Phenylketonuria (PKU)

Phenylketonuria (PKU) definition and facts*

*Phenylketonuria (PKU) medically edited by: Charles Patrick Davis, MD, PhD

  • Phenylketonuria (PKU) is a disease that's inherited that increases the levels of phenylalanine in the blood. If left untreated, high phenylalanine levels can cause intellectual disability and other problems. Phenylketonuria disease was discovered in 1934 by Dr. A. Folling.
  • Signs and symptoms of PKU vary from mild to severe depending upon phenylalanine levels. Infants appear normal until a few months old when they start to show
    • intellectual disability,
    • seizures,
    • delayed development,
    • behavior problems, and
    • psychiatric disorders.
  • Children with untreated PKU also may have lighter skin and hair than other family members, and also may develop a musty or mouse-like odor due to excess phenylalanine. These are classic PKU symptoms.
  • Less severe forms of PKU (sometimes termed variant PKU and non-PKU hyperphenylalaninemia may develop less severe symptoms with a smaller risk of brain damage.
  • Women with PKU that have uncontrolled phenylalanine levels can have babies that are at significant risk for intellectual disability, low birth weight, heart defects, microcephaly, behavior problems, and slow growth.
  • PKU occurs in about 1/10,000 to15,000 births, but classic PKU symptoms are rarely seen because of newborn screening tests for the disease.
  • Mutations in the PAH gene cause phenylketonuria. Some mutations in this gene allow the enzyme to retain some activity resulting in variant PKU and/or non-PKU hyperphenylalaninemia.
  • PKU is inherited in an autosomal recessive pattern so that both copies of the gene in the cell have mutations. Consequently, the autosomal recessive condition in both parents carry one copy of the mutated gene and parents typically do not show symptoms. Symptoms of PKU develop in their children when both parents donate one autosomal recessive condition to the newborn.
  • PKU has many different names, for example, phenylketonuria, PKU, Folling Disease, Folling's Disease, and phenylalanine hydroxylase deficiency disease.
  • PKU does not shorten a person's life expectancy, with or without treatment.
  • Blood tests for PKU is required for infants (newborns) in all 50 states.
  • There is a 1 in 4 chance of having a PKU infant when both parents are genetic carriers. The approximate incident rate of PKU in the US is 0.01%. This means about 74 infants every day are diagnosed with PKU.
  • PKU patients usually are treated in a special metabolic disease clinic with restrictions on foods that contain phenylalanine and are given amino acids, vitamins, minerals, and other micronutrients along with close monitoring of phenylalanine levels.
  • A PKU diet bans meat, fish, chicken, eggs, nuts, beans, milk, and other dairy products. People with PKU also need to avoid aspartame, a type of peptide composed of aspartic acid and phenylalanine.

What is phenylketonuria?

Phenylketonuria (commonly known as PKU) is an inherited disorder that increases the levels of a substance called phenylalanine in the blood. Phenylalanine is a building block of proteins (an amino acid) that is obtained through the diet. It is found in all proteins and in some artificial sweeteners. If PKU is not treated, phenylalanine can build up to harmful levels in the body, causing intellectual disability and other serious health problems.

Medically Reviewed by a Doctor on 7/28/2016

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