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Phenylketonuria (PKU)

Medically reviewed by Last updated on July 17, 2020.

What is Phenylketonuria (PKU)?

Harvard Health Publishing

Phenylketonuria (PKU) is a rare genetic (inherited) disorder that can cause abnormal mental and physical development if not detected promptly and treated appropriately.  

Normally, when a person eats foods that contain protein, special chemicals called enzymes break down these proteins into amino acids. The amino acids are then broken down further by other enzymes. A person with PKU does not have enough of the specific enzyme that breaks down the amino acid phenylalanine. Therefore, any phenylalanine in the food someone eats cannot be digested properly and collects in the body. 

Too much phenylalanine in the body causes problems with the brain and other organs. Damage from a buildup of phenylalanine can begin within the first month of life and, if undetected and/or untreated, PKU results in severe mental retardation, hyperactivity, and seizures. 

PKU affects about 1 baby in every 13,000 to 19,000 births. To be born with PKU, a baby has to have inherited the PKU gene from both parents. Most often, the parents do not know that they carry the gene.


Newborn babies with PKU usually do not show any symptoms, although they may be irritable, vomit or feed poorly . If babies are untreated they start to show developmental delays at a few months of age and the delays can become very severe.  

Children with PKU have lower levels of melanin, the substance that gives color to hair and skin. That's because when phenylalanine is broken down, one of its products is used to make melanin. As a result, children with PKU often will have pale skin, blond hair and blue eyes. Dry skin; eczema; and a "musty" odor resulting from the buildup of phenylalanine in hair, skin and urine are also common. Other signs and symptoms may include irritability, muscle stiffness, seizures, a small head and short stature.


In the United States, all newborn babies should be screened for PKU routinely within the first week of life. Infants must have had a protein meal before being tested for PKU. A very small amount of blood is taken by pricking the baby's heel. If abnormally high amounts of phenylalanine are found, additional blood tests may be necessary to confirm the diagnosis.

Expected Duration

PKU is an inherited disorder that lasts throughout a person's lifetime.


PKU occurs when a baby inherits 2 copies of the specific gene that causes PKU, 1 from each parent. Each parent usually has only 1 copy of the gene and therefore doesn't have PKU. Because the parents don't know they are carrying the gene, there is nothing they can do to prevent their babies from having the disorder. 

A woman who has PKU herself and is pregnant must strictly control her phenylalanine levels before and during pregnancy to avoid causing damage to her unborn child. High levels of phenylalanine in a pregnant woman can cause her child to have slow growth, developmental delays, small head size and other disorders. With careful monitoring and control, women with PKU can give birth to healthy children. A woman with PKU can pass the PKU gene to her child, but the child will not develop PKU unless another copy of the gene is inherited from the father.


The only treatment for PKU is to avoid eating foods that contain phenylalanine. Babies are put on a special formula that does not contain phenylalanine.  

Phenylalanine is found in most protein-containing foods, so people with PKU are advised to follow a special low-protein diet. They should avoid high-protein foods such as: 

  • Meat 
  • Poultry (chicken and turkey) 
  • Fish 
  • Eggs 
  • Milk and cheese 
  • The artificial sweetener aspartame (NutraSweet).  

Each person's protein needs vary over a lifetime. Children with PKU need to be sure they get enough protein to grow and develop properly. However, they must always avoid having too much phenylalanine in their body at any time. The levels of phenylalanine in the blood need to be monitored throughout the person's lifetime.

When To Call a Professional

At your baby's first checkup with a doctor, make sure that the genetic screening tests that are recommended after birth have been done and are normal. Call your baby's doctor if your baby: 

  • Seems to be less active than you expected 
  • Is missing normal developmental milestones 
  • Displays other signs and symptoms of PKU. 

Older children who have been diagnosed with PKU may need to check their phenylalanine levels more often and watch their diet more closely if they experience any of the following: 

  • Learning disabilities 
  • Irritability 
  • Hyperactivity 
  • Tremors.


The outlook depends on how early an infant with PKU is diagnosed and begins the special diet. It also depends on how strictly and consistently the diet is followed throughout life. Infants with PKU who are identified within the first few days after birth and are put on a strict diet before 3 weeks of age have the best prognosis. They usually do not experience severe developmental delay or mental retardation.

Learn more about Phenylketonuria (PKU)

Associated drugs

External resources

National Institute of Child Health & Human Development
National Institutes of Health

March of Dimes

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.