Xylose transport in the human jejunum

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Abstract
D-Xylose transport in the human jejunum was studied in vivo using a standard intestinal perfusion technique, and also in vitro in human jejunal brush border membrane vesicles. Initiald-xylose concentrations were linearly related tod-xylose absorption rates, a finding consistent with passive diffusion. Perfusion ofd-xylose with varyingd-glucose concentrations were aimed at examiningd-xylose—d-glucose jejunal cotransport.d-Xylose absorption rates from a 30 mMd-xylose perfusate did not change significantly when 10, 30, or 60 mM glucose were added (−3.0±0.62 vs −3.34±0.71, −3.82±0.81, and −4.56±0.72 mM/30 cm/hr, respectively; minus indicates net absorption) suggesting an absence of a cotransport system. In brush border membrane vesicles, xylose uptake was partially inhibited byd-glucose and phlorizin. These data suggest that jejunald-xylose absorption, at concentrations used clinically, is by passive diffusion, which process completely overrides a minord-glucose cotransport component. Thed-xylose tolerance test, therefore, reflects jejunal mucosal surface area and mucosal permeability tod-xylose and not nutrient carbohydrate absorption.