Phenylketonuria

Description

Phenylketonuria (PKU) is caused by a biochemical abnormality which is inherited by a child from both parents. At birth the baby appears to be quite normal, but the condition can be discovered by a screening blood test. This test, if positive, indicates that there is too much of a substance called phenylalanine in the baby's blood. Phenylalanine is an amino acids or one of the 'building blocks' of certain proteins and is a natural part of all protein foods (such as meat, fish, poultry, egg, milk and cheese and also in certain cereals and vegetables such as wheat, rice, peas, beans and lentils) and is essential in any normal diet.

Excess phenylalanine in the body is changed into another amino acid called tyrosine by an enzyme called phenylalanine hydroxylase . Tyrosine is needed by the body to make many important substances such as hormones, chemical messengers for the brain and the brown pigment melanin. In Phenylketonuria, because the enzyme phenylalanine hydroxylase is absent or working very poorly, the body cannot change phenylalanine into tyrosine. It is this that results in high concentrations of phenylalanine in the blood and a deficiency of tyrosine and some of the important chemicals made from tyrosine.

Symptoms and Signs

Before early screening and treatment became available and high levels of phenylalanine were allowed to continue in the blood, the normal growth and development of the baby's brain was affected and in some cases the children became severely mentally abnormal.

Diagnosis and Treatment

Diagnosis is by blood tests and urine analysis. To treat PKU, the amount of phenylalanine in the diet must be reduced so that the body has just enough for growth and tissue repair but no excess. All protein foods contain considerable amounts of phenylalanine, so they must be severely restricted. Nevertheless, everyone - including those with PKU - needs to eat a certain amount of protein.
In the PKU diet, special products usually called protein substitutes which have little or no phenylalanine, have been developed. These will provide the protein which a PKU child needs. This product will contain some extra tyrosine and all of the other amino acids needed for normal growth. The amount required will be carefully tailored by the dietitians and doctors to meet a child's specific needs.

The information in this page is presented in summarised form and has been taken from the following source(s):
1. National Society for Phenylketonuria (NSPKU) The Child with PKU by Professor Cockburn and Brenda Clark of Yorkhill Hospital in Glasgow and Linda Tyfield of Southmead General Hospital in Bristol: http://web.ukonline.co.uk/nspku/