Phenylketonuria (PKU) Symptoms & Causes

Here at Boston Children’s Hospital, our multidisciplinary team is ready to help you and your child with the challenges of having phenylketonuria(PKU). Learning more about PKU will help your child and whole family learn to live a healthy, active life with PKU.

What is PKU?

PKU is a genetically inherited metabolic disorder in which the body lacks the enzyme, phenylalanine hydroxylase (PAH), which is responsible for metabolizing the amino acid called phenylalanine. PAH normally breaks down phenylalanine into another amino acid called tyrosine. In people with PKU, phenylalanine builds up in the blood and then crosses into the brain, where it is toxic and causes damage.

Will my child be OK?
If your child begins treatment within in the first weeks of life, and strictly follows the low-phenylalanine diet as prescribed by your doctor, your child should be able to live a normal life and not be limited intellectually or physically.

What are the risks if it’s not treated right away?

If PKU is not treated early in infancy, there is a high risk the child will develop intellectual disabilities. The longer the child goes untreated, the higher the risk.

Is it curable?
There currently is no cure for PKU, but the condition is controllable through proper diet and supplements.

What kinds of diets do children with PKU need to adopt?

PKU is caused by the body’s inability to metabolize phenylalanine, which is found in high protein foods such as poultry, meat, eggs and dairy products. The PKU diet includes foods that are low in protein, such as fruits and vegetables, as well as low protein products made especially for people with PKU, such as low protein bread and pasta.  A special medical food that contains other amino acids but no phenylalanine is a critical part of the PKU diet.

Is there a medication that may help children with PKU?

The PAH enzyme that changes phenylalanine to tyrosine uses a biochemical co-factor called BH4, which helps the PAH enzyme make this change. Some children with PKU who are on a diet may benefit from a medication form of BH4, called Kuvan, which can lower phenylalanine levels in the blood, and may allow a patient to eat more natural sources or protein. Your child might be prescribed Kuvan  to determine if it will make the diet easier.

How common is PKU?

About one in every 15,000 babies in the United States is born with PKU.

Who is at risk for PKU?

PKU is present in almost all societies but is very rare among those of African-American heritage and certain Asian countries (e.g., Japan, Thailand, Korea). Those of Irish heritage as well as those with Turkish background have the highest risk..

Signs and symptoms

The signs and symptoms of PKU are not apparent at birth, but babies with PKU do show signs within a few months. Before signs are apparent, however, the brain may already be  damaged, so treatment needs to begin during the first weeks of life before this happens  Symptoms of PKU can include:

  • IQ loss
  • severe intellectual disabilities
  • skin rashes (eczema)
  • pale skin and blue eyes
    • Phenylalanine usually turns into melanin which gives color to hair and skin tone.
    • PKU prevents phenylalanine from turning into melanin.
  • seizures

Do symptoms go away after treatment?

If treatment is administered within the first few weeks after birth, there should be no symptoms if the diet that is prescribed is strictly followed. Symptoms might develop if the patient stops following the diet.


Q: How serious is PKU?

A: PKU is very serious if treatment is not started for within the first 2 to 3 weeks of life. Although symptoms may not be visible, exposure to high levels of phenylalanine after 2 to 3 weeks can have long lasting negative effects such as intellectual challenges. Untreated PKU can lead to intellectual disabilities. If PKU is treated right away and your child strictly follows the prescribed low-phenylalanine diet, your child should enjoy a normal quality of life.

Q: What is maternal PKU?

A: If a woman with PKU is not following a strict low phenylalanine diet during pregnancy, her baby may suffer from intellectual disabilities, heart defects and a small head (microcephaly), even though her baby may not inherit PKU. Women looking to become mothers should follow a low-phenylalanine diet beginning before pregnancy to prevent their baby from suffering from PKU in the mother.

Q: What is treatment like?

A: The main treatment for PKU is following a strict diet that eliminates high protein food such as, meat and dairy products and introduces a special formula. Additionally, your child needs to come to Children’s regularly for PKU checkups and to test his blood phenylalanine. We will work with you and your child to establish a diet he can be on for life.

Q: What are some potential side effects of the PKU diet?

A: If your child follows the diet correctly to get all the nutrients he needs, there should not be any side effects.

Q: If my child has PKU, what are the odds that he or she will have a baby with PKU?

A: The answer depends on who your child has a baby with. PKU is a recessive disease, meaning that the child with PKU gets two abnormal genes, one from each parent. If your child has a baby with someone who does not have a PKU gene (has two normal genes) none of the babies will have PKU.  If your child has a baby with someone who carries a gene for PKU (meaning that the person does not have PKU but has one gene for PKU and one normal gene) they have a 50% chance of having a baby with PKU.  However, if the person also has PKU, all of their babies will have PKU. .

What are potential side effects from treatment??

Following the strict diet can be difficult, and may lead to stress and anxiety. If it is followed correctly there should not be any physical effects on the body.

How severe could my child’s intellectual disabilitiesbe if he lapses from the diet?

If your child lapses from the diet, this could result in intellectual disabilities depending on how long the lapse is, how old your child is and the severity of your child’s PKU.

Questions to ask your doctor

The PKU diet is complicated and hard to manage. It’s a good idea to ask as many questions as you can during your appointment.

Here are some questions to get you started:

  • What exactly can my child eat and not eat?
  • What supplements will he take?
  • What advice can you give on helping my child stick to his diet?
  • What’s a simple way my child can explain PKU to his friends?
  • Will my child need to avoid certain medications if he gets sick?
  • How can I help my child cope?