William Howard Hay (December 14, 1866 – 1940) was an American physician and director of The East Aurora Sun and Diet Sanatorium.

In this period Hay was a member of the Medical Advisory Board of the Defensive Diet League of America, and campaigned against the use of aluminum cooking utensils,[1] vivisection[5] and vaccination for smallpox.

The Hay diet was popular around that time and many restaurants offered 'Hay-friendly' menus; followers of his dietary advice, who called themselves “Hayites”, included Henry Ford.

[8] Hay was criticized in the Journal of the American Medical Association as a food-faddist[1] and later for advocating that a patient with type 1 diabetes stop taking insulin.

[9] Physician Logan Clendening described the Hay Diet as a "half-baked unscientific food fad" [10] Jeffrey M. Pilcher a Professor of Food History has noted that: [Hay] believed that carbohydrates and proteins should never be eaten at the same meal because the body uses alkaline enzymes to digest carbohydrates whereas acids work on proteins.