The discovery of phenylketonuria

In 1934, two severely mentally retarded children were examined by Dr Asbjørn Følling. He proved, by classical organic chemistry, that they excreted phenylpyruvic acid in their urine. The substance was also found in the urine of eight additional mentally retarded patients. Based on these observations, oligophrenia phenylpyrouvica (later termed phenylketonuria) was described as a new inborn error of metabolism. Følling later showed the pattern of an autosomal recessive genetic disease, probably caused by a block in phenylalanine metabolism, and that asymptomatic heterozygote carriers of the trait could be detected by phenylalanine loading. The stepwise elucidation and the line of reasoning are described. Phenylketonuria was the first inborn error of metabolism shown to affect the mind, and its importance as a model disease is emphasized. The article finally gives some insight into aspects of the personality of the discoverer.