• Phenylketonuria (PKU), «FEHN uhl KEE tuh NYUR ee uh», is a hereditary disease that results in intellectual disability unless treated during early infancy. The bodies of people with PKU accumulate too much phenylalanine, one of the amino acids that make up proteins. PKU is caused by a mutation (change) in a gene that alters the function of the enzyme phenylalanine hydroxylase. This enzyme normally converts phenylalanine to the amino acid tyrosine. In PKU, phenylalanine consumed in food is not properly converted to tyrosine, and phenylalanine accumulates. Excess phenylalanine interferes with the growth, development, and function of the brain.
    PKU is detected in infants by measuring phenylalanine in blood during the first weeks of life. An abnormally high level of phenylalanine may indicate the presence of PKU. Further laboratory tests confirm the diagnosis.
    Treatment of PKU must begin early in infancy and is continued for life. To limit the intake of phenylalanine, physicians prescribe a low-protein diet that includes fruits and certain vegetables. Dairy products, meats, and most cereals must be avoided. The artificial sweetener aspartame, which contains phenylalanine, must also be avoided. In the United States and many other countries, labeling on containers of soft drinks and other foods sweetened with aspartame must carry a warning to people with PKU.
    Specially prepared foods allow PKU patients to get adequate nutrition on a low-protein diet. Women with PKU must follow this diet before and during pregnancy to protect the developing brain of an unborn baby. In some PKU patients, daily doses of a vitamin called tetrahydrobiopterin (BH4) can reduce blood phenylalanine levels and ease dietary restrictions.

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