How it works
Lean body mass is everything in your body that is not fat — muscle, bone, organs, and water.
Lean mass = Weight × (1 − Body fat %)
Why lean mass matters
When you lose weight, you want to lose fat while preserving muscle. Tracking lean body mass helps you see whether your weight loss is coming from fat or muscle.
How to protect lean mass
Eat enough protein, do resistance training, and lose weight at a moderate pace. Very fast weight loss tends to burn more muscle.