Why lean body mass matters for metabolic rate
Adipose (fat) tissue has a low resting metabolic rate of roughly 4–10 kcal/kg/day. Skeletal muscle has approximately 13 kcal/kg/day, and organs such as the brain, liver, and heart have substantially higher rates. Total body weight conflates these different tissues, while lean body mass more directly reflects metabolically active tissue.
The advantage of Katch-McArdle becomes meaningful at the extremes: a highly muscular 100 kg athlete and an obese 100 kg individual with the same height and age will get similar Mifflin-St Jeor results but substantially different Katch-McArdle results if their LBM differs by 20+ kg.