What is Phenylketonuria (PKU)?

There are 20 essential amino acids in the body. Phenylalanine is one of these.

These essential amino acids need to be supplied from external sources such as food. They are termed “essential” as they are needed for the body to function properly and grow normally.

However, too much of an amino acid may be bad for the body. An overdose of Phenylalanine causes phenylalanine hydroxylase, an enzyme that is present in the liver, to convert it into another amino acid called tyrosine.

If there is lack of deficiency of this enzyme there is resultant increase in build up of phenylalanine in the tissues of blood and blood especially in the brain.

If this amino acid builds up in the brain there may be irreversible and sometimes severe brain damage. This condition is called Phenylketonuria or PKU for short. (1)

Causes of PKU

Babies born with PKU are rare but they have a genetic defect that makes them incapable of breaking down phenylalanine.

The condition is inherited and often is passed down in families.

For the baby to acquire the condition both parents have to carry the defective gene. However, in spite of having the defective gene, the parents do not have the condition.

This method of transmission of the disease from parents with defective genes without the disease who transmit it to the baby is called “autosomal recessive trait”.

The parents who carry the faulty gene are called carriers of the condition. If both parents carry the defective gene there is a 25% chance that the baby will get the condition. The defective gene is carried on the 12th chromosome. (2, 3, 4, 5)

Phenylalanine is present in foods that contain protein and in some artificial sweeteners. Build up of the amino acid in the brain leads to mental retardation and other serious problems.

Mothers who have high blood levels of phenylalanine while carrying their babies in the womb also have a high risk of having the baby with:

  • mental retardation
  • developmental delay
  • small head size (microcephaly)
  • skin rash or eczema
  • fair skin due to lack of skin pigments
  • heart problems.

Babies with PKU may also have a musty body odor due to excessive phenylalanine in the body.

Frequency of PKU

PKU affects 1 in 10,000 to 1 in 15,000 newborns in the United States.

Since 1960’s all babies are screened for PKU. Thus, severe cases of PKU and its consequences are rarely seen these days. (3)

Further Reading

Last Updated: Jun 12, 2023

Dr. Ananya Mandal

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.


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