|---|---|
| Body weight | total body mass |
| Fat mass | percentage-based fat estimate |
| Fat-free mass | everything not fat |
| Lean mass | often close to FFM, but context and source vary |
Use lean mass for training focus and body fat percentage for fat change interpretation.
Meaningful change by block
| Block length | Meaningful FFM shift |
|---|---|
| 2 weeks | small noise range, avoid hard conclusions |
| 6 weeks | possible directional signal with adherence support |
| 12 weeks | likely stable trend if measurement is reliable |
What distorts FFM readings
| Distortion source | Why it changes the reading | Interpretation rule |
|---|---|---|
| Glycogen refill or depletion | glycogen stores carry large water shifts | compare under similar training and carb conditions |
| Sodium swing | extracellular water can rise or fall quickly | do not treat one scan after a high-sodium day as tissue change |
| Hard training block | inflammation and muscle damage alter water handling | allow recovery before comparing scans |
| Menstrual cycle phase | body-water distribution changes across the cycle | compare the same phase when possible |
| Device method difference | DEXA, BIA, and formulas estimate different things | compare within the same method, not across devices |
Benchmark examples
| Scenario | Setup | Interpretation |
|---|---|---|
| Beginner recomposition | progressive training + protein stable | visible gain can be measurable over several blocks |
| Dieting phase | reduced intake and training maintained | small loss can occur if intake and recovery dip |
| Long cut | aggressive deficit | prioritize fat loss and avoid over-interpreting FFM drops |
Measurement method rules
| Method | Best use | Main weakness |
|---|---|---|
| DEXA | periodic trend checks over long blocks | hydration, glycogen, and scheduling still affect it |
| Bioimpedance | frequent same-routine checks | highly sensitive to water and timing noise |
| Skinfolds and tape | coaching trend support | operator skill and site consistency change accuracy |
| Formula estimate | rough planning baseline | too coarse for short-block tissue claims |
Formula remains straightforward.
FFM = Body weight × (1 − Body fat fraction)
Use FFM as a trend measure, not as proof of muscle gain from a single reading. The cleaner the routine around measurement, the more useful the number becomes.