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    PKU (Phenylketonuria)

    PKU (phenylketonuria) is an inherited disorder of body chemistry that, if untreated, causes intellectual disabilities. Fortunately, through routine newborn screening, almost all affected newborns are diagnosed and treated early, allowing them to grow up with normal intelligence.

    At least 1 baby in 25,000 is born with PKU in the United States (1). The disorder occurs in all ethnic groups, although it is more common in individuals of Northern European and Native American ancestry than in those of African-American, Hispanic and Asian ancestry.

    What is PKU?
    Individuals with PKU cannot process a part of protein called phenylalanine, which is present in most foods. Because of a genetic abnormality, affected individuals lack or have very low levels of an enzyme (phenylalanine hydroxylase or PAH) that converts phenylalanine to other substances the body needs. Without treatment, phenylalanine builds up in the bloodstream and causes brain damage and intellectual disabilities.

    How does PKU affect a child?
    Children born with PKU appear normal for the first few months. If untreated, by 3 to 6 months they begin to lose interest in their surroundings. By the time they are 1 year old, they appear obviously developmentally delayed. Children with untreated PKU often are irritable and have behavioral problems. They may have a musty odor about them, and they may have dry skin, rashes or seizures. They usually are physically well developed and tend to have blonder hair than their siblings.

    Who gets PKU?
    Genes come in pairs. To inherit PKU, a child must receive two abnormal PAH genes (that regulate the production of the enzyme), one from each parent who has a mutation (change) in one PAH gene. A parent who has one abnormal PAH gene is called a "carrier." A carrier has one normal PAH gene and one PAH gene that contains a mutation. A carrier's health is not affected in any known way.

    When both parents are carriers, there is:

    • A 1-in-4 (25 percent) chance that both will pass one abnormal PAH gene on to a child, causing the child to be born with PKU.
    • A 2-in-4 (50-50) chance that the baby will inherit one abnormal PAH gene from one parent and the normal gene from the other, making it a carrier like its parents.
    • A 1-in-4 (25 percent) chance that both parents will pass on the normal gene. The baby will neither have the disease nor be a carrier.

    These chances are the same for each pregnancy.

    Are all babies tested for PKU?
    All states and U.S. territories screen for PKU. Babies are tested before they leave the hospital. The PKU test was the nation's first newborn screening test. Developed with the help of the March of Dimes, the test has been routinely administered since the 1960s, sparing thousands of children from intellectual disabilities (2).

    How is the test done?
    The baby's heel is pricked, and a few drops of blood are taken. (The same blood sample can be used to screen for a number of other inborn errors of body chemistry.) The blood sample generally is sent to a regional medical laboratory to find out if it has more than a normal amount of phenylalanine. Findings are sent to the health care professional responsible for the baby's care. If results are abnormal, more tests are done to determine whether the baby has PKU or if there is some other cause of high phenylalanine levels.

    Occasionally, a case of PKU can be missed when the test is performed before 24 hours of age. For this reason, some experts recommend that infants whose initial test was taken within the first 24 hours of life be tested again at 1-2 weeks of age (3).

    Can PKU symptoms be prevented?
    Yes. Intellectual disabilities can be prevented if the baby is treated with a special meal plan that is low in phenylalanine. This meal plan should be started as soon as possible after birth, ideally within the first seven to 10 days of life (2).

    At first, the baby is fed a special formula that contains protein but no phenylalanine. Breast milk or infant formula is used sparingly to supply only as much phenylalanine as the baby needs and can tolerate. Later, certain vegetables, fruits, some grain products (for example, certain cereals and noodles) and other low-phenylalanine foods are added to the meal plan. No regular milk, cheese, eggs, meat, fish and other high protein foods are ever allowed. Because protein is essential for normal growth and development, the child must continue to have one of the special formulas that is high in protein and essential nutrients, but contains little or no phenylalanine. Diet drinks and foods that contain the artificial sweetener aspartame (which contains phenylalanine and is sold as Nutrasweet or Equal) must be strictly avoided.

    Children and adults with PKU require follow-up care at a medical center or clinic that specializes in this disorder. The meal plan for each person must be individualized, depending upon how much phenylalanine can be tolerated. All affected persons need regular blood tests to measure phenylalanine levels. Testing for babies may be as frequent as once a week for the first year of life, and then once or twice a month throughout childhood.

    Individuals with PKU must remain on a restricted meal plan throughout childhood and adolescence and generally for life (although some relaxation of the meal plan may be possible as the person ages) (2). Until the 1980s, health care providers believed that children with PKU could safely discontinue their special meal plan around age 6 when brain growth was completed. However, studies since then have found that discontinuance of the meal plan before age 8 can lead to a decrease in IQ, and discontinuance after age 12 may lead to learning disabilities and behavioral problems (2, 4).

    Parents of children with PKU and affected adults should discuss their meal plan and treatment questions with health care professionals at a PKU clinic.

    What is maternal PKU?
    There are an estimated 3,000 women of childbearing age with successfully treated PKU in the United States (5). Most discontinued their special meal plan in childhood because, at that time, most doctors believed it was safe to do so.

    If these young women aren't following their eating plan, their blood phenylalanine levels are very high when they become pregnant. During pregnancy, high blood levels of phenylalanine in the mother can cause serious problems in the fetus. About 90 percent of their babies will have intellectual disabilities, and about 70 percent will have a small head size (microcephaly) (6). Many will have heart defects and low birthweight. Because most of these babies do not inherit PKU, but are suffering from brain damage caused by their mothers' high phenylalanine levels during pregnancy, they cannot be helped by the PKU eating plan.

    Fortunately, there is a way to help prevent intellectual disabilities and other problems in babies of women with PKU. Women with PKU need to resume their special eating plans at least three months before pregnancy and continue the eating plan throughout pregnancy. The Maternal PKU International Study found that women whose blood phenylalanine levels were under control before conception, or by 8 to ten weeks of pregnancy at the latest, were as likely to have healthy babies as women without PKU (7). At age 7, the IQs of their children did not differ from those of children of women without PKU (7). Women with PKU need at least weekly blood tests throughout pregnancy to make sure blood phenylalanine levels are not too high.

    The March of Dimes urges all women who know or suspect that they were treated for PKU as children to contact their health care provider or clinic before they attempt to conceive, so that their blood phenylalanine levels can be measured and they can begin the special eating plan, if necessary.

    Occasionally, a woman has undiagnosed PKU that can pose a risk to her baby. These women, who generally were not screened as newborns, usually are slightly affected, and may be diagnosed only following the birth of a baby with PKU-related birth defects. In order to help prevent these birth defects, some doctors recommend screening women who may be at risk of PKU, such as those with a family history of the disorder, so that affected women can start the PKU eating plan before pregnancy.

    Can drugs be used to manage PKU?
    In December 2007, the Food and Drug Administration (FDA) approved Kuvan (sapropterin dihydrocholoride), the first drug to help manage PKU (8). The drug helps reduce blood phenylalanine levels in individuals with PKU by increasing the activity of the PAH enzyme.

    Kuvan is effective only in individuals who have some PAH activity. Individuals who take this drug must continue to follow a phenylalanine-restricted eating plan and have blood tests to measure phenylalanine levels.

    What is new in PKU research?
    Researchers continue to study the long-term outcome for children who were born from treated maternal PKU pregnancies to determine whether there is an increased risk of learning disabilities, especially among children whose mothers' blood phenylalanine levels were not well controlled in the early weeks of pregnancy.

    Researchers also are studying the benefits of a nutritional supplement called BH4 in individuals with PKU. Others are developing a genetically engineered version of the missing enzyme. Both approaches eventually may allow affected individuals to eat a meal plan that approximates normal. Researchers also are exploring the possibility of treating PKU using gene therapy.

    For more information


    1. American College of Medical Genetics. Newborn Screening: Toward a Uniform Screening Panel and System. Final Report, 3/8/05.
    2. National Institutes of Health Consensus Development Statement. Phenylketonuria: Screening and Management. Washington, D.C., October 16-18, 2000.
    3. Kaye, C.I. and the American Academy of Pediatrics Committee on Genetics. Newborn Screening Fact Sheets. Pediatrics, volume 118, 2006, e934-963.
    4. Mitchell, J.J. and Scriver, C.R. Phenylalanine Hydroxylase Deficiency. Gene Reviews, updated 3/29/07.
    5. American College of Obstetricians and Gynecologists (ACOG) Committee on Genetics. Maternal Phenylketonuria. Committee Opinion, number 230, January 2000, reaffirmed 2004.
    6. Koch, R., and de la Cruz, F. The Maternal Phenylketonuria Collaborative Study: New Developments and the Need for New Strategies—Preface. Pediatrics, volume 112, number 6, part 2, December 2003, page 1513.
    7. Koch, R., et al. The Maternal Phenylketonuria International Study: 1984-2002. Pediatrics, volume 112, number 6, part 2, December 2003, pages 1523-1529.
    8. Food and Drug Administration (FDA). FDA Approves Kuvan for Treatment of Phenylketonuria (PKU), December 13, 2007.

    August 2007 (R March 2008)

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