Phenylketonuria (PKU) is a genetic condition that is passed to a baby by its parents.

Genetic mutation

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.

Autosomal recessive mutation

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:

• 1 in 4 chance that your baby will receive a pair of normal genes
• 1 in 2 chance that your baby will receive one normal gene and one mutated gene and become a carrier of PKU without developing symptoms
• 1 in 4 chance that your baby will receive a pair of mutated genes and develop PKU

Phenylketonuria (PKU) is diagnosed in newborn babies after a routine blood test.

Newborn screening

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.

Regular blood tests

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:

• six months of age or younger should have their blood tested once a week
• between six months and four years of age should have their blood tested once every two weeks
• over four years of age, and all adults, should have their blood tested once a month

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) 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.

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 which 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 will remain well and their natural intelligence will be unaffected.

Adults with PKU

It is generally accepted that adults with PKU probably function best while on the low-protein diet.

However, unlike children, high phenylalanine levels are not known to cause permanent damage to the brain in adults with PKU.

For this reason, clinicians differ in opinion over whether it is 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 attempt to come off it. They may find that as a result, they don’t function as well – for example, they may lose concentration and have a slower reaction time. Others don’t seem to have any problems.

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

Any patients who attempt to come off the diet should be supported by their clinician and should be followed up regularly, to monitor their condition for any complications that might arise.

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.

Treated PKU

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:

• depression – feelings of extreme sadness that last for weeks or months
• anxiety disorders – mild or severe feelings of unease, such as worry or fear, that can be mild or severe
• phobias, particularly agoraphobia (intense fear of crowded places or enclosed public places)
• low self-esteem

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.

Untreated PKU

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:

• behavioural difficulties, such as frequent temper tantrums and episodes of self-harm
• fairer skin, hair and eyes than siblings without the disease (as phenylalanine is involved in the body’s production of melanin, the pigment responsible for skin and hair color)
• eczema
• recurrent vomiting
• jerking movements in arms and legs
• tremors
• epilepsy
• musty smell on the breath, skin and urine

Phenylketonuria (PKU) can be successfully treated with a low-protein diet and dietary supplements.

Low-protein diet

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:

• green foods – safe to eat because they contain low levels of protein, although they should not be eaten to excess
• amber foods – contain moderate levels of protein and must be carefully weighed; they will be eaten in different quantities according to your blood phenylalanine level
• red foods – high protein foods that should never be eaten under any circumstances

Green foods

Examples of green foods include:

• fresh fruit – with the exception of bananas, figs and dates
• most vegetables – with the exception of asparagus, bean sprouts, broccoli, Brussels sprouts, and cauliflower

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.

Amber foods

Examples of amber foods include:

• cornflakes
• chips
• baked beans
• milk – a specially designed formula can be used as a milk substitute for babies and children
• rice

Red foods

Examples of red foods include:

• all types of meat
• eggs
• fish
• cheese

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.

If you do choose to return to a normal diet, it is important that you still attend regular follow-up appointments so you can be monitored for any complications that might arise, be kept informed of any relevant new evidence and be supported to go back on to a low-protein diet if you so choose.

Dietary supplements

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.

Aspartame

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:

• sugar substitutes, such as artificial sweeteners often used in tea and coffee
• diet versions of fizzy drinks
• chewing gum
• squashes and cordials
• some alcopops

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.