Guaran effect on rat intestinal absorption. A perfusion study.

Among the components of dietary fiber, the soluble polysaccharides, primarily guaran and pectin, have been found to impair intestinal absorption. Little is known, however, about the mechanism of this effect. The direct action of guaran on small intestinal absorption was evaluated by a single-pass perfusion technique. Guaran in the perfusate (1-7 g/L) inhibited small intestinal absorption of actively transported compounds, such as alpha-methyl-D-glucoside, cycloleucine, and taurocholate, and also of the passively permeating solutes 2-deoxy-D-glucose and urea. Viscosity-related inhibition by guaran was found to depend on the rate of perfusion and was only detectable at perfusion rates below 0.4-0.5 ml/min. Higher perfusion rates abolished and even reversed the inhibitory effect. The observed alterations of absorption rates caused by guaran were completely reversible after switching to a guaran-free perfusate. The concentration-dependent absorption of D-glucose and alpha-methyl-D-glucoside at perfusion rates of 0.4 and 0.2 ml/min, respectively, revealed an increase in the transport constant and essentially unaltered maximal transport capacity in the presence of guaran. Additionally, net water absorption changed to secretion upon addition of guaran. When pectin and carrageenan were used in solutions of comparable viscosity, their effect was similar to that of guaran. The results suggest a general mechanism by which soluble, viscosity-enhancing polysaccharides influence the intestinal absorption of nutrients. The most likely explanation appears to be an increase in the unstirred layer resistance to diffusion. Under our experimental conditions, this occurred at low perfusion rates, but was increasingly counteracted by raising the rate of perfusion.