What is phenylketonuria?

Phenylketonuria (PKU) is a genetic (inherited) condition in which levels of phenylalanine build up in the blood. Phenylalanine is an amino acid; amino acids are the building blocks of proteins. Many food protein sources contain phenylalanine.

If it is not treated, PKU causes serious complications, such as intellectual disabilities. In the United States, one in 10,000 to 15,000 babies is born with PKU every year.

Who is at risk of developing phenylketonuria?

While anyone can develop PKU, certain ethnic groups are more at risk, especially people of Native American or European descent.

What causes phenylketonuria?

PKU is caused by the buildup of phenylalanine in the body. Your body uses this amino acid to make other proteins. If you have PKU, your body can’t break down phenylalanine. If it is not treated, this buildup damages the brain and causes PKU symptoms.

What are the symptoms of phenylketonuria?

A child who is born with PKU and is not treated will have intellectual and developmental problems. He or she will also have less pigmentation in the skin.

People living with classic PKU (also called severe PKU) and who are not on a metabolic diet or being treated have the following symptoms:

  • Seizures or shaking
  • Eczema
  • Skin and hair discoloration
  • Microcephaly (small head size)
  • A musty odor to the breath, skin or urine (caused by extra phenylalanine)
  • Heart defects

Children born with moderate or mild cases of PKU may have less-severe intellectual disabilities. Some children with mild PKU don’t have any symptoms.

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