What is phenylketonuria (PKU)?

Phenylketonuria (PKU) is a genetic disorder in which a baby lacks or has very low levels of the enzyme phenylalanine hydroxylase (PAH). This enzyme is necessary to convert phenylalanine, an amino acid, into another amino acid called tyrosine after protein is eaten. If PKU is not treated soon after birth, phenylalanine builds up in the bloodstream and brain tissue, causing mental retardation and central nervous system problems. If PKU is treated soon after birth, all or most of these problems can usually be prevented.

Each year, about 250 U.S. children get PKU.¹ It is more common in whites and Native Americans than in blacks, Hispanics, and Asians.²

What causes PKU?

PKU is an autosomal recessive disease. This means that to have the disease, you must inherit the gene from each parent. If you inherit the gene from only one parent, you are a carrier of PKU but do not have the disease.

What are the symptoms?

Symptoms usually develop within a few months after birth, when phenylalanine has built up in a baby's system from consuming the protein in formula or breast milk. Before birth, the mother's body filters out the excess phenylalanine for the baby (fetus).

Early symptoms of PKU include a musty odor to the skin, hair, and urine. Babies may lose weight from vomiting and frequent diarrhea, and may be irritable, have skin problems, and be sensitive to light.

If phenylalanine levels increase, an older child may develop unusual behavior, such as screaming episodes, repetitive rocking, head banging, and arm biting. The child may also have growth and developmental delays, frequent seizures, and, if not treated, will become severely mentally retarded.

How is PKU diagnosed?

All babies born in the United States are tested for PKU within a few days after birth. Phenylalanine levels are measured in a blood sample taken from the baby's heel. The test may be repeated within the first week or two after birth.

If you have a family history of phenylketonuria, talk with your health professional about genetic testing. If you have a child with PKU, it may be helpful to get genetic counseling if you are considering having another child.

How is it treated?

The main treatment for PKU is a lifelong, special reduced-protein diet. This helps prevent phenylalanine from building up in the body. But because phenylalanine is needed for normal growth and development, it cannot be completely eliminated from the diet. Early diagnosis and treatment usually prevent injury to the baby's brain and other long-term problems.

Women of childbearing age who have PKU must carefully control their phenylalanine levels. Babies born to mothers who have high phenylalanine levels during pregnancy are at risk for mental retardation, having an unusually small head (microcephaly), physical growth problems, and congenital heart disease.

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