The Hay Diet is a nutrition method developed by the New York physician William Howard Hay in the 1920s. It claims to work by separating food into three groups: alkaline, acidic, and neutral. (Hay’s use of these terms does not completely conform to the scientific use, i.e., the pH of the foods.) Acid foods are not combined with the alkaline ones. Acid foods are protein rich, meat, fish, dairy, etc., and “alkaline” the carbohydrate-rich starch foods like rice, grains and potatoes. It is also known as the food combining diet, and many authors have written books expressing its merits and selling recipes.
A similar theory, called nutripathy, was developed by Gary A. Martin in the 1970s. Others who have promulgated alkaline-acid diets include Edgar Cayce, D. C. Jarvis, Robert Young, Herman Aihara, and Victor A. Marcial-Vega.
The food-combining diet has been the subject of one peer-reviewed randomized clinical trial, which found no benefit from the diet in terms of weight loss.