Phenylketonuria (PKU)

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Topic Overview

What is phenylketonuria (PKU)?

Phenylketonuria (PKU) is a rare genetic disorder in which the body cannot break down an amino acid called phenylalanine (say "fehn-uhl-AL-uh-neen"), which is a part of protein. This substance is found in breast milk, many types of baby formula, and most foods, especially those with a lot of protein, such as meat, eggs, and dairy products. If PKU is not treated, phenylalanine can build up in the blood and lead to intellectual disability and problems with the central nervous system (brain and spinal cord).

The good news is that early treatment can prevent all or most problems. Babies born with PKU need to start treatment with special formula soon after birth.

PKU is more common in whites and First Nations than in blacks, Hispanics, and Asians.

What causes PKU?

PKU is passed down through families. For a baby to have the disease, he or she must get (inherit) the PKU gene from both parents. The father and mother may not have PKU or even know that PKU runs in their families. If a baby gets the gene from only one parent, he or she is a carrier of the PKU gene but does not have the disease.

If you have a child with PKU and are thinking about having another baby, you may want to get genetic counselling. If you have a family history of PKU, talk with your doctor about genetic testing if you want to find out whether you carry the gene.

What are the symptoms?

If PKU is not found and treated soon after birth, symptoms usually start to appear within a few months after birth. (It takes time for the phenylalanine to build up in the baby's body.)

Early symptoms of PKU in a baby may include:

  • A musty odour to the skin, hair, and urine.
  • Skin problems.
  • Being sensitive to light.

Without early treatment, the child may have growth problems, developmental delays, seizures, and severe intellectual disability.

How is PKU diagnosed?

Screening is available to all newborns within a few days after birth.footnote 1 If the PKU screening test shows that your baby has a phenylalanine problem, the doctor will do further testing to check whether your baby has PKU.

Finding and starting treatment for PKU early usually can prevent brain damage and other long-term problems.

What treatment do children with PKU need?

The main treatment for PKU is a lifelong reduced-protein diet. Problems are less likely to occur if your baby starts a PKU diet by age 3 weeks. Your baby's doctor can help you choose a formula that doesn't contain phenylalanine. You may be able to feed your baby some breast milk, but talk to your baby's doctor first. As your child gets older, a registered dietitian can help you choose the right foods and recipes.

The medicine sapropterin (Kuvan) may help lower phenylalanine levels in some children who have PKU. More studies are needed to find out the health effects from long-term use. With or without the medicine, your child still needs to follow a PKU diet.

Your child will need regular blood tests. He or she may get tested as often as once or more a week for the first year and then once or twice a month throughout childhood.

It's normal to feel a wide range of emotions when your child has an illness like PKU. You may want to join a support group. Your doctor can also help.

What treatment do adults with PKU need?

People who have PKU need to follow a reduced-protein diet throughout their lives. For a few people, treatment with sapropterin (Kuvan) may help when used with a PKU diet. Talk to your doctor about the pros and cons of this medicine.

You'll need regular blood tests to check your phenylalanine level. If the level builds up, it can affect your IQ and your ability to learn, think, and understand.

Any woman with PKU who is planning to have a baby needs to be very careful to control her phenylalanine levels. Babies born to mothers who have high levels of phenylalanine are at risk for having a very small head, intellectual disability, growth problems, and heart problems. Having regular blood tests and following the PKU diet before and during pregnancy can help protect the baby.

Frequently Asked Questions

Learning about phenylketonuria (PKU):

Being diagnosed:

Getting treatment:

Other Places To Get Help


Canadian PKU and Allied Disorders
Canadian Paediatric Society: Caring for Kids



  1. Community Paediatrics Committee, Fetus and Newborn Committee (2011). Your newborn: Bringing baby home from the hospital. Available online:

Other Works Consulted

  • Somaraju UR, Merrin M (2012). Sapropterin dihydrochloride for phenylketonuria. Cochrane Database of Systematic Reviews (12).
  • Aminoff M (2009). Neurologic disorders. In RK Creasy et al., eds., Creasy and Resnik's Maternal-Fetal Medicine, 6th ed., pp. 1089–1112. Philadelphia: Saunders.
  • Committee on Genetics, American Academy of Pediatrics (2008). Maternal phenylketonuria. Pediatrics, 122(2): 445–449.
  • Durkin MS, et al. (2008). Childhood cognitive disability. In RB Wallace et al., eds., Wallace/Maxcy-Rosenau-Last Public Health and Preventive Medicine, 15th ed., pp. 1173–1183. New York: McGraw-Hill.
  • Feillet F, et al. (2010). Challenges and pitfalls in the management of phenylketonuria. Pediatrics, 126(2): 333–341.
  • Kaye CI (2006, reaffirmed 2011). Newborn screening fact sheets. Pediatrics, 118(3): e934–e963.
  • Poustie VJ, Wildgoose J (2010). Dietary interventions for phenylketonuria. Cochrane Database of Systematic Reviews (1).
  • Sapropterin (Kuvan) for phenylketonuria (2008). Medical Letter on Drugs and Therapeutics, 50(1287): 43–44.
  • Schwahn BC (2011). Hyperphenylalaninemias. In CD Rudolph et al., eds., Rudolph's Pediatrics, 22nd ed., pp. 561–563. New York: McGraw-Hill.
  • Thomas J, pet al. (2014). Inborn errors of metabolism. In WW Hay Jr et al., eds., Current Diagnosis and Treatment: Pediatrics, 22nd ed., pp. 1106–1133. New York: McGraw-Hill.


ByHealthwise Staff
Primary Medical ReviewerJohn Pope, MD - Pediatrics
Thomas M. Bailey, MD - Family Medicine
Adam Husney, MD - Family Medicine
Specialist Medical ReviewerChuck Norlin, MD - Pediatrics

Current as ofApril 21, 2015