The Hay diet — food combining’s most famous system — was devised in the 1930s by the American physician William Howard Hay. It classifies foods as acid-forming, alkaline-forming, or neutral, and forbids eating concentrated proteins and concentrated starches at the same meal, on the theory that the body cannot digest both together efficiently. Followers claim better digestion, more energy, and weight loss. Physiology disagrees: the stomach and pancreas secrete the full range of digestive enzymes for every mixed meal, and the body’s acid-base balance is tightly regulated regardless of “alkaline” foods. Like all food-combining schemes, the Hay diet’s benefits come from the calorie restraint its rules impose — not from the chemistry it imagines.