Many molecules that are considered to be "dietary fiber" are so because humans lack the necessary enzymes to split the
glycosidic bond and they reach the large intestine. Many foods contain varying types of dietary fibers, all of which contribute to health in different ways. Dietary fibers make three primary contributions: bulking, viscosity and fermentation. Different fibers have different effects, suggesting that a variety 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. 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. 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.
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. In the upper gastrointestinal tract, these compounds consist of bile acids and di- and monoacyl
glycerols which solubilize
triacylglycerols and cholesterol. 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.
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==