Phenylketonuria (PKU) is a serious, congenital, lifelong metabolic disease caused by a defect in the phenylalanine hydroxylase (PAH) enzyme. The role of PAH is to break down excess phenylalanine in the body. Phenylalanine is a necessary part of the human diet and is naturally occurring in all kinds of dietary protein.
In people without PKU, the PAH enzyme converts any excess phenylalanine in the body into the amino acid tyrosine. However, in people suffering from PKU, the lack of PAH enzyme activity allows phenylalanine to accumulate in blood and tissues to abnormally high levels, which is toxic to the brain.
If left untreated, PKU in children and young people leads to irreversible brain damages. In adults, high level of Phe in the blood and tissues causes classic intoxication symptoms such as concentration difficulties, irritability, headache, etc. Permanent neurological damages may also occur if adults are exposed to high blood phenylalanine for longer time, but this is still subject to investigation.
PKU can be diagnosed at birth and is commonly included in newborn screening programs in most developed countries. All newborns diagnosed must immediately follow a very strict, lifelong protein-limited diet in order to develop normally. The diet severely restricts or eliminates foods high in protein, such as bread and pasta, egg, all dairy products, all types of meats and fish, legumes, etc., while starchy foods, such as potatoes and corn, are acceptable in controlled volumes although the quantity of Phe consumed from these foods must be monitored.