William Prout (1784–1850) specialised in organic chemistry. He was the first to recognise the organic deterioration of tissues, and he revealed that the gastric juices of animals contain hydrochloric acid. He published papers on urine and digestion that contributed to the understanding of metabolism. He also demonstrated that the main food groups (carbohydrate, protein and fats) were transformed into blood and tissues, ‘primary assimilation’, by a vital principle that also controlled the removal of unwanted parts from the system, ‘secondary assimilation’. Prout was born into a well-established Gloucestershire family. His early education was neglected in favour of farming, but at the age of 17 he left to study medicine at Edinburgh University.