What is PKU?

Re-printed from: Information Sheet, PKU Handbook, Australasian Society for Inborn Errors of Metabolism, 1996

Phenylketonuria (PKU) is a disorder which prevents the normal breakdown of a part of food protein. However with treatment children with PKU can grow and develop normally.

How is PKU detected?

PKU is a condition in which the body is unable to break down one of the protein building blocks from the diet. These building blocks are called amino acids and one of them is phenylanlanine. In PKU the phenylalanine cannot be processed (metabolised) normally and builds up in the blood and tissues. The high phenylalanine level in the blood can prevent the brain from developing as it should. Progressive mental retardation results if the condition is not treated from early infancy.


Phenylalanine --------//-------------> Tyrosine


The phenylalanine hydroxylase enzyme does not work properly and causes a block in processing (metabolism).


Why did my child get PKU?

PKU is an inherited disorder which occurs when both parents have a gene causing PKU and each passes it on to their baby. (Genes are the particles of heredity which occur in all cells in the body, and form the 'blue-prints' for all the bodily processes, and traits such as hair colour, blood group and so on).

A parent who has one copy of a 'PKU' gene is a carrier, and is perfectly healthy, having also a normal gene as well as a 'PKU' gene in each cell. When both parents are carriers there is a one in four chance that their baby will inherit a 'PKU' gene from each parent, and so be born with PKU.

This type of inheritance is called automal recessive inheritance, and is explained more fully in the information sheet "PKU Inheritance". Most families cannot 'trace' PKU in any of their relatives.

There is nothing the parents of a child with PKU could have done to have prevented their child having PKU.


Is my Child Already Damaged (before the diet was started)?

Your baby would have had normal phenylalanine levels while in the womb. This is because the mother's metabolism can clear phenylalanine before it accumulates. Only after birth does the phenylalanine level rise. Because of early diagnosis and treatment no detectable damage will have occurred. Children treated early will grow and develop quite normally provided the treatment is followed.

Will my child be alright?

Children with PKU treated from early infancy grow up normally. A special low phenylalanine diet, and regular blood tests to measure the phenylalanine level, are necessary.

The diet is essential in childhood to prevent damage to the growing brain. Continuing the diet during adolescence and adult life gives the best outcome in school performance, concentration and ability to think clearly.

Apart from needing a diet, children with PKU should be treated exactly as other children. They are neither more or less likely to get colds and coughs or other illness.

Routine immunizations should be given at the usual times. Most medicines can be given safely but check with your PKU clinic doctor.


What can be done?

The diet for PKU is essential. If left untreated, PKU can lead to mental retardation.

It may seem complicated at first because of having to measure foods, but the foods chosen are like a very strict vegetarian diet. The PKU clinic team will support and help you with the diet.

Your child should be treated like any other apart from the diet. Parents should avoid being over sympathetic or protective. Dietary control is easier if the whole family are consistent about it and never give even a taste of the very high phenylalanine foods. In the first few years a child learns the taste and to like foods he/she is given to eat. He/she will learn to like the foods that are important for good dietary control such as vegetables, fruit and the low protein products. A child who has never tasted a food, even a tiny bit, will find it easier to keep to a diet in later years. He/she will probably dislike the high protein foods.

 Will there be any problems in Adulthood?

Adults with PKU lead normal productive lives, with continued treatment.

PKU has no effect on fertility, either in males or females.

In a woman with PKU high phenylalanine levels at conception and during pregnancy can damage the baby. This is know as 'Maternal PKU'. Planned pregnancies, with strict dietary control before conception and throughout the pregnancy, are necessary to prevent damage to the baby.

Basics principles of the low phenylalanine diet for PKU.

A healthy diet contains proteins for growth and repair, carbohydrates and fats for energy, activity and growth, and essential fats, vitamins and minerals to keep the person healthy and well. The diet for PKU provides all these for your child, whilst being low in phenylalanine.

People with PKU are 'normal' apart from needing a special diet which is low in phenylalanine. Initially some of the diet foods will be unusual to you, however your child will grow up enjoying their taste.

Some phenylalanine is needed for growth and making the body's protein but only in small measured amounts. This achieves normal growth while keeping the blood phenylalanine level in a 'safe' range.


If dietary phenylalanine intake is controlled carefully from early infancy the harmful effects of PKU will be prevented and your baby will grow up healthy. This makes the diet well worth while.