Absorbed nutrients follow three possible pathways in the body. They may be incorporated intact into body components (such as proteins), converted into physiologically essential materials (as in the conversion of tyrosine into thyroxine) or broken down with the release of energy and the formation of elementary excretory products. A pathological condition clearly attributable to a disturbance of one of these metabolic patterns is described as a ‘disease of metabolism.’ Genetically determined diseases of metabolism are call ‘inborn errors of metabolism.’