Phenylketonuria Dr Trisha Macnair Phenylketonuria (PKU) is an inherited enzyme deficiency that can result in brain damage if not detected early.

In this article

What is it? What are the symptoms?

Who's affected? How's it diagnosed?

What's the treatment? Advice and support

Phenylalanine is an essential amino acid, which means the body must have it to grow and function properly. Too much can be harmful, so the body uses the enzyme phenylalanine hydroxylase to convert it into tyrosine.

In PKU, levels of this enzyme are low. This means only limited amounts if phenylalanine can be converted into tyrosine. As a result, phenylalanine build ups in the blood and other body tissues, particularly the brain, where it can cause severe and irreversible damage.

If PKU isn't detected early, progressive developmental delay and severe learning difficulties can occur. Other problems may include epilepsy and eczema.

Children with PKU typically have very fair hair and their urine has a musty smell.

PKU affects about one in 10,000 people. Classic PKU and several less common variants are all inherited in an autosomal recessive fashion. The PKU gene is found on chromosome 12.

All newborn babies are screened for PKU using a heel-prick blood sample. Antenatal screening is also available.

There's no cure for PKU but starting a low-phenylalanine diet as soon as possible after birth can help to avoid the harmful effects.

Phenylalanine is found in many proteins, so protein-rich foods such as meat must be avoided and replaced with 'safe' proteins, usually in the form of specially formulated substitutes. This diet should be followed in the long term.

Women with PKU must follow this diet while pregnant or they risk exposing their unborn children to high levels of the amino acid. This can lead to miscarriage, birth defects and brain damage, even if the children don't inherit PKU themselves.

National Society for Phenylketonuria
Tel: 020 8364 3010
Email: info@nspku.org
Website: www.nspku.org

This article was last medically reviewed by Dr Rob Hicks in July 2006.