What is phenylketonuria?

Phenylketonuria, or PKU, is a rare genetic disorder. People with this disorder lack a substance in the body called phenylalanine hydroxylase (PAH). This substance is an enzyme needed to break down an amino acid called phenylalanine. Phenylalanine is a 'building block' that we get from our food that is required for proper growth and development. Because people with PKU lack the enzyme PAH, the amino acid, phenylalanine builds up in the blood, brain and other tissues of the body. When phenylalanine builds up, it can damage the body and harm the brain. If untreated, PKU can eventually cause serious problems including intellectual disabilities, seizures, and behavioral problems. The main form of the disorder, classic PKU, is usually identified through newborn screening so treatment is often started before symptoms begin. People get phenylalanine mostly through what they eat. Phenylalanine is found in all foods containing protein and in some artificial sweeteners. PKU is treated by a low-protein diet and the use of Medical Foods (including specialized formulas) that contain very little or no phenylalanine. PKU is caused by changes (alterations) in the PAH gene and is passed on in a family in an autosomal recessive manner. There is a mild form called non-PKU hyperphenylalaninemia. These people have a mild buildup of phenylalanine in the body, but often do not require treatment.

SOURCE: Emory University - Department of Human Genetics in collaboration with ThinkGenetic • • DATE UPDATED: 2019-01-23


Mitchell JJ. Phenylalanine Hydroxylase Deficiency. GeneReviews website. Accessed June 5, 2016.

Phenylketonuria. Genetics Home Reference website. Accessed June 5, 2016.

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