Phenylketonuria (PKU)

A rare inherited metabolic disorder in which the enzyme phenylalanine hydroxylase is defective or absent, preventing the normal conversion of phenylalanine to tyrosine. This leads to phenylalanine accumulation, which is neurotoxic at high concentrations. PKU is managed by a strict low-phenylalanine diet. Products containing aspartame (which metabolises to phenylalanine) must carry mandatory "Contains Phenylalanine" or "Phenylketonurics: Contains Phenylalanine" warnings in most markets. Individuals with PKU require L-Tyrosine supplementation (Section 45) as an essential amino acid since they cannot synthesise it from phenylalanine.