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


High levels of phenylalanine can also cause eczema.  


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:






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.

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.

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.




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.


 Symptoms of phenylketonuria   

Most babies with phenylketonuria (PKU) appear healthy at birth.

Symptoms usually only develop because of complications that arise if the condition isn't treated properly.

 Treated PKU 

If treatment is started within the first three weeks of life, people with PKU shouldn't 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 can be related to an increased phenylalanine level in the blood and may improve with stricter dietary treatment.

There's 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 – 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's 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 PKU and the low-protein diet. 

 Untreated PKU 

A wide range of symptoms can occur if treatment for PKU isn't started or is delayed. The most common is a learning disability. Usually, the more treatment is delayed, the more severe the disability becomes.

Even if PKU isn't diagnosed during newborn screening, it's worth starting dietary treatment later in life, because it's 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 condition (phenylalanine is involved in the body's production of melanin, the pigment responsible for skin and hair color)


recurrent vomiting

jerking movements in arms and legs



musty smell on the breath, skin and urine

Causes of phenylketonuria  

Phenylketonuria (PKU) is a genetic condition that's 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. Lack of the PAH enzyme means that a chemical called phenylalanine, which is found in some foods, can't 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) to develop the condition.

You won't develop PKU if you only receive one mutated copy of the gene from one of your parents, but you'll carry the mutated gene.

If you're a carrier of the mutated gene and you conceive a baby with a partner who's 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


 Diagnosing phenylketonuria  

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

 Newborn screening   

All newborn babies are routinely s  creened 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 phenylalanine levels 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 PKU. You'll also be referred to a dietician, who will advise you about changes that need to be made to your baby’s diet.

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're responding to treatment. Only a drop of blood is needed, and it can be collected at home and sent to the hospital by post.

It's 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 to have regular blood tests throughout their life. 

You may be able to have training so that you can carry out your child’s blood tests, or be able to test yourself if you have PKU. This will make testing more convenient.


Treating phenylketonuria  

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.

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 shouldn't be eaten to excess

amber foods – contain moderate levels of protein and must be carefully weighed; they'll 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

low-fat and low-protein butter and margarine 

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 won't have to pay full price for them.

Amber foods

Examples of amber foods include: 

potatoes, including chips

cereals, including oats and cornflakes

baked beans

milk – a specially designed formula can be used as a milk substitute for babies; a low-protein milk is also available for older people 



Red foods

Examples of red foods include:

all types of meat




nuts and seeds

flour-based foods – such as bread, cakes and biscuits

Quorn and tofu

There's some disagreement about whether a low-protein diet should be continued by adults. Some experts believe that the diet should be for life, while others feel that people should decide for themselves after a discussion with their doctor.

If you choose to return to a normal diet, it's important that you still attend regular follow-up appointments, so you can be monitored for any complications that might arise.

The appointments will also keep you informed about any relevant new evidence, and ensure that you're supported should you choose to go back on to a low-protein diet.

Dietary supplements

If you have PKU, as well as sticking to a low-protein diet, you 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), which replace the protein that's missing from the diet and are essential for the body to grow and remain healthy.

Supplements are available in various 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, because it's 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 children's cold and flu remedies. It's a legal requirement for any medication that contains aspartame to state it on the patient information leaflet that comes with the medication.

Phenylketonuria and pregnancy

Women with PKU have to take particular care during pregnancy, because high levels of phenylalanine can damage the unborn baby.

Providing that phenylalanine levels are strictly controlled during pregnancy, problems can be avoided and there's no reason why a woman with PKU shouldn't be able to have a normal, healthy baby.

It's recommended that all women with PKU plan their pregnancies carefully. You should aim to follow a strict diet and monitor your blood twice a week before becoming pregnant. It's best to try to conceive once phenylalanine levels are within the target range for pregnancy.

During pregnancy, you'll be asked to provide blood samples three times a week and will be in frequent contact with your dietician. As soon as your baby is born, phenylalanine control can be relaxed and there's no reason why, if you have PKU, you can't breastfeed your baby.

Contact your PKU doctors and dieticians as soon as possible if you become pregnant when your phenylalanine levels aren't adequately controlled. If your phenylalanine levels can be brought under control within the first few weeks of your pregnancy, the risk of damage to the baby should be small. However, your pregnancy will need to be monitored very carefully.