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Editor,—I thank Dr Craig for raising this issue but I do not see any conceptual error. The apparent inconsistency that he points out in our paper ((2001) Gut 48:28–33; OpenUrlCrossRefPubMedWeb of Science) is due to the controversy surrounding transcellular permeation of mannitol, as well as of other monosaccharides.1-1 While transcellular permeation of mannitol is well known, its use for osmotic shrinkage of membrane vescicles1-2 and as an extracellular fluid volume marker suggests that, at least in part, mannitol diffuses through the intercellular tight junctions. Thus it seems justified talking of permeability for mannitol. One of the reasons for its use in combination with cellobiose is the different molecular sizes of the two probes: the smaller size of mannitol allows its passage through the small tight junctions of the villi while the larger cellobiose passes through the larger tight junctions of the crypts.

Finally, we did find an increase in cellobiose excretion after fluorouracil (5-FU) that was in part prevented by oral glutamine. Although this difference did not reach statistical significance, overall the data indicate increased intestinal permeability after 5-FU, partially prevented by oral glutamine.