PKU Overview

Phenylketonuria (PKU) is an inherited genetic disorder that affects approximately 1 in 15,000 people in the United States.  A person with PKU does not produce enough of a certain liver enzyme called phenylalanine hydroxylase (PAH). This enzyme is needed to process the amino acid phenylalanine (Phe). Phe is found in food items that contain protein. Usually when people eat protein, it is broken down into different amino acids that are then processed and used for growth and repair of body tissue. Individuals with PKU, however, cannot process Phe in this way. Instead, Phe builds up in the blood and and can become toxic to the brain.

When a person with PKU consumes protein-rich foods, his or her body cannot break down most of the Phe. This leads to high levels of Phe in the blood. This high level of Phe causes cognitive as well as behavioral problems. It can also lead to insufficient tyrosine (Tyr) levels, an amino acid that is a precursor to neurotransmitters and hormones.

Restricting dietary Phe intake is important for a person with PKU to maintain good health. Individualized treatment plans are developed for each person with PKU based on the amount of Phe the person can consume without increasing his or her Phe levels. The main treatment for PKU is a low-Phe diet that includes:


  • Medical formula that people with PKU must consume every day to take in protein without Phe
  • Special low-protein (“low-pro”) medical foods that can replace common foods such as pasta, bread, and cheese to ensure those on the low-Phe diet get enough calories each day

Some people also take a medication called Kuvan® (sapropterin dihydrochloride). When this medication is used together with a low-Phe diet, it can help people with PKU maintain metabolic control.

All people with PKU should remain on a low-Phe diet to ensure Phe levels are within the recommended range of 120–360 μmol/L (2–6 mg/dL). People with PKU who relax their diet in the teen or adult years report issues with executive function such as with difficulties with memory and planning, and psychiatric symptoms such as depression and anxiety.

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