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Phenylketonuria (PKU) is a rare condition in which the body does not properly break down (metabolize) an amino acid called phenylalanine.

Alternative Names



Phenylketonuria (PKU) is inherited, which means it is passed down through families. Both parents must pass on the defective gene in order for a child to have PKU. This is called an autosomal recessive trait.

PKU occurs when a person is missing an enzyme called phenylalanine hydroxylase, which is needed to break down phenylalanine. Phenylalanine is 1 of the 8 essential amino acids found in foods that contain protein.

Without the enzyme, high levels of phenylalanine and two closely-related substances build up in the body. These substances are harmful to the central nervous system and cause brain damage. If the proteins containing phenylalanine are not avoided, PKU can lead to mental retardation by the end of the first year of life. Older children may develop movement disorders (such as athetosis) and hyperactivity.

Phenylalanine plays a role in the body's production of melanin, the pigment responsible for skin and hair color. Children with phenylketonuria often have lighter complexions than brothers or sisters without the disease. If the condition is untreated or foods containing phenylalanine are not avoided, a "mousy" odor may be detected on the breath and skin and in urine. The unusual odor is due to a build up of phenylalanine substances in the body.


  • Skin rashes (eczema)
  • Microcephaly
  • Tremors
  • Jerking movements of the arms or legs (spasticity)
  • Unusual positioning of hands
  • Seizures
  • Hyperactivity
  • Delayed mental and social skills
  • Mental retardation
  • A distinctive "mousy" odor to the urine, breath, and sweat
  • Light complexion, hair, and eyes

Exams and Tests

Tests include an enzyme assay to determine if the parents carry the defective gene responsible for PKU. During pregnancy, a chorionic villus sample may be taken to screen the unborn baby for PKU.

PKU is a treatable disease that can be easily detected with a simple blood test. Most states require a PKU screening test for all newborns. The test is generally done with a heelstick shortly after birth.


Treatment involves a diet that is extremely low in phenylalanine, particularly when the child is growing. However, sticking to the diet is difficult because phenylalanine products often have poor taste.

The diet must be strictly followed to prevent or reduce mental retardation. This requires close supervision by a registered dietitian or doctor, and cooperation of the parent and child. Those who continue the diet into adulthood have better physical and mental health.

Phenylalanine occurs in significant amounts in milk, eggs and other common foods. The artificial sweetener NutraSweet (aspartame) also contains phenylalanine. Any products containing aspartame should be avoided by children with this disorder.

A special infant formula called Lofenalac is made for infants with PKU. It can be used throughout life as a protein source that is extremely low in phenylalanine and balanced for the remaining essential amino acids.

Tetrahydrobiopterin (BH4) has been used in mild cases as an experimental treatment. Taking supplements to replace the long chain fatty acids missing from a standard phenylalanine-free diet may be helpful.

Outlook (Prognosis)

The outcome is expected to be very good if the diet is closely followed, starting shortly after the child's birth. If treatment is started after 3 years or if the disorder remains untreated, brain damage will occur.

Possible Complications

Severe mental retardation occurs if the disorder is untreated. ADHD (attention-deficit hyperactivity disorder) appears to be the most common problem seen in those who do not stick to a very low-phenylalanine diet.

When to Contact a Medical Professional

Call your health care provider if your infant has not been tested for PKU. This is particularly important if anyone in your family has the disorder.


Genetic counseling is recommended for prospective parents with a family history of PKU. The carrier state for PKU can be detected by enzyme assays, and PKU can be diagnosed prenatally.

It is very important that women with PKU closely follow a strict low-phenylalanine diet both before becoming pregnant and throughout the pregnancy, since build-up of phenylalanine will damage the unborn baby even if the child has not inherited the defective gene.

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