Sugar Transport and Fructose Metabolism in Human Intestine In Vitro*

Abstract

Present knowledge regarding sugar transport by human intestine has been derived primarily by extrapolation of data from other species. Previ-ous investigations, performed chiefly with everted sacs of hamster intestine, show that a sugar re-quires a specific structure before it can be trans-ported by intestine against a concentration gradi-ent, a process referred to as "active transport" (1, 2). Thus, in the hamster, the necessary re-quirements for a monosaccharide to be actively transported are a pyranose ring, a D-configura-tion, a hydroxyl group in the glucose configura-tion at carbon 2, and a methyl or substituted methyl group at carbon 5. Sugars that lack one of these features, or possess a large substituent on some part of their structure, are not actively trans-ported. Fructose, lacking these requirements, is therefore not transported against a gradient. Nonetheless, fructose disappears from the rat in-testinal lumen at a rate intermediate between that for sugars such as glucose or galactose, which are actively transported, and sugars such as mannose or pentoses, which are not (3). Since passive dif-fusion of a sugar through the intestinal wall is de-pendent upon the difference between its intracel-lular and extracellular concentration (2), the enhanced rate of fructose disappearance as com-pared to mannose or pentoses may be accounted *Submitted for publication January 8, 1965; accepted