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Home Knowledge Center Wellness Library Phenylketonuria (PKU)

Phenylketonuria (PKU)

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 treatment is not started soon after birth, phenylalanine levels rise and can cause lasting brain and nervous system damage, such as intellectual disability.

Symptoms of PKU usually appear within a few months of birth, after phenylalanine has built up in a baby's blood. Before birth, the mother's body filters out the excess phenylalanine for the baby (fetus).

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.

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 intellectual disability and other developmental problems.

Current as of: March 20, 2025

Author: Ignite Healthwise, LLC Staff

Clinical Review Board
All Ignite Healthwise, LLC education is reviewed by a team that includes physicians, nurses, advanced practitioners, registered dieticians, and other healthcare professionals.

This information does not replace the advice of a doctor. Ignite Healthwise, LLC, disclaims any warranty or liability for your use of this information. Your use of this information means that you agree to the Terms of Use. Learn how we develop our content.

To learn more about Ignite Healthwise, LLC, visit webmdignite.com.

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