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What is Phenylketonuria or PKU?

Updated: Feb 21


Mother and baby with PKU

What is PKU?


PKU: A quick snapshot

Phenylketonuria or PKU for short is a rare and serious genetic condition that affects the cognitive development of 1 in 10,000 babies born in the UK.


PKU affects the body's protein metabolism in such a way that the individual cannot break down a type of amino acid called phenylalanine. In PKU, the amino acid phenylalanine is not broken down into tyrosine which means it starts to build up in the blood. This is a serious condition as a high level of phenylalanine is toxic, causing serious brain damage and cognitive development.


In healthy individuals, phenylalanine is broken down into tyrosine by the enzyme phenylalanine hydroxylase, or PAH for short.


How phenylalanine builds up in PKU:

diagram of PKU


In PKU there is the absence of this PAH enzyme meaning the body cannot break down phenylalanine. PKU treatment involves restricting the amount of phenylalanine that enters the body. Therefore, a strict PKU diet is required. These diets are complex and require a specialist metabolic dietitian's input.


Phenylalanine is found in high-protein foods but also in some bread, cereals, fizzy drinks, fruits and vegetables. Those following a PKU diet would need to seek specialist help from both their GP and a specialist metabolic dietitian. Often your medical team will prescribe you specialist formula drinks and foods and amino acid supplements to help support you.

The science of PKU metabolism in detail:


Let's look at how our bodies typically process phenylalanine.


In our liver, we have an enzyme called phenylalanine hydroxylase or PAH.

When we eat protein-rich foods our bodies break down those proteins into their amino acids (building blocks of protein), and one of those amino acids is phenylalanine. This phenylalanine is then metabolised in our liver by the PAH enzyme into another amino acid called tyrosine. This means that our bodies regulate how much phenylalanine is in our blood so that it doesn't become toxic.


Tyrosine is the amino acid that is produced from the breakdown of phenylalanine. The body needs Tyrosine as it is responsible for the following neurotransmitters:


  • Dopamine: Dopamine regulates your reward and pleasure centres. This important brain chemical is also important for memory and motor skills.


  • Adrenaline and noradrenaline: These hormones are responsible for the fight-or-flight response to stressful situations. They prepare the body to “fight” or “flee” from a perceived attack or harm.


A PKU diagnosis can range from mild to severe. The scale of PKU diagnosis will be dependent on the activity of the phenylalanine hydroxylase or PAH enzyme.

Having more or less PAH will affect how much phenylalanine the person will be able to break down.


Classical PKU is classified as the most severe form of PKU. Individuals with classical PKU will have almost zero PAH enzyme in the body requiring that individuals follow a strict daily restriction of phenylalanine in their diets and supplementation for tyrosine is also required.


How do you test for PKU?


Newborn blood test for PKU

In the UK, babies are screened for PKU in the first few days after birth. You may remember that your baby was given a heel prick blood test. This heel prick test is the blood test that screens for a number of genetic conditions - PKU is one of these.




How is PKU diagnosed?

In the UK, babies approximately 1 week after birth are screened for 9 conditions. One of these is PKU.


How did I get PKU?

PKU is a rare genetic condition that is inherited in an autosomal recessive pattern meaning that both your mother and father are carrying a copy of the mutated PKU gene. If both parents are carriers and if they have a baby they have a 1 in 4 chance of that child having PKU. Often as your mother or father only have one copy of the mutated gene it means they have no symptoms and they are just a carrier.



genetic code in PKU

How autosomal recessive genetics works:


If you're a carrier of the altered gene in PKU and you have a baby with a partner who's also a carrier, your baby has:


  • a 1 in 4 chance of inheriting the condition

  • a 1 in 2 chance of being a carrier of PKU

  • a 1 in 4 chance of receiving a pair of normal genes


Remember both you and your partner have to both be carriers of the PKU mutated gene. The gene for PKU is found at Chromosome 12q24.1



What does living with PKU mean?

Nowadays most children are screened for the condition at birth which means that effective treatment commences as soon as possible and as a result often no symptoms are experienced.


The aim of treatment in PKU is to reduce blood phenylalanine levels. If these blood phenylalanine levels are not kept within healthy levels then these children develop profound, irreversible intellectual disability and seizures.


Fortunately in the developed world symptomatic PKU is rarely seen due to newborn screening and early treatment but newborn screening is still uncommon in some regions of the Middle East, Asia, and Africa.


Treatment needs to be initiated by week 6 of life to ensure normal outcomes


Living with PKU

Thankfully living with PKU is significantly improving thanks to research innovations. There are various treatment options that range from low PKU food products to new medical treatments and gene therapeutics.


Living with PKU is lifelong and can be a burden for the individual as it requires a lifelong low phenylalanine blood level to ensure the buildup does not cause permanent toxic effects. It's important to have a good support network as it's not uncommon for children or adults to find the PKU diets burdensome and hard to stick with.


What are the PKU treatments?


Diet therapy

If you have PKU you have to follow a low phenylalanine diet as the body cannot break down phenylalanine to tyrosine. Depending on your PKU diagnosis following a PKU diet often means that the body may not have enough tyrosine so often that is added as a supplement.


How do I learn about a low PKU diet?

Following a low PKU diet is not just as simple as restricting high-protein foods such as meat, fish, and dairy. Phenylalanine is in some vegetables, bread, and cereals and each item has varying amounts.


foods to avoid in PKU

It is essential that you speak with a qualified specialist metabolic dietitian that has experience with PKU to help you set your PKU diets. The BDA has a number of freelance dietitians that could help you.


Your dietitian will set you a target amount of exchanges or amounts of phenylalanine to consume in a day.


For example here are some exchanges in fruits and vegetables:


The exchange amounts of most fruit and vegetables are calculated from their phenylalanine (but not protein) content. 1 exchange = 50 mg phenylalanine.


  • Peas (fresh frozen) 25g = 1 exchange

  • Boiled spinach 25g = 1 exchange

  • Baby spinach 25g = 1 exchange

  • Spring greens 35g = 1 exchange

  • Choi sum 35g = 1 exchange

  • Sweet Corn kernels 35g = 1 exchange

  • Corn on the cob 4cm = 1 exchange

  • Baked beans 20g = 1 exchange

  • Broad beans 20g = 1 exchange

  • Frozen mixed vegetables 30g = 1 exchange

  • Beansprouts 60g = 1 exchange

  • Broccoli 60g = 1 exchange

  • Rocket 35g = 1 exchange

  • Kale 35g = 1 exchange

  • Boiled potato 80g = 1 exchange

  • Roast potato 55g = 1 exchange

  • Chips or french fries 45g = 1 exchange

  • Jacket potato 80g = 1 exchange


Individuals following a PKU diet also have to avoid aspartame as this is converted to phenylalanine in the body. Aspartame is found in fizzy drinks and as a sweetener in some processed foods. It is best to check the labels with the support of your metabolic dietitian.


For further information on PKU diets and dietary exchanges visit the NSPKU organisation.


Non-dietary treatments for PKU

Thankfully PKU research has come a long way over the last 20 years and there are now various medications, enzyme therapies, and gene therapies that are being researched to help reduce the burden of following a PKU DIET. Speak to your GP and specialist dietitian to find out what other non-dietary treatments are suitable for your circumstances. For more detailed information visit the PKU organisation.


The Health nutritionist is a nutrition and dietetic clinic supporting individuals across a wide range of dietary conditions. Our team has specialist knowledge and many years of experience in nutrition and dietetics. If you would like to contact one of our specialist team members please complete our nutrition assessment form and we will be happy to arrange a free discovery call to advise on the nutrition consultation process.










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