Phenylketonuria (PKU) is a rare genetic condition that is present from birth. The body is unable to break down a substance called phenylalanine, 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 – but if it 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.
What happens?
The body takes in phenylalanine in the form of protein in food. Phenylalanine is normally broken down in the body by an enzyme called phenylalanine hydroxylase.
In people with PKU, this enzyme does not work properly because of a genetic mutation (alteration in their DNA). As a result, phenylalanine levels in their blood, and other tissues, rise.
Read more about the causes of phenylketonuria.
How common is it?
PKU is rare – it is 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 is very rare in black and Japanese people.
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 constant care.
The main treatment for PKU is a low-protein diet that avoids foods such as meat, eggs and dairy products, and controls the intake of many other foods (such as potatoes and cereals). In addition, patients must take an amino acid supplement to ensure that they are getting all of the nutrients required for normal growth and good health.
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 will remain well and their natural intelligence will be unaffected.
In adults, unlike children, high phenylalanine levels are not known to cause permanent damage to the brain. Things such as concentration and reaction time may be affected, but this can be reversed with a low-protein diet. Some adults remain on the diet because they feel that they function better when their phenylalanine levels are lower.
However, there are many adults who return to a normal diet and the resulting high phenylalanine level causes them no obvious problems.
Read more about the treatment of phenylketonuria.
