Chapter 4

2009 Cengage-Wadsworth

Chapter 4

Fiber


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


Fiber and Plants• >95% of fibers from cell wall

– Primary wall– Secondary wall

• Fiber-related components– Cellulose– Hemicellulose– Lignin– Pectins– Suberin– Cutin


Chemistry & Characteristics of Dietary & Functional Fibers• Varieties:

– Cellulose– Hemicellulose– Pectins– Lignin– Gums -glucans

– Fructans– Resistant starch– Chitin & chitosan– Polydextrose &

polyols– Psyllium– Resistant dextrins


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


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


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


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


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


-Glucans• Dietary & functinal fibers• Homopolymers of glucopyranose

units• Water-soluble, highly fermentable• Sources: cereal brans, especially

oats & barley


Fructans--Inulin, Oligofructose, & Fructooligosaccharides

• Dietary fibers• Fructose units in chains of varying

length• Prebiotics• Sources: chicory, asparagus,

onions, garlic, artichokes, tomatoes, bananas


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


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


Polydextrose & Polyols• Polydextrose

– Polysaccharide of glucose & sorbitol units that have been polymerized at high temperatures & partial vacuum

• Polyols– Polyglycitol & malitol– Found in syrups


Psyllium• Functional fiber• Mucilage from husk of psyllium

seeds• Used as laxative

– Must ingest plenty of fluids


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


Selected Properties & Physiological Effects of Fiber

• Important properties include:– Solubility in water– Water-holding capacity & viscosity– Adsorption or binding ability– Degradability/fermentability


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


Solubility in Water• Soluble

– Delay gastric emptying

– Increase transit time

– Decrease nutrient absorption

• Insoluble– Decrease transit

time– Increase fecal bulk


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


Adsorption or Binding Ability• Diminished absorption of lipids• Increased fecal bile acid excretion• Lowered serum cholesterol

concentrations (hypocholesterolemic properties)

• Altered mineral & carotenoid absorption


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


Degradability/Fermentability• Nonfermentable fibers

– Detoxification– Increased fecal volume (bulk)


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


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


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


Perspective 4

Phytochemicals & Herbal Supplements in Health &

Disease


Phytochemicals & Herbals• Phytochemicals• Echinacea• Garlic• Ginkgo biloba• Ginseng• Milk thistle• St. John’s wort• Regulation of herbal supplements

Chapter 4

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