Inherited disorders: Phenylketonuria

Last updated: Wednesday, July 10, 2024

Patients with phenylketonuria (PKU) are unable to break down the amino acid phenylalanine because the enzyme responsible, phenylalanine hydroxylase, is reduced or absent due to genetic variation. This allows phenylalanine to build up to potentially toxic levels. Nerve cells in the brain are particularly sensitive to this amino acid and they can become damaged when exposed to high levels.

As mentioned on the page for cystic fibrosis, in the UK, the families of infants are offered newborn screening within the first week of life for PKU. 

Symptoms of untreated PKU can include behavioural difficulties, epilepsy, tremors, jerky movements, eczema, vomiting, and reduced pigment of the skin, hair and eyes. The main treatment is a controlled, low-protein diet to reduce intake of phenylalanine, but patients also have to take amino acid supplements to make sure they're getting all the nutrients required for normal growth and good health. Some patients may also be eligible for a relatively new treatment called sapropterin

Chemical structure for aspartame (left) and phenylalanine (right)

People with PKU must avoid the sweetener aspartame because it is converted to phenylalanine by the body. Aspartame is found in some foods and drinks, and also in some medicines. As a pharmacist you may be asked to check whether a medicine contains aspartame, or to advise on an aspartame-free alternative. It is a legal requirement for any medicine that contains aspartame to state it on the patient information leaflet. You can use the 'Advanced search' function of the emc to search for medicines that don't contain aspartame.