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

The body uses amino acids to build proteins. Phenylalanine is needed for normal growth in infants and children and for normal protein production. But if too much builds up in the body, the brain is affected. Phenylketonuria (PKU) that's not treated can lead to developmental delays and permanent intellectual disability.

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 have their blood tested for signs of PKU. Treatment for PKU involves following a strict diet with 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. The special diet should start as soon as PKU is diagnosed and continue for the rest of the person's life.