Phenylketonuria (PKU) Facts

Phenylketonuria (PKU) is a genetic disorder that causes the amino acid phenylalanine to build up in the blood. This results in severe and irreversible mental retardation, especially in infants, if not treated early.

There is little information available on the incidence of PKU. In the United States, research reports:

  • PKU occurs in 1 out of every 13,500 to 19,000 births.
  • A higher incidence in whites and Native Americans.
  • A lower incidence in blacks, Hispanics, and Asians.

Cause and Incidence

In PKU, the child lacks or has very low levels of the enzyme phenylalanine hydroxylase (PAH). PAH is necessary to convert phenylalanine into another amino acid, tyrosine. If phenylalanine is not converted, it builds up in the bloodstream and disturbs the metabolism, resulting in mental retardation. Phenylalanine is a component of protein, so when protein is consumed, phenylalanine builds up in those people with PKU.

At birth, babies with PKU have no signs or symptoms of the disorder because the mother's body filters out phenylalanine for her unborn baby before birth. Symptoms appear after the baby starts consuming protein (from either breast milk or formula), and the resulting phenylalanine builds up in his or her system, usually by a few months of age. If PKU is not treated, it usually causes progressively severe mental retardation and other problems affecting the nervous system. The baby will not be able to regain the lost mental ability.

PKU is an autosomal recessive disease. This means that to have the disease, a person must inherit a gene from both parents. If a person inherits the gene from only one parent, he or she is a carrier. This means that the person carries the gene for PKU, but does not have the disease.


In the United States, babies are routinely tested for PKU shortly after birth so that early diagnosis and treatment are possible and so that injury to the baby's brain is usually prevented. Treatment for PKU consists of a special diet containing reduced amounts of protein (which contains phenylalanine). Phenylalanine is essential for normal growth and development and cannot be eliminated from the diet.

If PKU is treated by 3 weeks of age, the baby may not have any permanent brain damage. Rarely, children may have learning or behavior problems (such as slower language development, hyperactivity, anxiety, and poor concentration) despite early treatment.

Maternal PKU

Women of childbearing age who have PKU must be very careful. High blood levels of phenylalanine during pregnancy can result in babies with mental retardation, small heads (microcephaly), retarded physical growth, and heart-related risks such as congenital heart disease

If you have a family history of phenylketonuria, talk with your health professional about genetic testing. If you have a child with PKU, seek genetic counseling before attempting another pregnancy.


Signs and symptoms of phenylketonuria (PKU) include:
  • A musty odor to the skin, hair, and urine.
  • Vomiting and diarrhea, leading to weight loss.
  • Irritability.
  • Skin problems, such as sensitivity to light (photosensitivity), dry skin, or itchy skin rashes (eczema).

If phenylalanine rises above an acceptable level, the child almost always:

  • Develops unusual behavior, such as screaming episodes, repetitive rocking, head banging, and arm biting (common in older children).
  • Grows and develops at a slower rate than normal.
  • Has frequent seizures.
  • Becomes severely mentally retarded.

Children with PKU tend to be blond with fair skin and blue eyes. This is because PKU affects the synthesis of melanin, which provides pigment (color) to the skin, eyes, and hair.

Some people with PKU have more severe symptoms than others because of variations in the disease.

If you think your new baby has symptoms of phenylketonuria, talk with your health professional about whether the baby was tested adequately. Early diagnosis and treatment for PKU can prevent mental retardation. See the Exams and Tests section of this topic for more information on testing.

Source: Healthwise, Inc.

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