Editing Dietary fiber (section)

===Upper gastrointestinal tract===

Following a meal, the stomach and upper gastrointestinal contents consist of
* food compounds
* complex lipids/[[micelles|micellar]]/[[aqueous]]/[[hydrocolloid]] and [[hydrophobic]] phases
* [[hydrophilic]] phases
* solid, liquid, colloidal and gas bubble phases.<ref name=Eastwood92/>
[[Micelle]]s are colloid-sized clusters of molecules which form in conditions as those above, similar to the critical micelle concentration of detergents.<ref name=Carey>{{cite journal |vauthors=Carey MC, Small DM, Bliss CM |title=Lipid digestion and absorption |journal=Annu Rev Physiol |volume=45 |issue= |pages=651–77 |date=1983 |pmid=6342528 |doi=10.1146/annurev.ph.45.030183.003251 }}</ref>
In the upper gastrointestinal tract, these compounds consist of bile acids and di- and monoacyl [[glycerol]]s which solubilize [[triacylglycerol]]s and cholesterol.<ref name=Carey/>

Two mechanisms bring nutrients into contact with the epithelium:
# intestinal contractions create turbulence; and
# convection currents direct contents from the [[lumen (anatomy)|lumen]] to the epithelial surface.<ref name=Edwards>{{cite journal | vauthors = Edwards CA, Johnson IT, Read NW | title = Do viscous polysaccharides slow absorption by inhibiting diffusion or convection? | journal = European Journal of Clinical Nutrition | volume = 42 | issue = 4 | pages = 307–12 | date = April 1988 | pmid = 2840277 }}</ref>
The multiple physical phases in the intestinal tract slow the rate of absorption compared to that of the suspension solvent alone.
# Nutrients diffuse through the thin, relatively unstirred layer of fluid adjacent to the epithelium.
# 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]].<ref name=Edwards/>
# 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.<ref name=Edwards/>
# The presence of mucus or fiber, e.g., pectin or guar, in the unstirred layer may alter the viscosity and solute diffusion coefficient.<ref name=Eastwood92/>

Adding viscous polysaccharides to carbohydrate meals can reduce [[post-prandial]] blood glucose concentrations. Wheat and maize but not oats modify glucose absorption, the rate being dependent upon the particle size. The reduction in absorption rate with guar gum may be due to the increased resistance by viscous solutions to the convective flows created by intestinal contractions.

Dietary fiber interacts with pancreatic and enteric enzymes and their substrates. Human pancreatic enzyme activity is reduced when incubated with most fiber sources. Fiber may affect [[amylase]] activity and hence the rate of hydrolysis of starch. The more viscous polysaccharides extend the mouth-to-[[cecum]] transit time; guar, [[tragacanth]] and pectin being slower than wheat bran.<ref>{{cite journal |vauthors=Schneeman BO, Gallaher D |title=Effects of dietary fiber on digestive enzyme activity and bile acids in the small intestine |journal=Proc Soc Exp Biol Med |volume=180 |issue=3 |pages=409–14 |date=December 1985 |pmid=2417249 |doi=10.3181/00379727-180-42197 }}</ref>