Translations:Dietary fiber/37/en

Two mechanisms bring nutrients into contact with the epithelium:

1. intestinal contractions create turbulence; and
2. convection currents direct contents from the lumen to the epithelial surface.

The multiple physical phases in the intestinal tract slow the rate of absorption compared to that of the suspension solvent alone.

1. Nutrients diffuse through the thin, relatively unstirred layer of fluid adjacent to the epithelium.
2. Immobilizing of nutrients and other chemicals within complex polysaccharide molecules affects their release and subsequent absorption from the small intestine, an effect influential on the glycemic index.
3. Molecules begin to interact as their concentration increases. During absorption, water must be absorbed at a rate commensurate with the absorption of solutes. The transport of actively and passively absorbed nutrients across epithelium is affected by the unstirred water layer covering the microvillus membrane.
4. The presence of mucus or fiber, e.g., pectin or guar, in the unstirred layer may alter the viscosity and solute diffusion coefficient.