Phenylketonuria (PKU) is an inherited disorder in which the body cannot break down an amino acid called phenylalanine, which is a part of protein. If PKU is not treated soon after birth, phenylalanine levels rise and can cause permanent brain and nervous system damage, such as mental retardation.

Symptoms of PKU usually appear within a few months of birth, after a baby has started drinking formula or breast milk and phenylalanine has built up in his or her blood. Early symptoms may include:

• A musty odor to the skin, hair, and urine.
• Weight loss from vomiting and frequent diarrhea.
• Irritability.
• Skin problems.
• Sensitivity to light.

Screening for PKU is routinely done shortly after birth, making early diagnosis and treatment possible. Early treatment, within the first few weeks of birth, may prevent permanent brain damage. Rarely, some children who receive treatment will have learning or behavior problems. Left untreated, PKU causes progressively more severe mental retardation.

People with PKU must follow a diet low in protein throughout life. Women of childbearing age with PKU must carefully manage their phenylalanine levels to prevent harm to their baby should they become pregnant. Babies born to mothers who have high phenylalanine levels during pregnancy are at risk for mental retardation and other developmental problems.