of dietary fibers contribute to overall health. some fibers contribute through one primary mechanism. for instance, cellulose and wheat bran provide excellent bulking effects, but are minimally fermented. alternatively, many dietary fibers can contribute to health through more than one of these mechanisms. for instance, psyllium provides bulking as well as viscosity. bulking fibers can be soluble (e.g. psyllium) or insoluble (e.g. cellulose and hemicellulose). they absorb water and can significantly increase stool weight and regularity. most bulking fibers are not fermented or are minimally fermented throughout the intestinal tract. viscous fibers thicken the contents of the intestinal tract and may attenuate the absorption of sugar, reduce sugar response after eating, and reduce lipid absorption (notably shown with cholesterol absorption). their use in food formulations is often limited to low levels, due to their viscosity and thickening effects. some viscous fibers may also be partially or fully fermented within the intestinal tract (guar gum, beta-glucan, glucomannan and pectins), but some viscous fibers are minimally or not fermented (modified cellulose such as methylcellulose and psyllium). fermentable fibers are consumed by the microbiota within the large intestines, mildly increasing fecal bulk and producing short-chain fatty acids as byproducts with wide-ranging physiological activities. resistant starch, inulin, fructooligosaccharide and galactooligosaccharide are dietary fibers which are fully fermented. these include insoluble as well as soluble fibers. this fermentation influences the expression of many genes within the large intestine, which affect digestive function and lipid and glucose metabolism, as well as the immune system, inflammation and more. fiber fermentation produces gas (majorly carbon dioxide, hydrogen, and methane) and short-chain fatty acids. isolated or purified fermentable fibers are more rapidly fermented in the fore-gut and may result in undesirable gastrointestinal symptoms (bloating, indigestion and flatulence). dietary fibers can change the nature of the contents of the gastrointestinal tract and can change how other nutrients and chemicals are absorbed through bulking and viscosity. some types of soluble fibers bind to bile acids in the small intestine, making them less likely to re-enter the body; this in turn lowers cholesterol levels in the blood from the actions of cytochrome p450-mediated oxidation of cholesterol. insoluble fiber is associated with reduced risk of diabetes, but the mechanism by which this is achieved is unknown. one type of insoluble dietary fiber, resistant starch, may increase insulin sensitivity in healthy people, in type 2 diabetics, and in individuals with insulin resistance, possibly contributing to reduced risk of type 2 diabetes. not yet formally proposed as an essential macronutrient, dietary fiber has importance in the diet, with regulatory authorities in many developed countries recommending increases in fiber intake. physicochemical properties dietary fiber has distinct physicochemical properties. most semi-solid foods, fiber and fat are a combination of gel matrices which are hydrated or collapsed with microstructural elements, globules, solutions or encapsulating walls. fresh fruit and vegetables are cellular materials. the cells of cooked potatoes and legumes are gels filled with gelatinized starch granules. the cellular structures of fruits and vegetables are foams with a closed cell geometry filled with a gel, surrounded by cell walls which are composites with an amorphous matrix strengthened by complex carbohydrate fibers. particle size and interfacial interactions with adjacent matrices affect the mechanical properties of food composites. food polymers may be soluble in and/or plasticized by water. the variables include chemical structure, polymer concentration, molecular weight, degree of chain branching, the extent of ionization (for electrolytes), solution ph, ionic strength and temperature. cross-linking of different polymers, protein and polysaccharides, either through chemical covalent bonds or cross-links through molecular entanglement or hydrogen or ionic bond cross-linking. cooking and chewing food alters these physicochemical properties and hence absorption and movement through the stomach and along the intestine upper gastrointestinal tract following a meal, the stomach and upper gastrointestinal contents consist of food compounds complex lipids/micellar/aqueous/hydrocolloid and hydrophobic phases hydrophilic phases solid, liquid, colloidal and gas bubble phases. micelles are colloid-sized clusters of molecules which form in conditions as those above, similar to the critical micelle concentration of detergents. in the upper gastrointestinal tract, these compounds consist of bile acids and di- and monoacyl glycerols which solubilize triacylglycerols and cholesterol. two mechanisms bring nutrients into contact with the epithelium: intestinal contractions create turbulence; and 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. 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. 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. the presence of mucus or fiber, e.g., pectin or guar, in the unstirred layer may alter the viscosity and solute diffusion coefficient. 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. colon the colon may be regarded as two organs, the right side (cecum and ascending colon), a fermenter. the right side of the colon is involved in nutrient salvage so that dietary fiber, resistant starch, fat and protein are utilized by bacteria and the end-products absorbed for use by the body the left side (transverse, descending, and sigmoid colon), affecting continence. the presence of bacteria in the colon produces an 'organ' of intense, mainly reductive, metabolic activity, whereas the liver is oxidative. the substrates utilized by the cecum have either passed along the entire intestine or are biliary excretion products. the effects of dietary fiber in the colon are on bacterial fermentation of some dietary fibers thereby an increase in bacterial mass an increase in bacterial enzyme activity changes in the water-holding capacity of the fiber residue after fermentation enlargement of the cecum is a common finding when some dietary fibers are fed and this is now believed to be normal physiological adjustment. such an increase may be due to a number of factors, prolonged cecal residence of the fiber, increased bacterial mass, or increased bacterial end-products. some non-absorbed carbohydrates, e.g. pectin, gum arabic, oligosaccharides and resistant starch, are fermented to short-chain fatty acids (chiefly acetic, propionic and n-butyric), and carbon dioxide, hydrogen and methane. almost all of these short-chain fatty acids will be absorbed from the colon. this means that fecal short-chain fatty acid estimations do not reflect cecal and colonic fermentation, only the efficiency of absorption, the ability of the fiber residue to sequestrate short-chain fatty acids, and the continued fermentation of fiber around the colon, which presumably will continue until the substrate is exhausted. the production of short-chain fatty acids has several possible actions on the gut mucosa. all of the short-chain fatty acids are readily absorbed by the colonic mucosa, but only acetic acid reaches the systemic circulation in appreciable amounts. butyric acid appears to be used as a fuel by the colonic mucosa as the preferred energy source for colonic cells. cholesterol metabolism dietary fiber may act on each phase of ingestion, digestion, absorption and excretion to affect cholesterol metabolism, such as the following: caloric energy of foods through a bulking effect slowing of gastric emptying time a glycemic index type of action on absorption a slowing of bile acid absorption in the ileum so bile acids escape through to the cecum altered or increased bile acid metabolism in the cecum indirectly by absorbed short-chain fatty acids, especially propionic acid, resulting from fiber fermentation affecting the cholesterol metabolism in the liver. binding of bile acids to fiber or bacteria in the cecum with increased fecal loss from the entero-hepatic circulation. one action of some fibers is to reduce the reabsorption of bile acids in the ileum and hence the amount and type of bile acid and fats reaching the colon. a reduction in the reabsorption of bile acid from the ileum has several direct effects. bile acids may be trapped within the lumen of the ileum either because of a high luminal viscosity or because of binding to a dietary fiber. lignin in fiber adsorbs bile acids, but the unconjugated form of the bile acids are adsorbed more than the conjugated form. in the ileum where bile acids are primarily absorbed the bile acids are predominantly conjugated. the enterohepatic circulation of bile acids may be altered and there is an increased flow of bile acids to the cecum, where they are deconjugated and 7alpha-dehydroxylated. these water-soluble form, bile acids e.g., deoxycholic and lithocholic are adsorbed to dietary fiber and an increased fecal loss of sterols, dependent in part on the amount and type of fiber. a further factor is an increase in the bacterial mass and activity of the ileum as some fibers e.g., pectin are digested by bacteria. the bacterial mass increases and cecal bacterial activity increases. the enteric loss of bile acids results in increased synthesis of bile acids from cholesterol which in turn reduces body cholesterol. the fibers that are most effective in influencing sterol metabolism (e.g. pectin) are fermented in the colon. it is therefore unlikely that the reduction in body cholesterol is due to adsorption to this fermented fiber in the colon. there might be alterations in the end-products of bile acid bacterial metabolism or the release of short chain fatty acids which are absorbed from the colon, return to the liver in the portal vein and modulate either the synthesis of cholesterol or its catabolism to bile acids. the prime mechanism whereby fiber influences cholesterol metabolism is through bacteria binding bile acids in the colon after the initial deconjugation and dehydroxylation. the sequestered bile acids are then excreted in feces. fermentable fibers e.g., pectin will increase the bacterial mass in the colon by virtue of their providing a medium for bacterial growth. other fibers, e.g., gum arabic, act as stabilizers and cause a significant decrease in serum cholesterol without increasing fecal bile acid excretion. fecal weight feces consist of a plasticine-like material, made up of water, bacteria, lipids, sterols, mucus and fiber. feces are 75% water; bacteria make a large contribution to the dry weight, the residue being unfermented fiber and excreted compounds. fecal output may vary over a range of between 20 and 280 g over 24 hours. the amount of feces egested a day varies for any one individual over a period of time. of dietary constituents, only dietary fiber increases fecal weight. water is distributed in the colon in three ways: free water which can be absorbed from the colon. water that is incorporated into bacterial mass. water that is bound by fiber. fecal weight is dictated by: the holding of water by the residual dietary fiber after fermentation. the bacterial mass. there may also be an added osmotic effect of products of bacterial fermentation on fecal mass. effects of fiber intake preliminary research indicates that fiber may affect health by different mechanisms. color coding of table entries: both applies to both soluble and insoluble fiber soluble applies to soluble fiber only insoluble applies to insoluble fiber only effects increases food volume without increasing caloric content to the same extent as digestible carbohydrates, providing satiety which may reduce appetite. attracts water and forms a viscous gel during digestion, slowing the emptying of the stomach, shortening intestinal transit time, shielding carbohydrates from enzymes, and delaying absorption of glucose, which lowers variance in blood sugar levels lowers total and ldl cholesterol, which may reduce the risk of cardiovascular disease regulates blood sugar, which may reduce glucose and insulin levels in diabetic patients and may lower risk of