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Phenylketonuria (PKU)

PKU is a serious and rare inherited metabolic condition. Patients with PKU lack an enzyme called phenylalanine hydroxylase. This means that they are unable to break down an amino acid called phenylalanine. As a result, levels of phenylalanine build up in the blood and brain. Unmanaged, this can lead to brain damage which causes learning and behavioural difficulties.1

Around 1 in 10,000 babies born in the UK have PKU.2

PKU does not usually cause any symptoms if treatment is started early. Possible symptoms of unmanaged PKU include behavioural difficulties, fair skin and hair, vomiting, eczema, abnormal movements in the arms and legs, tremors, epilepsy and a musty smell to the breath, skin and urine.2


How is PKU diagnosed?

At around 5 days old, babies are offered newborn blood spot screening to test for PKU and many other conditions. This involves pricking the baby’s heel to collect a few drops of blood to test.1 High levels of phenylalanine will be detected in the blood.2 Thanks to newborn screening, PKU can be detected early in the first few days of life.


How is PKU managed?

Early management is absolutely vital in order to prevent brain damage and learning difficulties.

Management of this condition is through a carefully controlled low protein diet that contains measured amounts of phenylalanine containing foods, protein substitutes and supplements. Foods high in protein must be avoided such as meat, fish, eggs, cheese, bread and pasta.1 Any food or drink containing aspartame must also be avoided as this is converted to phenylalanine in the body. Examples of aspartame containing products include sugar substitutes, chewing gum, squash/cordial, diet versions of fizzy drinks and some medications.2

Together, these measures will help the patient meet their protein, energy, vitamin and mineral requirements. Many protein substitutes are available on prescription.

A baby with PKU will require a carefully regulated milk intake (may be formula or breastmilk), according to specialist metabolic dietetic input.1

Patients will require frequent blood tests to monitor their condition, as well as regular developmental checks and dietetic input. Diet is adjusted accordingly.1

With early diagnosis and careful management, most children with PKU are able to live healthy lives.2

Women with PKU must adhere strictly to their management plan with close dietetic input if they are pregnant or considering becoming pregnant as high phenylalanine levels can harm an unborn child.2


References PKU

1. British Inherited Metabolic Diseases Group. Temple Guide. PKU. April 2020. Available from; https://www.bimdg.org.uk/store/temple//B20030_TEMPLE_PKU_A5_DIGITAL_-_Final_841093_23062020.pdf (Accessed 19th Oct 2021)
2. https://www.nhs.uk/conditions/phenylketonuria/ (Assessed 19th Oct 2021)

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