Human jejunal unstirred layer: evidence for extremely efficient luminal stirring.

Previous studies employing the osmotic transient technique have suggested that the human jejunal lumen is poorly stirred with a resultant unstirred layer thickness of approximately 600 microns. However, assuming negligible epithelial resistance to glucose absorption, we recently estimated that the unstirred layer thickness seemingly could not exceed 48 microns in the human jejunum. Because disaccharidases are located on the brush border, the rate of disaccharide hydrolysis can be used to determine unstirred layer thickness, independent of epithelial transport. In the present report, we utilized published hydrolysis data for sucrose and maltose to calculate the unstirred layer thickness in the normal human jejunum. This calculation indicated that the unstirred layer thickness was only approximately 35 microns, less than one-fifteenth of previously reported 600-microns values obtained with the osmotic transient technique. Diffusion through a 600-microns unstirred layer would be the rate-limiting step in absorption of all rapidly transported compounds. In contrast, with a 35-microns unstirred layer, variations in epithelial function or luminal stirring could readily influence the absorption.