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Phenylketonuria (PKU)
Find information on phenylketonuria causes, symptoms, diagnosis, and treatment.

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What is Phenylketonuria?

Phenylketonuria (PKU) is a rare genetic condition in which the body cannot break down the essential amino acid phenylalanine. Amino acids are the building blocks of protein. Phenylalanine is found in all proteins and in some artificial sweeteners. Your body uses the enzyme phenylalanine hydroxylase to convert phenylalanine to tyrosine, a nonessential amino acid. Tyrosine is necessary for your body to make neurotransmitters like epinephrine, norepinephrine, and dopamine.

Without treatment for PKU, phenylalanine would build up in your body, and the buildup would cause damage to your nervous system and brain.

According to the U.S. National Library of Medicine’s Genetic Home Reference (NLM), the occurrence of PKU varies among ethnic groups worldwide. It is rare in the United States—just one in every 10,000 to 15,000 newborns will have PKU (NLM).

Prompt identification of the disorder and a commitment to lifelong dietary modifications can treat the effects of PKU and prevent brain damage. (NLM) For this reason, all newborns in the U.S. are screened for PKU.

What Causes PKU?

PKU is inherited in an autosomal recessive pattern. This means both parents must pass on a defective version of the gene PAH for their child to inherit the disorder. PAH provides instructions for making phenylalanine hydroxylase. Each parent has two copies of the gene PAH. If one copy of your PAH gene is altered, you will not have any symptoms, but you will be a carrier of the altered gene.

What Are the Symptoms of PKU?

Symptoms of PKU 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 his or her life. If the child is not treated for PKU during this time, he or she will begin to develop symptoms such as:

  • intellectual disabilities or mental retardation
  • seizures
  • tremors or jerky hand and leg movements
  • hyperactivity
  • stunted growth
  • eczema
  • a distinct odor in breath, skin, or urine that is often described as musty
  • lighter skin, hair, and eye color than their family members

A less severe form of this disorder is called variant PKU or non-PKU hyperphenylalaninemia (having too much phenylalanine Children with this form of the disorder may have only mild symptoms, but they will need to follow a special diet to prevent mental retardation.

In rare cases in which the disorder was not diagnosed at birth and treatment was not started quickly, symptoms of PKU can include:

  • irreversible brain damage and mental retardation within the first few months of life
  • behavioral problems and seizures in older children

Once a specific diet and other necessary treatments are started, symptoms diminish. People with PKU who properly manage their diet don’t show any symptoms.

How Is PKU Diagnosed?

Since the 1960s, hospitals in the U.S. have been routinely screening newborns for PKU using a blood sample. A healthcare professional will use 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 will be done after the baby has been drinking milk for at least 24 hours to prevent a false negative result. The test is normally done when the baby is a few days old, and still in the hospital.

If you don’t deliver your baby in a hospital, you will need to schedule these tests with your pediatrician.

Additional blood and genetic tests will be performed to confirm the initial results and presence of the PAH gene mutation responsible for PKU. These tests are often done within six weeks of the baby’s birth.

If a child or adult displays symptoms of PKU, such as developmental delays, your doctor may order the blood test to confirm the diagnosis. This test will involve taking a sample of blood and analyzing it for the presence of the enzyme needed to break down phenylalanine.

How Is PKU Treated?

The main component of your treatment will be eating a special diet that limits your intake of foods containing phenylalanine (mainly proteins). To ensure you have adequate protein for your body’s needs, you will also consume PKU formula. This is a medical food that contains all the amino acids that your body needs except phenylalanine.

You will have to follow this diet and consume PKU formula throughout your life to manage your symptoms. In the past, doctors felt that adults could go off the PKU diet in adolescence. Today people with PKU are strongly encouraged to maintain a strict diet for life for their health and mental functioning.

Infants and babies with PKU cannot be fed breast milk and must consume a special formula, often known as Lofenalac.


Every individual’s tolerance for phenylalanine is different, and you will need to work closely with healthcare professionals like a dietitian to maintain a proper balance of nutrients while you limit your phenylalanine intake.

It is important to keep track of your phenylalanine levels by keeping records of the amount of phenylalanine in the foods you eat throughout the day. Specific low-protein and PKU-friendly foods are often available at specialty stores. These will increase the variety of food in your diet and make the lifestyle easier to manage over time.

Additional research is being done on supplementing the PKU diet with fatty acids or other supplements. You should talk with your dietician and doctor before taking any dietary supplements.


The FDA recently approved sapropterin (Kuvac) for the treatment of PKU. Sapropterin increases your tolerance to phenylalanine. It doesn’t work for everyone, so you will need to work with your healthcare professionals to monitor your levels of phenylalanine to see if this medication should be part of your treatment. If sapropterin is effective for you, you must take it for life and continue to follow a low-PKU diet in order to manage your PKU symptoms.

Insurance Coverage

To date, 37 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 to you. If you do not have medical insurance, you can check with your local health departments to see what options are available to help you afford PKU formula.

Pregnancy and PKU

If you are a woman with PKU, there are additional risks if you fail to follow a low-PKU diet during your childbearing years. They include a high risk of miscarriage if you do not maintain a low PKU diet during pregnancy. There is also a high risk of your baby being exposed to high levels of phenylalanine. That can lead to mental retardation, heart defects, low birth weight, small head, and behavioral problems.

Even if your unborn baby does not have PKU, a mother with PKU should maintain the diet her doctor set for her before and during pregnancy.

Long-Term Outlook

The long-term outlook for someone with PKU is very good if a limited-phenylalanine diet is started early and maintained from shortly after his or her birth throughout life. If treatment is delayed or the condition remains untreated, brain damage occurs. This can lead to mental retardation by the first year of a child’s life.

In addition, PKU is a lifelong condition that you will have to mange throughout your entire life.

Can PKU Be Prevented?

This is a genetic condition and cannot be prevented. An enzyme assay can be done to see if individuals planning to have children carry the genes for PKU. Sampling is also done during pregnancy to screen unborn babies for PKU.

If you have PKU, you can prevent any of the symptoms from appearing by following a low-phenylalanine diet throughout your life.

Written by: Cindie Slightham
Edited by:
Medically Reviewed by:
Published: Aug 20, 2012
Published By: Healthline Networks, Inc.
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