Phenylketonuria (PKU)

Phenylketonuria (also called PKU) is a condition in which your body can’t break down an amino acid called phenylalanine. All babies born in the United States are tested for PKU through the newborn screening program in their state.

What is phenylalanine? 

Phenylalanine is an essential amino acid. Amino acids are building blocks for proteins. Our bodies need amino acids for proper growth and development but we cannot make them on our own. We need to get them from food sources. Phenylalanine is found in most foods that contain protein. This includes beef, poultry, fish, soy products, eggs, cheese, etc.

Once phenylalanine is in the body, it is converted into tyrosine, another amino acid. Tyrosine is then used by the body in a variety of ways, including the formation of chemicals that are necessary for your brain to function properly.

Why is phenylalanine harmful for people with PKU?

If your baby is born with PKU, she cannot break down phenylalanine. Phenylalanine then builds up in the blood and interferes with normal brain development. Without treatment, babies born with PKU begin to have signs of the condition at about 6 months of age. These include:

• Jerky movements in arms and legs
• Seizures
• Skin rashes
• Small head size
• Developmental delays and behavioral problems

What causes PKU?

PKU is inherited. This means it’s passed from parent to child through genes. A gene is a part of your body’s cells that stores instructions for the way your body grows and works. Genes come in pairs—you get one of each pair from each parent. Sometimes a change in a gene can cause it to not work correctly. This change is known as a mutation.

Your baby has to inherit a mutation for PKU from both parents to have PKU. If she inherits the mutation from just one parent, your baby is called a PKU carrier. A PKU carrier has one copy of the mutation but doesn’t have PKU.

How is PKU treated?

If your baby is diagnosed with PKU, then she will need to be on a special diet that significantly reduces the amount of phenylalanine she consumes. Ideally the diet would begin in the first few days of life. Babies who have PKU may never show symptoms if they are transitioned to a low-phenylalanine diet soon after birth.

If your baby is diagnosed with PKU, she will need to maintain a low-phenylalanine diet for life. If she were to stop controlling her dietary intake of phenylalanine, changes in the brain would occur, even well into adulthood. Women who have PKU and wish to become pregnant need to be on a very well controlled diet in order to protect their baby during pregnancy.

If you have any questions about this topic or other pregnancy and newborn health issues, please email the Pregnancy and Newborn Health Education Center at askus@marchofdimes.org.

Tags: newborn screening, phenylalanine, phenylketonuria, PKU