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2009 Cengage-Wadsworth
Definitions of Dietary Fiber & Functional Fiber
• Dietary fiber - nondigestible CHO & lignin that are intact & intrinsic in plants
• Functional fiber - nondigestible CHO that are isolated, extracted, or manufactured & known to have physiological benefits
2009 Cengage-Wadsworth
Fiber and Plants
• >95% of fibers from cell wall– Primary wall– Secondary wall
• Fiber-related components– Cellulose– Hemicellulose– Lignin– Pectins– Suberin– Cutin
2009 Cengage-Wadsworth
Chemistry & Characteristics of Dietary & Functional Fibers• Varieties:
– Cellulose– Hemicellulose– Pectins– Lignin– Gums -glucans
– Fructans– Resistant starch– Chitin & chitosan– Polydextrose &
polyols– Psyllium– Resistant dextrins
2009 Cengage-Wadsworth
Cellulose
• Dietary & functional fiber• Long, linear polymer of 1-4 linked
glucose units• Main component of plant cell walls• Sources: bran, legumes, nuts,
peas, root vegetables, cabbage family, outer covering of seeds, apples
2009 Cengage-Wadsworth
Hemicellulose
• Dietary fiber• Heterogenous group of polysaccharide
substances• Sugars in backbone & side chains
– Xylose, mannose, galactose (backbone)– Arabinose, glucoronic acid, galactose (side
chains)
• Sources: bran, whole grains, nuts, legumes, some vegetables & fruits
2009 Cengage-Wadsworth
Pectins
• Dietary & functional fiber• Complex group of polysaccharides
called galacturonoglycans• Backbone = galacturonic acid• Cell wall & middle lamella in plants• Water-soluble, gel-forming• Sources: apples, strawberries,
citrus
2009 Cengage-Wadsworth
Lignin
• Dietary & functional fiber• Highly-branched polymer
composed of phenol units with strong bonding
• Structural components of plants - found in stems, seeds, bran layer
• Sources: wheat, mature root vegetables, fruits with edible seeds
2009 Cengage-Wadsworth
Gums (Hydrocolloids)
• Dietary & functional fibers• Group of substances secreted at
site of plant injury– Tree exudates: gum arabic, gum
karaya, gum ghatti– Shrub exudate: gum tragacanth
• Composed of sugars & derivatives• Sources: oatmeal, barley, legumes
2009 Cengage-Wadsworth
-Glucans
• Dietary & functinal fibers• Homopolymers of glucopyranose
units• Water-soluble, highly fermentable• Sources: cereal brans, especially
oats & barley
2009 Cengage-Wadsworth
Fructans--Inulin, Oligofructose, & Fructooligosaccharides
• Dietary fibers• Fructose units in chains of varying
length• Prebiotics• Sources: chicory, asparagus,
onions, garlic, artichokes, tomatoes, bananas
2009 Cengage-Wadsworth
Resistant Starch
• Starch that cannot be digested by humans
• Types– RS1 - plant cell walls– RS2 - ungelatinized starch granules– RS3 - retrograde starch from cooking &
cooling or extruding foods– RS4 - chemically modified starch
• RS1 & RS2 = dietary fibers, RS3 & RS4 = functional fibers
2009 Cengage-Wadsworth
Chitin & Chitosan
• Chitin – Amino-polysaccharide polymer
containing 1-4 linked glucose units– Cells walls of some lower plants
• Chitosan – Deacetylated form of chitin;
polysaccharide made of glucosamine & N-acetyl glucosamine
2009 Cengage-Wadsworth
Polydextrose & Polyols
• Polydextrose– Polysaccharide of glucose & sorbitol
units that have been polymerized at high temperatures & partial vacuum
• Polyols– Polyglycitol & malitol– Found in syrups
2009 Cengage-Wadsworth
Psyllium
• Functional fiber• Mucilage from husk of psyllium
seeds• Used as laxative
– Must ingest plenty of fluids
2009 Cengage-Wadsworth
Resistant Dextrins
• Functional fibers• Generated by treating cornstarch
with heat & acid & then with amylase
• Glucose polymers containing 1-4 & 1-6 glucosidic bonds & 1-2 & 1-3 bonds
2009 Cengage-Wadsworth
Selected Properties & Physiological Effects of Fiber
• Important properties include:– Solubility in water– Water-holding capacity & viscosity– Adsorption or binding ability– Degradability/fermentability
2009 Cengage-Wadsworth
Solubility in Water
• Classifications:– Soluble - dissolve in hot water– Insoluble - don’t dissolve in hot water
• Water-soluble: some hemicelluloses, pectin, gums, -glucans
• Water-insoluble: cellulose, lignin, some hemicelluloses, chitosan, chitin
2009 Cengage-Wadsworth
Solubility in Water
• Soluble– Delay gastric
emptying– Increase transit
time– Decrease nutrient
absorption
• Insoluble– Decrease transit
time– Increase fecal bulk
2009 Cengage-Wadsworth
Water-Holding/Hydration Capacity & Viscosity
• Ability of fiber to bind water• Delayed (slowed) gastric emptying• Reduced mixing of gastrointestinal
contents with digestive enzymes• Reduced enzyme function• Decreased nutrient diffusion rate--
attenuation of the blood glucose response
• Altered small intestine transit time
2009 Cengage-Wadsworth
Adsorption or Binding Ability
• Diminished absorption of lipids• Increased fecal bile acid excretion• Lowered serum cholesterol
concentrations (hypocholesterolemic properties)
• Altered mineral & carotenoid absorption
2009 Cengage-Wadsworth
Degradability/Fermentability
• Fermentable fibers– Fermentable fibers as prebiotics– Short-chain fatty acid generation– Increased water & sodium absorption
in the colon– Mucosal cell proliferation– Provision of energy– Acidification of luminal environment
2009 Cengage-Wadsworth
Degradability/Fermentability
• Nonfermentable fibers– Detoxification– Increased fecal volume (bulk)
2009 Cengage-Wadsworth
Roles of Fiber in Disease Prevention & Management
• Hypoglycemic & hypolipidemic effects of soluble fibers– For diabetes & high serum cholesterol
• Insoluble, nonfermentable fibers for GI diseases– Diverticular disease, gallstones, IBS,
constipation• Health claims for fiber
2009 Cengage-Wadsworth
Roles of Fiber in Disease Prevention & Management
• Mechanisms– Adsorb bile acids, promote excretion– Increase fecal bulk– Provide fermentable substrates for bacteria in
colon– Shortened fecal transit time– Decreases interluminal pH– Fermentation may release fiber-bound
calcium– Butyric acid appears to hinder cancer cells– Insoluble fibers bind carcinogens
2009 Cengage-Wadsworth
Recommended Fiber Intake
• Adequate Intakes– Adults:
• 19-50: 25 g for women, 38 g for men• 51 or >: 21 g for women, 31 g for men
– Children:• 1-3: 19 g; 4-8: 25 g• Girls 9-18: 26 g• Boys 9-13: 31 g; boys 14-18: 38 g