Lean mass is everything in the body that isn't fat. Skeletal muscle, bones, organs, water. Muscle dominates the modifiable portion.
Mechanism: Lean mass is the primary site of glucose disposal, the largest pool of amino acids, and a hormonal endocrine organ in its own right (myokines). Lean mass loss is sarcopenia; it accelerates after 50 unless trained against.
Use case: Track via DEXA (most accurate) or InBody/Withings (good enough for trends). Goal: maintain or grow lean mass into 70s and beyond.
Caveats: Scale weight alone is useless without body composition. A 'thin' person with low lean mass is at higher mortality risk than a heavier person with high lean mass.