DNAPhenylketonuria or PKU is a genetic condition preventing the body from breaking down the amino acid phenylalanine and two similar substances commonly found in protein. Without the enzyme needed to breakdown phenylalanine, PKU causes a build-up of the amino acid, which can lead to developmental delay.

Risk Factors

Both parents must pass along the defective gene for offspring to develop PKU. If just one parent passes along the defective gene PKU will not develop. With two defective genes, the body is unable to produce the enzyme responsible for breaking down phenylalanine. Phenylalanine is present in protein.


One of the first symptoms of PKU in infants is stunted growth or smaller than average head size. Infants may also develop:

  • Jerky Movements
  • Tremors
  • Eczema or Skin Rash
  • Hyperactivity

Mental development delays can occur in the first 12 months if the condition goes unnoticed or untreated. Social and behavioral issues are also common in children with PKU.


The first few months of life are important for infants with PKU. If amino acid levels are left unchecked and/or dietary restrictions are not followed, brain damage may occur leading to mental retardation. Behavioral issues and seizures have also been noted in children with PKU.

Testing and Diagnosis

Newborn blood testing is the most common test for PKU. All infants born in a hospital setting are tested for PKU. If the infant is born outside of the hospital, testing can be completed one to two days after birth or later, if necessary. Testing requires a heel stick with a few drops of blood placed on a testing card.


PKU is a treatable condition. Infants are prescribed a special formula that does not contain phenylalanine. Dietary restrictions must be followed for life to prevent build-up of phenylalanine. Phenylalanine is also found in some over-the-counter supplements and food products that may not appear to be sources of protein, or the amino acid. Nutrition labels in the United States may note the presence of phenylalanine in a warning statement.

It is important for infants, children and adults with PKU to avoid foods high in protein, including:

  • Eggs
  • Milk
  • Cheese
  • Beans
  • Nuts
  • Beer
  • Peas
  • Chocolate Candy
  • All Meats
  • Aspartame

With early detection and dietary restriction, PKU will not shorten lifespan. If diagnosis is delayed and dangerous levels of phenylalanine buildup in the body in infancy, mental delays and brain damage may occur. Patients must follow proper diet for life to prevent side effects and symptoms of PKU.