What is PKU? Phenylketonuria, or PKU, is an inborn error of metabolism. People with PKU have to adhere to a strict diet that is low in protein, with the addition of a medical formula that provides all of the nutrients and vitamins that we don't get from food. We cannot eat meats, dairy, grains, and legumes. The successful PKU diet typically consists of ONLY fruits, vegetables, some fats, and PKU formula. Untreated PKU results in severe mental retardation, among other consequences. Because Fanni's diet has not been properly restricted, she does have some delays.