The 500 kcal/day rule for weight change
A deficit or surplus of 500 kcal/day translates theoretically to approximately 0.45 kg (1 lb) per week, based on the assumption that 1 lb of body fat contains approximately 3,500 kcal. This relationship is an approximation — it assumes all weight change is from fat, ignores water retention and lean mass changes, and does not account for metabolic adaptation.
Protein intake is prioritised in the macro suggestion because it has the highest thermic effect, supports muscle preservation during deficits, and has the highest satiety per calorie. The suggestion of 2.2 g/kg bodyweight is on the high end of the evidence range (1.6–2.2 g/kg) to account for conservative estimates.