Understanding PKU

What is PKU?

PKU is a rare, inherited metabolic condition which affects the way protein is broken down in the body. Protein is made up of individual building blocks called amino acids, which are usually separated by enzymes (acting like chemical scissors).

In PKU there is a lack of the enzyme phenylalanine hydroxylase (PAH), which usually breaks down the amino acid phenylalanine (Phe) and turns it into tyrosine (Tyr), another amino acid. Because this process is not working appropriately, high levels of Phe build up in the blood and brain.

If left unmanaged, the high level of Phe becomes toxic and can cause irreversible brain damage and developmental delay. It is therefore really important to closely follow the management plan as advised by your specialist team (see section on the management plan below). 

If left unmanaged, the high level of Phe becomes toxic and can cause irreversible brain damage and developmental delay. It is therefore really important to closely follow the management plan as advised by your specialist team (see section on the management plan below). 

How did I get PKU?

PKU is a genetic condition, inherited from both parents. There is nothing that could have been done to prevent you from having PKU, and no way of knowing that you had the condition until you received the positive newborn screening result from your hospital.

Everyone has two copies of the gene that make up the PAH enzyme. Each parent of a person with PKU will be a carrier of the genetic mutation associated with the condition, meaning that one copy of the gene is working and the other is not (you only need one copy to be working for the enzyme to function). Around one in fifty to one hundred people in the general population in the UK and Ireland are carriers of the PKU gene.

A person with PKU will inherit one copy of the non-working PAH gene from each parent. When both parents are carriers, there is a one in four (25%) chance of this happening with each pregnancy. 

What is the management plan for PKU?

To keep Phe levels in a safe range during infancy, childhood and into adulthood; the amount of Phe provided in the diet (in the form of protein) must be restricted but not completely taken away: the body cannot make Phe and needs a small amount for essential functions. Whilst following a special low protein diet, all of the other amino acids are then put back into the diet using a special protein substitute which must be taken every day.

Day-to-day management

Traffic light system

The special low protein diet for PKU is commonly described as using a ‘traffic light’ system.

RED: Do not eat – too high in protein

Foods such as meat, chicken, fish, eggs, milk, cheese, nuts, tofu, normal bread and pulses usually contain too much protein to be included in a PKU diet, except in very mild cases where the dietary restriction required is minimal.

AMBER: Contain some protein – measure for ‘exchanges’

This group of foods contains a lower amount of protein, and they are therefore useful for providing essential Phe in the diet. Examples of foods commonly used include rice, pasta, potato, sweetcorn and breakfast cereals – although food labels and special lists provided by your Dietitian mean that you can use a wider range of foods to suit your preferred diet. The amount of Phe / protein that you can tolerate will be monitored by your specialist team, and is measured in ‘exchanges’.

1 exchange = 50mg Phe = 1g protein

GREEN: Very low in protein – eat freely

These foods are very low in protein, and can be eaten in normal quantities thereby providing energy and variety in the PKU diet. They can be split into 2 groups. The first is those foods naturally low in Phe / protein such as fruits and some vegetables, fats, oils, herbs, spices and sugars. Your dietitian will give you lists of these foods. You will also be taught how to read food labels so that you can identify foods that are naturally low in protein on the supermarket shelves.

The second group consists of specially produced low-protein prescription foods, which are designed to be used as a low-protein alternative to some everyday foods. They include low-protein flour replacement, pasta, milk substitute, cereals, breads and biscuits. More information about low-protein foods can be found in the products section of the website.

Protein substitute

The protein substitute is an essential part of the PKU diet; and must be taken every day throughout your life. It contains all of the amino acids needed by the body (with the exception of Phe), vitamins, minerals and other essential nutrients. They come in many different forms, and your dietitian will guide you through which is appropriate for you, how much to take, and how often it needs to be taken. Spreading the protein substitute throughout the day is vital and also helps to keep Phe levels down. More information about protein substitutes can be found in the products section of the website.

It is important to always make sure that you have a good supply of protein substitute, and that it is in date. Try to make a monthly stock check part of your routine and contact your dietitian or home delivery company if you are running low. Nutricia Metabolics provide a convenient; free of charge home delivery service for protein substitute and low protein prescription foods. If you would like to use this service, speak to your dietitian and check out the link here.


Monitoring forms a very important part of the management of PKU. You will be taught how to take a bloodspot sample, which is then posted to the lab for analysis and shows the levels of Phe in the blood. Regular testing allows your dietitian to make necessary adjustments to the diet.

Try to make the blood monitoring part of your routine and let your specialist team know if you are running low on supplies. It really helps you get a clear picture of how well you are doing on your PKU diet.

Phe levels sometimes fluctuate due to periods of illness. However, make sure that you take your protein substitute frequently if possible as this can help to minimise any ups and downs. Your dietitian will also regularly update you with blood level results, and he/she can answer any questions that you may have.

Your specialist team will also undertake further monitoring when they see you in the outpatient clinic.

Managing illness

During illness, infection and/or loss of appetite, the body breaks down its own protein to use as energy. This releases Phe into the bloodstream and causes a sharp rise in blood Phe levels, which will return to normal once the illness has resolved.
If you are not well, aim to drink plenty of fluids including the protein substitute if possible (smaller, more frequent doses are often better). Extra calories can also be provided by using carbohydrate containing drinks, e.g. sugar containing squash or juice. Contact your specialist team for advice and for more information on what to do when you are not feeling well.

Where can I find more information and support?

  • Your specialist team: they should always be your first port of call for more information and any questions that you may have
  • The National Society for Phenylketonuria (NSPKU). Check out their website for more information on PKU, and for information on how to register as a member. A regular magazine, annual conference and organised events all help to keep you in touch with other families and what is going on in the PKU community
  • Also check out our social page for links to our Facebook, Instagram and Twitter 

NOTE: The dietary management for PKU varies for each person so all information presented here is for guidance purposes only. The information provided on this page is in no way intended to replace the care, advice and medical supervision of your healthcare professional. Your own dietitian and/or doctor will advise you on all aspects relating to management of PKU for you and your family. Always consult your healthcare professional before making any changes to your low-protein diet.