Fiber

Dietary Fiber(s) is comprised of indigestible carbohydrate polymers and lignin (phenolic polymers) that, for the most part, come from the cell walls of plants. Humans can’t digest the fiber because we don’t have the enzymes to do so (although some of the bacteria in our gut may). Our enzymes either can’t recognize the type of carbohydrate in the polymer or can’t hydrolyze the bond. Example; Cellulose is comprised of glucose polymers where the glucose molecules are joined by a β-1,4 link as opposed to the α-1,4 link recognized by amylase. Functional Fiber(s) is comprised of dietary fibers that have been purified or synthesized or modified and added to human foods. Functional fibers also have some physiological benefits. Some Some Dietary Fibers Functional Fibers Cellulose Cellulose Hemicellulose Pectin Pectin Lignin Lignin Gums Gums Chitin β-glucans Chitosan

Because some of the functions have definite health-benefits, the IOM has established an AI for total fiber: g fiber Adult Male 38 Adult Female 25 Really old men (>50) 31 Really old women (>50) 21 … and somewhat lesser amounts for boys & girls and small children…

Function of the different fibers is dependent on their water-soluble properties as well as their ability to bind particular compounds and digestibility by the bacteria in the gut. Water Soluble Insoluble Some hemicelluloses Lignin Pectin Cellulose Gums Some hemicelluloses β-glucans

Some General Effects Soluble Fibers - Can absorb some nutrients - bile acids fecal bile acids - lipids lipid absorption serum lipids - Can cause gel formation nutrient absorption transit time digestive function gastric emptying Some Soluble & Insoluble Fibers - Can bind water fecal volume transit time frequency of defecation - Can be fermented growth of beneficial bacteria acid in colon bile acid synthesis short-chain fatty acids lactate

Some specific examples: Lignins, gums, and pectins: absorb bile acids, fatty acids, cholesterol - conversion of bile acids to secondary bile acids by microflora may contribute to carcinogenesis - decrease in lipid absorption decreases serum levels and enhances liver uptake of LDL Pectins, gums, some hemicelluloses: high water-binding - increases fecal bulk and decreases transit time - decreases transluminal concentration of carcinogens / precarcinogens - decreases time for production of toxins and carcinogens Pectin, β-glucans, gums: “prebiotics” that stimulate growth of helpful bacteria - appears to inhibit initiation & proliferation of tumor cells - appears to attenuate production of carcinogens - appears to enhance production of butyric acid which reduces proliferation and differentiation of cancer cells Lignin can bind carcinogens to enhance elimination through feces

Major Dietary Sources Lignins: seeds of fruits & vegetables, wheat bran, root vegetables (carrots)… Pectins: strawberries, apples, citrus fruits, some in legumes and nuts… Gums: mostly as food additives with some in oatmeal, barley, and legumes… Hemicelluloses: bran, whole grains, nuts legumes… β-glucans: cereal bran… Cellulose: bran, legumes, nuts, root vegetables, seeds, apple peels… Obviously the food groups that include whole grains, nuts, seeds, and vegetables are the major sources of fiber; consumption of which is sadly lacking in the average American diet!