Phenylketonuria, also known as PKU, is a congenital alteration of metabolism caused by a lack of the enzyme phenylalanine hydroxylase, which results in the inability to metabolize the amino acid tyrosine from phenylalanine in the liver. It is a congenital disease with an autosomal recessive inheritance pattern.

What happens in the case of a child born with PKU?

The baby is born without problems, since until the moment of delivery it is his mother who is in charge of metabolizing all the compounds and she does it correctly, even if she is a carrier of erroneous information (capable of transmitting PKU).

When the baby begins to feed, the milk proteins will break down and release all the amino acids, each of which will be metabolized in its own way.

However, phenylalanine will have its way of transformation into tyrosine interfered, since PAH has not been formed well, and phenylalanine will begin to accumulate.

Tyrosine, on the other hand, will not be formed in sufficient quantity to be used in the synthesis of proteins that the baby’s body needs to grow and in the synthesis of neurotransmitters.

We will have a metabolic error. The balance that must exist in the body between all its compounds will have been broken for the metabolism to function properly.

This can have negative consequences for the child (a certain degree of developmental delay or irreversible brain damage), which would not be detected until a few months later. Hence the importance of an early diagnosis of PKU.


Treatment will have to be instituted quickly. This is based simply on preventing phenylalanine from accumulating in large quantities, restricting it in the child’s diet. The diet of children with PKU will be based on the restriction of natural proteins (milk, meat, fish, eggs and other foods that contain phenylalanine), replacing them with a special formula that contains all other amino acids (except phenylalanine), and that is supplemented with tyrosine to avoid its deficiency. In this way, the balance that had been broken by interrupting a metabolic pathway is regained and the entire metabolism works properly again. Both phenylalanine and tyrosine are essential amino acids for the formation of proteins that will constitute the body of the newborn, so the restriction of phenylalanine must be adequate for each child so that it reaches the correct concentration in blood and tissues.