Where the 3,500 calories per pound figure comes from
The 3,500 kcal/lb figure was introduced by Max Wishnofsky in 1958, calculated from the energy content of human adipose tissue. Pure fat contains approximately 9 kcal/g, but adipose tissue is roughly 87% lipid, giving approximately 3,500 kcal/lb after accounting for water and protein components.
The main limitation is that it implies a constant linear relationship between calorie deficit and weight loss, ignoring metabolic adaptation, water fluctuations, and lean mass changes.