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A to Z: Phenylketonuria

May also be called: PKU, Neonatal Phenylketonuria

Phenylketonuria (fen-ul-KEE-tuh-NUR-ee-uh) is a metabolic disorder caused by a defect in the enzyme that breaks down the amino acid phenylalanine (fen-ul-AL-uh-neen).

More to Know

Phenylalanine is one of the eight amino acids that you can only get from food sources. Usually, after someone eats, the body breaks down phenylalanine with an enzyme called phenylalanine hydroxylase (PAH) and uses the resulting products to make proteins. When people have phenylketonuria (PKU), it means they were born with an inherited defect in the gene that controls the production of PAH. This means their bodies can’t break down phenylalanine properly.

Phenylalanine is necessary for normal growth in infants and children and for normal protein production. However, if too much of it builds up in the body, brain tissue is affected and mental retardation occurs. Phenylalanine also affects melanin, the pigment responsible for hair and skin color, so kids with PKU often have fair skin and hair, and blue eyes.

A child with PKU may also have:

  • seizures
  • stunted growth
  • behavioral problems
  • skin rashes
  • a musty odor to the breath, skin, or urine due to too much phenylalanine in the body

All newborn babies in the United States are required to have their blood tested for signs of PKU. Treatment for PKU involves following a strict diet that should include the right balance of vegetables, fruits, grains, and fats. The child’s diet should be low in phenylalanine; this means no high-protein foods like milk, dairy, meats, eggs, nuts, soy, and beans. A person with PKU should also avoid the artificial sweetener aspartame. Special protein supplements may be taken to make up any deficiencies. The special diet should start as soon as PKU is diagnosed and continue for the rest of the person’s life.

Keep in Mind

With early diagnosis, close medical monitoring, and strict, lifelong adherence to the right diet, PKU is treatable. Kids and adults with properly treated PKU can usually expect a good outcome. With delayed treatment or no treatment, brain damage and some amount of mental retardation will occur.

All A to Z dictionary entries are regularly reviewed by KidsHealth medical experts.

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