What else is it called?

  • Folling’s disease
  • PAH deficiency
  • Phenylalanine Hydroxylase Deficiency
  • PKU

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What causes it?

PKU stands for Phenylketonuria. PKU is an inherited metabolic disorder.

Phenylketonuria means there is too much phenylalanine in the blood and too many phenylketones in the urine.

PKU affects the way your baby breaks down protein. Many foods contain protein. The body needs protein for growth and repair.

Protein is broken down into amino acids (building blocks of protein) by enzymes (which are like chemical scissors). Enzymes then further break the amino acids into smaller parts.

Metabolism refers to the chemical processes that occur inside the cells of the body.

In PKU, the body lacks an enzyme called phenylalanine hydroxylase. This means that the body is unable to break down an amino acid called phenylalanine. As a result, levels of phenylalanine build up in the blood and brain.

How common is it?

About 1 in 10,000 babies born in the UK has PKU.

What are the signs and symptoms?

The build-up of phenylalanine can lead to brain damage, with learning and behavioural difficulties. Early management can prevent brain damage and learning difficulties.

How is it diagnosed?

PKU is diagnosed by newborn screening. High levels of phenylalanine are found in the blood.

Can it be treated?

PKU is managed with the following special diet:

  • Limited high protein foods
  • Measured amounts of phenylalanine (protein) containing foods
  • A protein substitute
  • Low protein foods
  • Avoiding aspartame

These foods are high in phenyalanine (protein) and must be avoided: meat, fish, eggs, cheese, bread, pasta, nuts, seeds, soya, Quorn and tofu. Any food or drink containing aspartame should also be avoided.

In babies, a restricted amount of phenylalanine (protein) is given from breast milk or measured amounts of infant formula. The amount given will be monitored regularly by your specialist metabolic dietitian.

Protein substitute is essential for metabolic control. It will help to meet your body’s protein, energy, vitamin and mineral requirements. It is available on prescription.

There are many low protein foods. These include fruit, many vegetables and prescribable low protein foods such as bread and pasta.

They provide:

  • a source of energy
  • variety in the diet

During any childhood illness, catabolism or protein breakdown occurs, causing blood phenylalanine levels to increase. It is important to continue with the usual diet as much as possible.

If you have PKU you will need to be monitored frequently. This includes:

  • Frequent blood tests to check phenylalanine
  • Height and weight
  • Developmental checks
  • Diet and medications are adjusted according to age, weight, and phenylalanine levels

Take home messages:

  • PKU is a serious inherited metabolic disorder
  • Damage can be prevented with a diet low in phenylalanine and a protein substitute
  • Remember to always give the correct amount of measured phenylalanine and protein substitute as prescribed by your metabolic centre
  • Regular blood spot tests are essential to monitor blood phenylalanine levels

Helpful hints:

  • Always ensure you have a good supply of your dietary products and protein substitute and that they are in date.
  • Your dietary products and protein substitute are prescribed by your GP. These are obtained via a pharmacy or home delivery.
  • Always ensure you have sufficient blood testing equipment and send samples on a regular basis.
  • Medications to control fever should be given as normally recommended – always keep supplies available.

Do my family need to be tested?

Humans have chromosomes composed of DNA. Genes are pieces of DNA that carry the genetic instruction. Each chromosome may have several thousand genes.

The word mutation means a change or error in the genetic instruction. We inherit particular chromosomes from the egg of the mother and sperm of the father.

The genes on those chromosomes carry the instruction that determines characteristics, which are a combination of the parents. PKU is an inherited condition. There is nothing that could have been done to prevent your baby from having PKU.

Everyone has a pair of genes that make the phenylalanine hydroxylase enzyme. In children with PKU, neither of these genes works correctly. These children inherit one non-working PKU gene from each parent.

Parents of children with PKU are carriers of the condition. Carriers do not have PKU because the other gene of this pair is working correctly.

When both parents are carriers, in each pregnancy the risk to the baby is as follows:

  • 25% chance (1 in 4) of PKU
  • 50% chance (1 in 2) for the baby to be a carrier of PKU
  • 25% chance (1 in 4) for the baby to have two working genes and neither have PKU or be a carrier

Relevant Organisations


This information is from the TEMPLE Guides which have been adapted by the Dietitians Group of the British Inherited Metabolic Disease Group (BIMDG) and is based on the original TEMPLE written by Burgard and Wendel. The TEMPLE Guides are supported by Nutricia. Dated February 2017.

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