Paracellular intestinal transport of six-carbon sugars is negligible in the rat

Background & Aims: Active d-glucose absorption has been theorized to increase convective flow and enhance tight junction permeability such that paracellular transport becomes the major mechanism of d-glucose absorption. This concept was tested in rats by measuring the absorption of four gavaged, nonmetabolizable six-carbon sugars (l-glucose, l-galactose, l-mannose, and d-mannitol) thought to be absorbed solely by the paracellular route. Methods: Uptake of gavaged probes was measured by recovery in 24-hour urine specimen collections. Results: l-glucose (71.2% ± 2.4%) absorption exceeded that of the other probes (1.4%-9%). Coadministration of 3.0 mol/L d-glucose, 0.22 mol/L d-glucose, or chow significantly reduced the absorption of l-glucose to 38.1% ± 7.2%, 61% ± 3.3%, and 53.6% ± 3.5%, respectively, but did not influence the absorption of the other six-carbon probes. Conclusions: (1) l-glucose seems to have a weak affinity for a d-glucose carrier and is not a marker of paracellular transport, and (2) paracellular transport accounts for a minimal fraction of d-glucose uptake; this fraction is not enhanced by ingestion of d-glucose or chow.