by Amy Marlow, MPH, RD, CDN
Phenylketonuria (PKU) is an inherited metabolic disorder in which a person does not have enough of the liver enzyme needed to break down phenylalanine. Phenylalanine is an amino acid found in proteins and some artificial sweeteners. With low levels of this enzyme, phenylalanine can build up in the blood and become toxic to the brain.
PKU is caused by a mutation of the gene responsible for making the enzyme that breaks down phenylalanine.
The only way to get PKU is to inherit the defective gene from both parents.
Risk is higher if a sibling has PKU.
PKU appears to be more common in babies born to parents of Turkish, Yemenite Jewish, northern and eastern European, Italian, and Chinese ancestry.
PKU symptoms can vary from mild to severe, depending on how much enzyme is made.
If detected and treated early, PKU symptoms may never appear. If untreated, one or more of the following symptoms of PKU may appear by the first birthday:
PKU may be diagnosed at birth by a routine newborn screening test which is required by law in all states. The hospital draws blood from the heel between 1-7 days after birth. If a high level of phenylalanine is detected in the blood, additional tests of the blood and urine may be done to confirm the diagnosis of PKU. If you have your baby at home, arrange for a PKU test and other standard screening tests to be done by a pediatrician after birth.
A baby with PKU will need to see a doctor who specializes in genetic disorders and a registered dietitian (RD) who can advise on the dietary treatment for the condition.
PKU is not curable, but it is treatable with a special diet beginning at birth and continuing throughout life.
Women with PKU must follow the diet strictly both before and during pregnancy. A phenylalanine level < 6 milligrams/deciliter (mg/dL) [< 363 micromole/liter (µmol/L)] must be maintained to prevent brain damage in the unborn baby.
Low-Phenylalanine Eating Pattern
This strict diet significantly reduces the intake of phenylalanine to a level that a person with PKU can handle without seeing harmful effects. Newborns require 40-60 mg/kg/day of phenylalanine to maintain adequate growth. For most older children and adults with PKU, the safe range is 200-400 mg of phenylalanine per day. The diet consists of:
Your doctor may advise medication. Tetrahydrobiopterin (also known as sapropterin dihydrochloride or BH4) may be used to reduce your phenylalanine levels.
Part of the treatment plan for PKU is regular monitoring of the level of phenylalanine in the blood. During infancy, the pediatrician may monitor the blood frequently, as often as once or twice a week. Over time, blood monitoring may not be needed as frequently. The treatment goal is to keep the level of phenylalanine in the blood in the safe range between 2-6 mg/dL (121-363 µmol/L). The doctor or registered dietitian may also perform an occasional diet analysis to calculate the amount of phenylalanine being consumed and to check for potential nutrient deficiencies.
There is no way to prevent PKU, but proper management can prevent complications.
Canadian PKU and Allied Disorders
Caring for Kids—Canadian Paedatric Society
Phenylketonuria (PKU) EBSCO DynaMed Plus website. Available at: http://www.dynamed.com/topics/dmp~AN~T114631/Phenylketonuria-PKU . Updated June 16, 2017. Accessed August 24, 2017.
Phenylketonuria. GeneReviews website. Available at: https://www.ncbi.nlm.nih.gov/gtr/conditions/C0031485/. Updated January 5, 2017. Accessed August 24, 2017.
Phenylketonuria: screening and management [National Institutes of Health Consensus Statement, October 16-18, 2000]. National Institutes of Health website. Available at: https://consensus.nih.gov/2000/2000Phenylketonuria113html.htm. Accessed August 24, 2017.
What is PKU? University of Washington PKU Clinic website. Available at
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Accessed August 24, 2017.
1/21/2014 DynaMed Plus Systematic Literature Surveillance http://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.
Last reviewed September 2018 by EBSCO Medical Review Board Michael Woods, MD, FAAP
Last Updated: 1/21/2014
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