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Phenylketonuria (PKU) is when a person does not have enough of the liver enzyme needed to break down an amino acid called phenylalanine. It can build up and harm the brain.


PKU is caused by a change in the gene that makes the enzyme that breaks down phenylalanine.

Risk Factors

The only way to get PKU is to have gotten the gene for it from both parents.


Symptoms depend on how much of the enzyme is made.

Problems may not happen when PKU is found early and treated. If it is not, there may be:

  • Seizures
  • Rash
  • Learning problems
  • Problems with behavior
  • A musty body odor
  • Light skin and hair coloring


PKU may be found at birth by a blood test that is done by law in all states. The test can also be done at any time. More blood and urine tests are done to confirm PKU.

A doctor who treats genetic problems will be needed. A dietitian will also be needed.


There is no cure. People must stay on a special diet throughout life. Pregnant women with PKU must stay on it to lower the risk of serious birth defects.

The level of phenylalanine in the blood will need to be checked with blood tests. This will help make sure it is in a safe range.

Fetus and mother blue
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Low-Phenylalanine Eating Pattern

This strict diet lowers phenylalanine to a level that a person with PKU can have without having problems. Its key points are:

  • Drink phenylalanine-free medical formula, which has calories, protein, vitamins, and minerals without phenylalanine. They are made for babies, children, and adults.
  • Limit starches, such as bread, potatoes, and beans.
  • Do not eat high-protein foods, such as milk, meat, fish, chicken, pork, eggs, beans, and nuts.
  • Do not eat foods or have drinks that are made with artificial sweeteners that have phenylalanine.


Pegvaliase-pqpz or tetrahydrobiopterin may be used to lower phenylalanine levels.


There are no methods to prevent PKU.


National PKU News  http://www.pkunews.org 

NIH Genetics Home Reference  http://ghr.nlm.nih.gov 


Canadian PKU and Allied Disorders  http://canpku.org 

Caring for Kids—Canadian Paediatric Society  http://www.caringforkids.cps.ca 


Phenylketonuria (PKU) EBSCO DynaMed website. Available at:  https://www.dynamed.com/condition/phenylketonuria-pku . Updated June 10, 2019. Accessed September 17, 2019.

Regier DS, Greene CL. Phenylalanine hydroxylase deficiency. GeneReviews 2017 Jan 5.

What is PKU? University of Washington PKU Clinic website. Available at http://depts.washington.edu/pku/about/whatispku.html. Accessed September 17, 2019.

1/21/2014 DynaMed Systematic Literature Surveillance  https://www.dynamed.com/topics/dmp~AN~T114631/Phenylketonuria-PKU : Lindegren ML, Krishnaswami S, Fonnesbeck C, et al. Adjuvant treatment for phenylketonuria (PKU). Comparative Effectiveness Reviews. 2012;56.

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