Introduction 

Phenylketonuria (PKU) is a rare genetic condition that's present from birth (congenital).

The body is unable to break down a substance called phenylalanine (see below), which builds up in the blood and brain. High levels of phenylalanine can damage the brain.

PKU is treated with a special low-protein diet, which reduces the levels of phenylalanine in the body and prevents brain damage.

PKU doesn't usually cause any symptoms if treatment is started early. If PKU isn't treated, damage to the brain and nervous system can lead to:

  • learning disabilities
  • behavioural difficulties
  • epilepsy 

High levels of phenylalanine can also cause eczema.  

Read more about the symptoms of phenylketonuria.

Phenylalanine

Phenylalanine is an amino acid (a "building block" of protein). The phenylalanine in your body comes from protein in your diet, particularly high-protein foods, such as:

  • meat
  • fish
  • eggs
  • cheese
  • milk

What happens?

The body takes in phenylalanine from the digestion of protein in the gut. Once it's entered the body, phenylalanine is either used to make proteins or removed by an enzyme (protein) called phenylalanine hydroxylase.

In people with PKU, phenylalanine hydroxylase doesn't work properly due to a genetic mutation (alteration in their DNA). As a result, phenylalanine levels in the blood and other tissues rise.

Read more about the causes of phenylketonuria.

Screening for PKU

All babies born in the UK and other developed countries are routinely screened for high phenylalanine levels. This is done using the heel prick test and is carried out during the first week of a baby’s life.

During the test, a blood sample is taken from the baby’s heel to test for a number of rare but serious conditions, including PKU.

If phenylketonuria is picked up, further tests will be arranged to confirm the diagnosis.

Read more about diagnosing phenylketonuria.

Treating PKU

Without treatment early in life, most people with PKU will develop severe learning difficulties and will require care throughout their lives.

The main treatment for PKU is a low-protein diet that completely avoids high-protein foods – such as meat, eggs and dairy products – and controls the intake of many other foods, such as potatoes and cereals.

In addition, people with PKU must take an amino acid supplement to ensure they're getting all of the nutrients required for normal growth and good health.

There are also a number of specially designed low-protein versions of popular products – such as flour, rice and pasta – specifically designed for people with PKU and related conditions to incorporate into their diets. Many of these are available on prescription.

If a high phenylalanine level is confirmed, a baby will immediately be started on a low-protein diet and amino acid supplements. Phenylalanine levels in the blood are regularly monitored by collecting blood from a finger prick onto a special card and sending it to a laboratory.

As long as a person with PKU sticks to a low-protein diet throughout childhood and their phenylalanine levels stay within certain limits, they'll remain well and their natural intelligence will be unaffected.

Adults with PKU

It's generally accepted that adults with PKU function best while on a low-protein diet.

However, unlike in children, high phenylalanine levels aren't known to cause permanent brain damage in adults with PKU.

For this reason, clinicians differ in opinion over whether it's strictly necessary for adults with phenylketonuria to continue with the low-protein diet for the rest of their life.

Some adults with PKU find it difficult to follow the low-protein diet and return to a normal diet. As a result, some may find that they don't function as well  for example, they may lose concentration or have a slower reaction time. Others don't seem to have any problems and lead a normal life.

Any adverse effects caused by coming off the low-protein diet can usually be reversed by going back on the diet.

Anyone who attempts to come off the diet should be supported by their clinician and have a regular follow-up to monitor their condition for any complications that might arise.

For women with PKU, it's essential that they return to a strict diet if they're considering becoming pregnant, because high phenylalanine levels can harm an unborn child.

Read more about treating phenylketonuria.




All newborn babies have a blood test to check for PKU 

How common is phenylketonuria?

Phenylketonuria (PKU) is rare. It's estimated to affect 1 in every 10,000 babies born in the UK.

Both sexes are affected equally by PKU. The condition is slightly more common in white people, particularly those of Celtic descent (Scottish, Irish and Welsh). It's very rare in black and Japanese people.

Page last reviewed: 02/10/2014

Next review due: 02/10/2016