Phenylketonuria (PKU)

Phenylketonuria affects around one in every 8,000 babies born in Scotland. It is a condition that is inherited when both parents are asymptomatic carriers of one active and one inactive copy of the gene. Inheriting two inactive copies of the gene causes deficiency of a liver enzyme known as phenylalanine hydroxylase which, in turn produces high levels of phenylalanine in all body fluids. Babies with PKU cannot metabolise phenylalanine, which is a component of all natural protein in every day foods, for example, milk, fish, eggs and cheese. If phenylalanine levels remain high this results in severe damage to the baby’s brain.


Babies diagnosed with this condition must be seen by a paediatrician as soon as possible and started on a special diet. Therapy for PKU relies on the partial elimination of phenylalanine from the diet. Since phenylalanine is present in all proteins, this requires the use of chemically-defined protein substitutes with a limited amount of natural protein. The effectiveness of therapy is monitored by regular blood samples which accurately measure the phenylalanine level. With prompt treatment, the baby is very likely to develop normally. The treatment is lifelong and effective management requires a multidisciplinary team comprising doctors, dietitians, clinical biochemists, psychologists and pharmacists.

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