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What is Phenylketonuria (PKU)?

Phenylketonuria [fen-ul-ke-toe-NU-re-uh], also known as PKU, is a genetic disorder that affects the body’s metabolism.

Metabolism [meh-TAB-uh-lih-zem] describes the body’s ability to convert food and drink into energy, as well as get rid of certain wastes made during that process. It’s a complex process that uses many different chemicals, enzymes, hormones, cells, and organs to work properly.

With PKU, the body isn’t able to make an important enzyme called PAH (phenylalanine hydroxylase). PAH helps the body process a chemical called Phe (phenylalanine). Phe is naturally found in foods with protein and is normally changed into other chemicals that the body can use. Without the enzyme, Phe levels in the body can become too high, which can lead to brain damage, mental disabilities, or other serious health problems.

PKU is a genetic condition, meaning it is inherited and is often present at birth. Babies born in hospitals in the U.S. are screened for PKU before going home. Babies born outside a hospital can be tested at their pediatrician’s office. Early detection and treatment of PKU can prevent life-long health problems.

A person with PKU will need to follow a low-Phe diet for the rest of their life. It may be necessary to work with a registered dietician nutritionist to plan a safe, healthy diet. 


In infants, symptoms of PKU may take several weeks to months to develop. By 6 months of age, early symptoms may include:

  • Irritability
  • A loss of interest in their surroundings
  • Developmental delays – the child doesn’t reach physical, mental, or social milestones when they are supposed to
  • Lighter skin and eye color
  • Skin problems, such as rashes
  • Odd smelling skin, sweat, and urine (pee)
  • Convulsions or seizures

If PKU is left untreated, it can cause:

  • Mental disabilities
  • Epilepsy
  • Behavior problems

When to See a Doctor

See a doctor if you see any symptoms of PKU in your infant or child.

Women planning to homebirth a child should plan to visit a pediatrician after delivery for PKU screening.


PKU is genetic, meaning it is passed down from parents to child. It’s possible for parents to carry the gene without having symptoms of PKU. A gene that tells the body how to make an enzyme gets changed (mutates).

Diagnosis and Tests

Babies born in hospitals in the U.S. are screened for PKU before going home. Babies born outside a hospital can be tested at their pediatrician’s office.

A PKU screening test uses a small blood sample taken from a newborn. The sample is tested in a lab to measure how much Phe is present in the blood. Blood is collected using a heel stick – a small needle pricks the bottom of the baby’s foot. Heel sticks cause very little pain and heal very fast.

PKU can also be tested for later in life, though this is not usually needed, as the condition is almost always found in the child at birth. Families with a history of PKU can have a prenatal test to check if the baby has PKU.


There is no cure for PKU but treatment can help manage the disorder. Treating PKU usually requires a special diet that is low in Phe.  

If you have PKU it’s avoid foods high in protein, such as:

  • Milk
  • Eggs
  • Cheese
  • Nuts
  • Soybeans
  • Beans
  • Chicken
  • Beef
  • Pork
  • Fish
  • Potatoes or other vegetables that have protein

Also avoid beverages sweetened with aspartame (Equal, Nutrasweet). Aspartame is made with Phe.

There are special formulas available for infants that contain nutrients that they need to stay healthy. There are also formulas available for older children and adults that can supply vitamins and minerals that are missing as a result of the PKU diet.

Every person with PKU has a different tolerance for Phe. It’s important to work with your doctor to figure out the right diet for your needs. Blood tests and visits to your doctor can help you tune your diet. It may be possible to eat more Phe as you get older, but it’s best to stay on the diet for the rest of your life.

It can be hard to keep the PKU diet. Support from family, friends, or a support group can help you stay strong.

There are some medicines that can help your body break down PKU, but it must still be used with the diet to prevent Phe buildup.

Other treatments are being researched, including:

  • Gene therapy
  • Amino acid supplementation
  • Enzyme replacement therapy


There is no way to prevent PKU.

If you’re a woman with PKU and you plan on having a child, it’s important to stay on the PKU diet during pregnancy to make sure that the high levels of Phe don’t harm the fetus. You can also talk to your doctor about genetic testing to find out if you can pass this condition on to your child.

Phenylketonuria, also known as PKU, is an amino acid metabolism disorder where the body can’t process a chemical called phenylalanine or Phe. Phe is naturally found in foods with protein. High levels of Phe in the body can cause brain damage and developmental disabilities. Newborns born in hospitals are screened for PKU. People with PKU need to eat a special diet that is low in protein.