The Dr. Hay diet claims to work by separating out food groups into those which are "alkaline, acid and neutral". The vast majority of the Medical community view the diet as pseudoscience with no basis in medicine. Some people believe that it has helped them achieve their required results. However, it is likely that they lost weight because they reduced their total energy intake.
It is also known as the "food combining" diet, and many authors have written books expressing its merits and selling recipes.