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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:
High levels of phenylalanine can also cause eczema.
Read more about the symptoms of phenylketonuria.
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.
PKU is rare. 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.
All babies born in 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.
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.
Most babies with phenylketonuria (PKU) appear healthy at birth. Symptoms usually only develop due to complications that arise if the condition is not treated properly.
If treatment is started within the first three weeks of life, people with PKU should not experience the severe learning difficulties associated with the condition.
Some children and adults with PKU experience mild to moderate problems with attention, planning, or problem solving. These problems are usually related to an increased phenylalanine level in the blood and will improve with stricter dietary treatment.
There is some evidence that adults with PKU may be more likely to develop mental health problems at some point compared with the population at large.
The most widely reported conditions in people with PKU are:
It is not clear whether the increased risk of developing these conditions is related to physical problems, such as a reduction in the neurotransmitters (brain chemicals) that affect mood, or psychological problems, such as worrying about the PKU.
Read more about treating phenylketonuria.
A wide range of symptoms can occur if treatment for PKU is not started or is delayed. The most common is a learning disability. Usually, the more that treatment is delayed, the more severe the disability becomes.
Intelligence is measured using a test known as the intelligence quotient (IQ) test. A person with average intelligence will score 100 in an IQ test (have an IQ of 100). If a person with severe PKU is never treated, they will usually have an IQ score of less than 50 and will require a lot of help with everyday activities.
Even if PKU is not diagnosed during newborn screening, it is worth starting dietary treatment later in life as it is sometimes possible to reverse some of the decline in intelligence. For example, in one reported case, a child’s treatment was delayed until they were two years old, they went from having a moderate learning disability to having average intelligence.
Other symptoms of untreated PKU include:
Phenylketonuria (PKU) is a genetic condition that is passed to a baby by its parents.
PKU is caused by a genetic mutation (change to the DNA) in the human phenylalanine hydroxylase (PAH) gene.
There are many different types of mutations that can affect the PAH gene, all of which result in the body not producing a functioning PAH enzyme.
Enzymes are proteins that speed up and control chemical reactions. The lack of the PAH enzyme means that a chemical called phenylalanine, which is found in some foods, cannot be processed properly by the body.
All the genes in your body come in pairs. You receive one half of the pair from your mother and the other half from your father.
The mutation that causes PKU is an autosomal recessive mutation. This means you need to receive two mutated copies of the gene (one from your mother and one from your father) in order to develop the condition.
You will not develop PKU if you only receive one mutated copy of the gene from one of your parents, but you will carry the mutated gene.
If you are a carrier of the mutated gene and you conceive a baby with a partner who is also a carrier, there is a:
Phenylketonuria (PKU) is diagnosed in newborn babies after a routine blood test.
All newborn babies are routinely screened for PKU using a blood test that's usually carried out about a week after birth.
A sample of your baby’s blood will be taken using a blood spot or heel prick test (sometimes known as a Guthrie test) and sent to a laboratory where levels of phenylalanine will be measured. If levels are unusually high, a second blood sample will be taken to confirm the diagnosis.
If the second test confirms high phenylalanine levels, your baby will be referred to a doctor who specialises in treating cases of PKU. You will also be referred to a dietician, who will advise you about changes that need to be made to your baby’s diet.
Read about treating phenylketonuria for more information.
If a diagnosis of PKU is confirmed, your child will need regular blood tests to measure levels of phenylalanine in their blood and assess how well they are responding to treatment.
It is recommended that children who are:
Someone with PKU will usually need regular blood tests throughout their life.
It may be possible for you to receive training so you can carry out your child’s blood tests, or test yourself if you have PKU. This will help to make testing more convenient.
Phenylketonuria (PKU) can be successfully treated with a low-protein diet and dietary supplements.
Your dietician will draw up a detailed dietary plan for your child that can be revised as your child grows and their needs change.
For more detailed information and advice, the National Society for Phenylketonuria (NSPKU) website has a dietary information booklet that is available to download. The NSPKU is the UK’s leading charity for people with PKU.
The NSPKU recommends a ‘traffic light’ approach to diet, where foods are categorised as follows:
Examples of green foods include:
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. Many of these special foods are available on prescription, so you will not have to pay the full price for them.
Examples of amber foods include:
Examples of red foods include:
There is some disagreement about whether a low-protein diet should be continued by adults. Some experts believe the diet should be for life, while others feel that patients should decide for themselves after a discussion with their doctor.
As well as sticking to a low-protein diet, people with PKU will also have to take regular dietary supplements. These contain essential nutrients absent from the low-protein diet.
In addition to vitamins and minerals, the supplements contain a mixture of chemicals called amino acids (not phenylalanine) that are essential for the body to grow and remain healthy.
Supplements are available in various different forms, including powders, drinks and tablets, and need to be taken three or four times a day.
People with PKU also have to avoid food products that contain aspartame, as it is converted into phenylalanine in the body. Aspartame is a sweetener found in:
All food products that contain aspartame or a related product should be clearly labelled.
There are also medications that contain aspartame, such as some cold and flu remedies for children. It is a legal requirement for any medication that contains aspartame to state it on the patient information leaflet that comes with the medication.
Important: Our website provides useful information but is not a substitute for medical advice. You should always seek the advice of your doctor when making decisions about your health.