Muscle, not body fat, is your metabolic engine and longevity drug. Dr. Gabrielle Lyon, a fellowship-trained geriatrician, explains why we're under-muscled, not overfat—and why the 0.8g/kg protein guideline comes from 1940s wartime rationing, not modern science. Building muscle through strategic protein intake and resistance training is the closest thing we have to a longevity hack. Skeletal muscle is the primary site for glucose disposal and metabolic regulation; diseases like insulin resistance and obesity begin in muscle weakness, not fat excess.