Theoretically, phenylketonuria requires lifelong dietary control.

Phenylketonuria is caused by a deficiency in phenylalanine hydroxylase in the metabolic pathway of phenylalanine, leading to disordered metabolism of phenylalanine in the liver. Phenylketonuria is the first genetic metabolic disease that can be managed through diet. Since natural foods all contain certain amounts of phenylalanine, once diagnosed, affected children should cease consuming natural diets and should be treated with a low-phenylalanine diet.

Treatment with a low-phenylalanine formula should continue at least until the age of 12. When the concentration of phenylalanine in the blood is controlled at an ideal level, natural foods can gradually be reintroduced in small amounts. The reintroduction of food should still follow the principles of low protein and low phenylalanine. The types and amounts of food that each child can add vary individually and are related to the severity of the enzyme deficiency.