SummaryThe changes induced by dietary xylitol in the gastrointestinal tract of the rat were investigated in relation to the phenomenon of vitamin‐sparing. Within 18 days of consuming a synthetic diet, deficient in thiamin, riboflavin and pyridoxine, rats ceased to grow and began to lose weight rapidly. If xylitol was then included in the diet (10% w/w), the effect of the vitamin‐deficient diet on growth was reversed. Moreover, within 3 days of the rats ingesting xylitol, the metabolism of this sugar polyol by the caecal microflora was increased 17‐fold and the caecal concentrations of thiamin and thiamin pyrophosphate were increased 5‐fold. Increases were also observed in the caecal size, the weight of the caecal contents, and the weight of the caecal wall. In contrast to the rapid changes observed within the caecum, liver thiamin pyrophosphate levels did not rise until 6–12 days after the feeding of xylitol, at which time the rats had begun to gain weight.The caecal contents were shown to contain facultative bacteria which have the ability to metabolise and grow on xylitol and which can, at the same time, synthesise thiamin. Species of the generaKlebsiella, SerratiaandMicrococcuswhich have this ability were isolated from the caecal contents of rats. It is assumed that coprophagy is the means by which the thiamin and other vitamins synthesised by enteral bacteria become available to the host, although some absorption from the caecum cannot be excluded.