Once a child with phenylketonuria is diagnosed, natural diets should be ceased and a low-phenylalanine diet treatment should be initiated. Treatment with a low-phenylalanine formula should continue at least until the age of 12. Breast milk is the ideal natural food for infants; therefore, although breastfeeding should be temporarily halted after diagnosis, it should not be completely stopped so that it can be promptly reintroduced once blood phenylalanine levels are controlled. When blood phenylalanine levels are controlled to an ideal concentration, gradually reintroduce small amounts of natural diet, preferably starting with breast milk, as it contains only one-third the phenylalanine content of cow's milk.