What is PKU?

Phenylketonuria (PKU) is a genetic inborn error of metabolism that is detectable during the first days of life with appropriate blood testing (newborn screening). The absence or deficiency of an enzyme that is responsible for processing the essential amino acid phenylalanine characterizes PKU. With normal enzymatic activity, phenylalanine is converted to another amino acid (tyrosine), which is then utilized by the body. However, when the phenylalanine hydroxylase enzyme is absent or deficient, phenylalanine abnormally accumulates in the blood and is toxic to brain tissue.

Without treatment, most infants with PKU develop mental retardation. Those with untreated PKU may also develop additional neurologic symptoms.

To prevent mental retardation, treatment consists of a carefully controlled, phe-restricted diet begun during the first days or weeks of life. Most experts suggest that a phe-restricted diet should be lifelong. A carefully maintained diet can prevent mental retardation as well as neurological, behavioral and dermatological problems. It is generally believed that keeping blood phenylalanine levels in the range of 2-6mg/dl is the safest, especially in infancy and early childhood. Frequent blood monitoring should be done to achieve this goal.

PKU is inherited as an autosomal recessive trait. In other words, two people who conceive a child must both be carriers of the gene in order for there to be a chance that their infant will have PKU. When both carriers conceive a child, there is a one in four or 25% chance for each pregnancy that the baby will have PKU. It is estimated that PKU occurs in one in 15,000 newborns in the United States. The incidence varies in other parts of the world.

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    DISCLAIMER: The information contained on this page is not intended to replace the advice of a metabolic medical professional.

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