urobilinogen

a colourless product of the reduction of the bile pigment bilirubin. Urobilinogen is formed from bilirubin in the intestine by bacterial action. Part of it is reabsorbed and returned to the liver; part of it is excreted in the faeces (a trace may also appear in the urine). When exposed to air, urobilinogen is oxidized to a brown pigment, urobilin.