Phenylketonuria (PKU) is a rare but potentially serious inherited disorder.
Our bodies break down the protein in foods, such as meat and fish, into amino acids, which are the «building blocks» of protein.
These amino acids are then used to make our own proteins. Any amino acids that are not needed are broken down further and removed from the body.
People with PKU cannot break down the amino acid phenylalanine, which then builds up in their blood and brain. This can lead to brain damage.
At around 5 days old, babies are offered newborn blood spot screening to test for PKU and many other conditions. This involves pricking your baby’s heel to collect drops of blood to test.
If PKU is confirmed, treatment will be given straight away to reduce the risk of serious complications. Treatment includes a special diet and regular blood tests.
With early diagnosis and the correct treatment, most children with PKU are able to live healthy lives.
About 1 in 10,000 babies born in the UK has PKU.
Symptoms of PKU
PKU does not usually cause any symptoms if treatment is started early.
Without treatment, PKU can damage the brain and nervous system, which can lead to learning disabilities.
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 who do not have the condition (phenylalanine is involved in the body’s production of melanin, the pigment responsible for skin and hair colour)
- repeatably being sick
- jerking movements in arms and legs
- musty smell to the breath, skin and urine
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 the nutrients required for normal growth and good health.
There are also several low-protein versions of common foods (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 are regularly monitored by collecting blood from a finger prick on to a special card and sending it to a laboratory.
A dietitian will create a detailed dietary plan for your child that can be revised as your child grows and their needs change.
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.
People with PKU must also avoid food products that contain aspartame, as it’s converted into phenylalanine in the body.
Aspartame is a sweetener found in:
- sugar substitutes such as the artificial sweeteners often used in tea and coffee
- diet versions of fizzy drinks
- chewing gum
- squash and cordial
- some alcopops
All food products that contain aspartame or a related product should be clearly labelled.
There are also medicines that contain aspartame, such as some children’s cold and flu remedies.
It’s a legal requirement for any medicine that contains aspartame to state it on the patient information leaflet that comes with the medicine.
Regular blood tests
A child with phenylketonuria 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.
You may be able to have training so you can do your child’s blood tests, or be able to test yourself if you have PKU. This will make testing more convenient.
It’s recommended that children who are:
- 6 months of age or younger should have their blood tested once a week
- between 6 months and 4 years of age should have their blood tested once every 2 weeks
- over 4 years of age should have their blood tested once a month
Someone with PKU will usually need to have regular blood tests throughout their life.
How PKU is inherited
The genetic cause (mutation) responsible for PKU is passed on by the parents, who are usually carriers and do not have any symptoms of the condition themselves.
The way this mutation is passed on is known as autosomal recessive inheritance. This means a baby needs to receive 2 copies of the mutated gene to develop the condition – 1 from their mother and 1 from their father.
If the baby only receives 1 affected gene, they’ll just be a carrier of PKU.
If you’re a carrier of the altered gene and you have a baby with a partner who’s also a carrier, your baby has:
- a 1 in 4 chance of inheriting the condition
- a 1 in 2 chance of being a carrier of PKU
- a 1 in 4 chance of receiving a pair of normal genes
Adults with PKU
Many adults with PKU find they function best while on a low-protein diet. The current advice is for people with PKU to remain on a low-protein diet for life.
Unlike in young children, there is not yet any evidence that high phenylalanine levels cause any permanent brain damage in adults with PKU.
Some adults with PKU may have higher phenylalanine levels because they find it difficult to follow the low-protein diet or have returned to a normal diet.
As a result, they may find they do not function as well. For example, they may lose concentration or have a slower reaction time.
These adverse effects can usually be reversed by going back on to a strict diet to bring the phenylalanine levels down again.
Anyone who returned to a normal diet should still be supported by their clinicians 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, as high phenylalanine levels can harm an unborn child.
PKU and pregnancy
Women with PKU must take particular care during pregnancy, as high levels of phenylalanine can damage their unborn baby.
Provided that phenylalanine levels are strictly controlled during pregnancy, problems can be avoided and there’s no reason why a woman with PKU should not 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 low-protein 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 3 times a week and will be in frequent contact with a dietitian.
As soon as your baby is born, phenylalanine control can be relaxed and there’s no reason why you cannot breastfeed your baby.
Contact your PKU doctor and dietitian as soon as possible if you become pregnant when your phenylalanine levels are not adequately controlled.
If your phenylalanine levels can be brought under control within the first few weeks of your pregnancy, the risk of damage to your baby should be small. But your pregnancy will need to be monitored very carefully.
Information about you
If you or your child has PKU, your clinical team will pass on information about you to the National Congenital Anomaly and Rare Diseases Registration Service (NCARDRS).
This helps scientists look for better ways to prevent and treat this condition. You can opt out of the register at any time.
Page last reviewed: 3 December 2019
Next review due: 3 December 2022
Phenylketonuria (commonly known as PKU) is an inherited disorder that increases the levels of a substance called phenylalanine in the blood. Phenylalanine is a building block of proteins (
The signs and symptoms of PKU vary from mild to severe. The most severe form of this disorder is known as classic PKU. Infants with classic PKU appear normal until they are a few months old. Without treatment, these children develop permanent intellectual disability. Seizures, delayed development, behavioral problems, and psychiatric disorders are also common. Untreated individuals may have a musty or mouse-like odor as a side effect of excess phenylalanine in the body. Children with classic PKU tend to have lighter skin and hair than unaffected family members and are also likely to have skin disorders such as eczema.
Less severe forms of this condition, sometimes called variant PKU and non-PKU hyperphenylalaninemia, have a smaller risk of brain damage. People with very mild cases may not require treatment with a low-phenylalanine diet.
Babies born to mothers who have PKU and uncontrolled phenylalanine levels (women who no longer follow a low-phenylalanine diet) have a significant risk of intellectual disability because they are exposed to very high levels of phenylalanine before birth. These infants may also have a low birth weight and grow more slowly than other children. Other characteristic medical problems include heart defects or other heart problems, an abnormally small head size (
Phenylketonuria (PKU) is a rare genetic condition that causes an amino acid called phenylalanine to build up in the body. Amino acids are the building blocks of protein. Phenylalanine is found in all proteins and some artificial sweeteners.
Phenylalanine hydroxylase is an enzyme your body uses to convert phenylalanine into tyrosine, which your body needs to create neurotransmitters such as epinephrine, norepinephrine, and dopamine. PKU is caused by a defect in the gene that helps create phenylalanine hydroxylase. When this enzyme is missing, your body can’t break down phenylalanine. This causes a buildup of phenylalanine in your body.
Babies in the United States are screened for PKU shortly after birth. The condition is uncommon in this country, only affecting about 1 in 10,000 to 15,000 newborns each year. The severe signs and symptoms of PKU are rare in the United States, as early screening allows treatment to begin soon after birth. Early diagnosis and treatment can help relieve symptoms of PKU and prevent brain damage.
PKU symptoms can range from mild to severe. The most severe form of this disorder is known as classic PKU. An infant with classic PKU may appear normal for the first few months of their life. If the baby isn’t treated for PKU during this time, they’ll start to develop the following symptoms:
- tremors, or trembling and shaking
- stunted growth
- skin conditions such as eczema
- a musty odor of their breath, skin, or urine
If PKU isn’t diagnosed at birth and treatment isn’t started quickly, the disorder can cause:
- irreversible brain damage and intellectual disabilities within the first few months of life
- behavioral problems and seizures in older children
A less severe form of PKU is called variant PKU or non-PKU hyperphenylalaninemia. This occurs when the baby has too much phenylalanine in their body. Infants with this form of the disorder may have only mild symptoms, but they’ll need to follow a special diet to prevent intellectual disabilities.
Once a specific diet and other necessary treatments are started, symptoms start to diminish. People with PKU who properly manage their diet usually don’t show any symptoms.
PKU is an inherited condition caused by a defect in the PAH gene. The PAH gene helps create phenylalanine hydroxylase, the enzyme responsible for breaking down phenylalanine. A dangerous buildup of phenylalanine can occur when someone eats high-protein foods, such as eggs and meat.
Both parents must pass on a defective version of the PAH gene for their child to inherit the disorder. If just one parent passes on an altered gene, the child won’t have any symptoms, but they’ll be a carrier of the gene.
Since the 1960s, hospitals in the United States have routinely screened newborns for PKU by taking a blood sample. A doctor uses a needle or lancet to take a few drops of blood from your baby’s heel to test for PKU and other genetic disorders.
The screening test is performed when the baby is one to two days old and still in the hospital. If you don’t deliver your baby in a hospital, you’ll need to schedule the screening test with your doctor.
Additional tests may be performed to confirm the initial results. These tests search for the presence of the PAH gene mutation that causes PKU. These tests are often done within six weeks after birth.
If a child or adult shows symptoms of PKU, such as developmental delays, the doctor will order a blood test to confirm the diagnosis. This test involves taking a sample of blood and analyzing it for the presence of the enzyme needed to break down phenylalanine.
People with PKU can relieve their symptoms and prevent complications by following a special diet and by taking medications.
The main way to treat PKU is to eat a special diet that limits foods containing phenylalanine. Infants with PKU may be fed breast milk. They usually also need to consume a special formula known as Lofenalac. When your baby is old enough to eat solid foods, you need to avoid letting them eat foods high in protein. These foods include:
To make sure that they still receive an adequate amount of protein, children with PKU need to consume PKU formula. It contains all the amino acids that the body needs, except for phenylalanine. There are also certain low-protein, PKU-friendly foods that can be found at specialty health stores.
People with PKU will have to follow these dietary restrictions and consume PKU formula throughout their lives to manage their symptoms.
It’s important to note that PKU meal plans vary person to person. People with PKU need to work closely with a doctor or dietitian to maintain a proper balance of nutrients while limiting their intake of phenylalanine. They also have to monitor their phenylalanine levels by keeping records of the amount of phenylalanine in the foods they eat throughout the day.
Some state legislatures have enacted bills that provide some insurance coverage for the foods and formulas necessary to treat PKU. Check with your state legislature and medical insurance company to find out if this coverage is available for you. If you don’t have medical insurance, you can check with your local health departments to see what options are available to help you afford PKU formula.
The United States Food and Drug Administration (FDA) recently approved sapropterin (Kuvan) for the treatment of PKU. Sapropterin helps lower phenylalanine levels. This medication must be used in combination with a special PKU meal plan. However, it doesn’t work for everyone with PKU. It’s most effective in children with mild cases of PKU.