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Home > Vitafriendspku > Your pku journey > Introduction to PKU

Introduction to PKU

What is PKU?

Like most metabolic disorders, PKU is an autosomal, recessive, inherited disorder.

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This definition might seem a bit complicated so to explain these terms:  


Autosomal means it is not sex-linked and is equally likely to occur in boys or girls.  

Recessive means that for a child to have PKU, they must receive a PKU gene from both parents. 

Inherited means it is passed down from a child’s parents. 


If someone is a carrier of the PKU gene, this means they have one PKU gene and one non-PKU gene. Carriers do not have PKU themselves and their children may or may not inherit PKU.

Recessive PKU Gene Diagram


If both parents are carriers, there is a 25% chance of the child having PKU, a 50% chance of them being a carrier for PKU, and a 25% chance of them not having the PKU gene at all. 

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What is protein?

Protein is a nutrient needed by the body. It helps to build, repair and maintain body cells and tissues. There are 20 building blocks called amino acids that make up protein. Some of these cannot be made in the body so they must come from the food we eat. These are called essential amino acids.

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Protein Building Block

Protein is made up of many building blocks called amino acids.

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What is phenylalanine?

Phenylalanine or phe for short, is one of these essential amino acids. It is this amino acid that a person with PKU cannot process correctly.

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Phe Building Block


Phe is found in all foods containing protein. In PKU, the build-up of phe is due to the deficiency of an enzyme (which acts like chemical scissors) called phenylalanine hydroxylase (PAH). PAH helps turn phe into another amino acid called tyrosine. The deficiency of PAH results in a build-up of phe which, in excessive amounts, is toxic to the brain. As phe cannot be converted to tyrosine, low tyrosine levels are seen. This may cause problems with chemical messages in the body and melanin (the pigment that gives human skin, hair, and eyes their colour) deficiency. 


PAH enzyme


PKU can be different from person to person. Some people have no or very little PAH enzyme activity – this is referred to as classical or severe PKU. Some individuals have more PAH enzyme activity - this is called moderate or mild PKU. In the case of mild PKU, the term ‘Hyperphenylalanaemia’ (Hyperphe for short) is sometimes used 

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What are the outcomes for my child if their PKU is managed well?

With early diagnosis and the right management, children with PKU can live healthy lives and long-term outcomes are good. If phe levels are well managed, your child will grow and develop similarly to other children their age.

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Keeping Phe levels within the recommended ranges means your child’s natural intelligence will be unaffected and they have as much chance as others to do well at school, go to university, have a career, be physically active and enjoy family life to the fullest.


For those of you who have a daughter with PKU, although it seems a long time away, in the future they might like to have a family of their own. Women with PKU are advised to keep their blood phe levels extremely well controlled before they plan to become pregnant, as well as throughout their pregnancy. By already being used to keeping their phe levels in range, sticking to a low protein diet and taking their protein substitute, means this will be easier to manage in adulthood.


The best way to make sure your child’s phe levels are in the recommended range is to help them stick to their low protein diet and make sure they are consistent with taking their protein substitute.

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Going to school

Children with PKU will attend school like all other children. The only extra consideration is that you will need to plan ahead a little bit more. If you would like more information on preparing for school, click here . When your child goes to school, there is no reason their phe levels will not stay well controlled. It is important that staff at school understand:

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  • what PKU is
  • why your child needs to follow a low protein diet
  • the importance of your child’s protein substitute


Having this understanding will help them to provide support in managing your child’s PKU. It may be helpful to ask your child’s dietitian to speak to the school, or even visit, to discuss the basic principles of PKU with staff. This should give them the confidence to manage your child’s needs and will provide an opportunity for them to ask any questions they might have. The Introduction to Phenylketonuria is a good way for school staff to gain a better understanding of PKU. It is likely there will be other children at school with specific dietary needs too so they will be familiar with learning about different dietary requirements.

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Keeping active

Your child can take part in any sports they choose. Like everyone, they should try to keep physically active. Being physically active helps to:

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  • manage weight
  • boost sleep quality
  • improve energy and mood


If your child is exceptionally active, then it is important that they get enough energy (calories) from their low protein diet. Your dietitian will be able to discuss this with you in more detail.

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Support for grandparents

Anyone who is involved in your child’s care needs to understand PKU. As well as school staff, families will often rely on grandparents to look after their children. It is important that grandparents know what PKU is and why their grandchild needs to follow a low protein diet, as well as the importance of taking their protein substitute.

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Having this knowledge will help them to support managing your child’s PKU. It may be helpful to ask them to come along to your child’s hospital/clinic appointment to ask any questions they may have and to develop their understanding of PKU. The Introduction to Phenylketonuria is a good way for them to gain a better understanding of PKU. This guide will help to reinforce the message that staying on a low protein diet keeps phe levels in recommended ranges, which allows for normal development and a healthy life.


Sometimes, it may be difficult for grandparents to feel like they are not able to give their grandchild the usual “treats” and other foods. If they are good cooks and bakers, ask them to try out low protein recipes so they can become familiar with low protein cooking and the selection of foods that are available to children with PKU. Perhaps they could become the nominated low protein birthday cake maker!

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Learn more about PKU

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