Phenylketonuria is primarily due to a deficiency of phenylalanine hydroxylase in the body, which prevents the conversion of phenylalanine to tyrosine. Due to the blockage of the metabolic pathway, secondary metabolic pathways are enhanced, leading to the deamination of phenylalanine and the production of large amounts of phenylpyruvic acid. Through oxidation, by-products such as phenylacetic acid, phenyllactic acid, and para-hydroxyphenylpyruvic acid are formed. The treatment mainly involves the use of low-phenylalanine formula milk. When the concentration in the blood reaches the desired level, natural foods can be gradually added in small amounts. Breast milk is the preferred choice as it contains only one-third of the phenylalanine content of cow's milk. For older infants and children, cow's milk, porridge, noodles, and eggs can be added. The principle for introducing foods should be based on low protein and low phenylalanine content, adjusted according to the phenylalanine concentration in the blood. Both too high and too low levels of phenylalanine can affect growth and development.