Hay diet

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. Acidic foods are protein rich, such as meat, fish, dairy, etc. Alkaline foods are carbohydrate rich, such as rice, grains and potatoes. It is also known as the food combining diet.

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, and Robert O. Young.

History
William Hay contracted Bright's disease (or what modern medicine refers to as Nephritis). Heart-dilated and near death, Hay began eating only natural foods; his calorie intake dropped and his health improved. Hay spent the following decade studying naturopathy and food combining to reduce the acid end-product of digestion. He found that fruits and vegetables produce an alkaline end-product when they are fully metabolized, while processed and refined foods left a high acidic environment after digestion. His theories went on to encompass food-combining; whereas incorrect combination would cause even alkaline foods to leave a less desirable acidic digestion end-product. ""Any carbohydrate foods require alkaline conditions for their complete digestion, so must not be combined with acids of any kind, as sour fruits, because the acid will neutralise. Neither should these be combined with a protein of concentrated sort as these protein foods will excite too much hydrochloric acid during their stomach digestion." - William Hay, How to Always Be Well"

The Hay System promoted the practice of eating three meals per day with meal one being alkaline foods only, meal two protein foods with salads, vegetables and fruit, and meal three comprising starchy foods with salads, vegetables and sweet fruit. There should be an interval of 4.0 to 4.5 hours between each meal. However, in 1935, Stewart Baxter showed that the pancreas secretes digestion enzymes simultaneously regardless of whether the food eaten is carbohydrates or protein. The theory that carbohydrate and protein rich foods should be eaten separately is considered "unfounded" because it ignores the fact that carbohydrate rich foods all contain significant amounts of protein. Eating protein separately from carbohydrates also tends to cause the body to burn the protein as an energy source rather than to build muscle.

American artist Man Ray was a notable follower of the Hay Diet.

Studies
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.