Dr William Hay came up with the Hay diet in 1911 when he observed his own health improve by following diet meal plans based around the concept of eating "fundamentally." As our diet plan review explains, the Hay diet essentially involves not combining protein and starch within the same serving. The reason for this is that proteins and carbohydrates require different enzymes to break them down so that the body can successfully absorb the nutrients. Followers of the Hay diet believe that protein-starch based recipes are difficult for the body to absorb, and cause intestinal problems such as obesity, constipation, indigestion and arthritis.

The recipes and diet meal plans of the Hay diet are rich in vegetables, salads and fruits. It recommends eating proteins, starches and fats in small quantities. Proteins need an acid medium for digestion, whilst carbohydrates require an alkaline medium. Dishes that are a mixture of high starch and high protein result in a situation in the digestive system where there is too much acid to digest the carbohydrates fully, and too much alkaline to allow the proteins to fully break down, which may result in health problems.

This nutritional theory is marketed as much more than a way to reduce weight. It sees itself as a programme that results in greater health, freedom from digestive problems, greater energy levels as well as weight loss. Critics of the programme claim there is no scientific evidence to support the claims that carbohydrates inhibit the digestion of proteins. However, celebrity followers of the Hay diet include Princess Diana, Elizabeth Hurley and Catherine Zeta Jones.