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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 will be discovered before the age of 1 month by a screening blood test which is taken from all babies born in the UK. If this special test is positive and a further blood test shows the baby to have PKU, treatment is started as soon as possible.
A positive blood test shows that there is too much of a substance called phenylalanine in the baby’s blood. 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.
Phenylalanine is a natural part of all protein foods and is essential in any normal diet. In PKU, the aim of treatment is to stop the build up of phenylalanine by using a special diet. When the diet is monitored carefully, it will allow the baby’s brain to grow and develop so that a normal lifestyle, apart from the dietary restrictions, can be expected.
Treatment for phenylketonuria has only been available since the late 1950’s so that, even today, doctors, dietitians, biochemists and other health professionals are still learning about PKU, particularly the way it is inherited and the long term outlook for treated adults with the condition. There are likely to be many questions you will want to have answered and the people looking after your baby will be able to answer most of them but there may be some which at the moment, cannot be answered with any certainty due to our incomplete understanding of PKU.


In children with PKU the protein food cannot be used properly. Foods which supply most of the protein in our diet include meat, fish, poultry, egg, milk and cheese but reasonable amounts of protein are found in certain cereals and vegetables such as wheat, rice, peas, beans and lentils. All of these proteins are made up of substances or ‘building blocks’ called amino acids.
There are many different types of amino acid and each protein food contains these in varying amounts. This is why each protein food looks and tastes different from another eg milk does not look and taste like meat.
Phenylalanine is one of these amino acids and it is this particular one which the PKU child cannot deal with properly.


To treat PKU, we must reduce the amount of phenylalanine in the diet 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. Your dietitian / doctor will recommend the most appropriate one for your child. 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 your child’s specific needs.


The simple answer is that your baby has inherited PKU from both of you. Your immediate response may be that neither you nor your partner have any knowledge of anyone in your families having PKU. In order that you can understand how PKU is inherited, we have to explain the way in which characteristics are passed on from parents to children.
We all know that children tend to have features such as the colour of hair and eyes which make them resemble one or other parent. All of our
inherited characteristics are controlled by the store of information contained within the cells of our bodies. These information stores are called genes and these genes have been copied from the set of father’s genes contained in the sperm and the set of mother’s genes contained in the ovum or egg. Therefore, from the original sperm and ovum which made your baby there will be a father’s gene and a mother’s gene controlling each inherited characteristic. Usually we need only one gene to control one activity in the cell and in the body, so that if a gene inherited from one parent is defective, the gene from the other parent will allow the cell and body to work normally.
In PKU both genes which control the production of the enzyme phenylalanine hydroxylase are faulty ie, the child with PKU has inherited one
faulty gene from the father and one from the mother.