Page 73 - Small Animal Clinical Nutrition 5th Edition

Macronutrients 73

plex carbohydrates (Table 5-4). Fibers differ from starches in rupted by other sugars (e.g., galactose, arabinose and rhamnose)
VetBooks.ir that fibers resist enzymatic digestion in the small intestine. As to form branches and kinks. Pectins are found in high levels in
fruits and vegetables such as apples, strawberries, raspberries,
a result, microbes in the colon usually ferment fibers. Fibers
carrots, broccoli, potatoes, sugar beet pulp and the skins of cit-
include cellulose, hemicellulose, pectin, gums and RS (See pre-
vious discussion.) and are unique because of the types of sugars rus fruits.Commercially,pectin is extracted from either apple or
they contain and the resultant chemical bonds. citrus waste after the manufacture of juice. Pectins are water
soluble, form viscous gels and are rapidly fermented by intestin-
Chemical Structure al bacteria.
Unlike starches, cellulose is a polymer of glucose units bonded Gum is a general term for the diverse group of viscous and
by β1,4-linkages (rather than α linkages) that are only broken sticky polysaccharides found in the seeds and exudates of
by microbial enzymes (Figure 5-11). Cellulose is the most plants. The sugars that make up gums are diverse, and the pre-
abundant polysaccharide in plants, forming the structure of cise structural information for some gums is unknown. Gum
plant cell walls. In plants, hydrogen bonds closely hold straight arabic, guar gum, gellan gum, konjac gum, carrageenan gum,
cellulose chains to one another forming orderly and compact psyllium gum, xanthan gum, carob gum, gum ghatti and gum
aggregates called fibrils. Cellulose fibrils contain regions that tragacanth are just a few of the gums used as thickening agents,
are highly crystalline and other regions that are more random water binders, stabilizers, emulsifiers and gelling agents in jams,
and amorphous. Cellulose is usually associated with hemicellu- pie fillings, confectionery products, sauces, salad dressings,
lose and lignin within plant cell walls. Water-soluble chemical canned meat products and moist pet foods. Depending on their
derivatives of cellulose, including carboxymethylcellulose, source and processing, gums have variable viscosity and solubil-
methylcellulose and hydroxypropylcellulose, are used as stabi- ity in water and thus have variable fermentation rates. Intestinal
lizers, thickeners and emulsifiers in pet and human foods such bacteria moderately to rapidly ferment most gums.
as ice cream, gravies, soups and beverages. Cellulose is also Lignin is not a carbohydrate in the strictest chemical defini-
added to weight-reduction foods for people and pets to dilute tion; however, it is often considered a fiber because it makes up
calories and provide bulk. Cellulose is not very water soluble the structural part of plant cell walls and is not digested by
but can have significant water-holding capacity and is slowly mammalian intestinal enzymes. Lignin is not a single chemical
fermented by microbes in the colon. compound but a series of compounds made from derivatives of
Hemicelluloses are composed primarily of glucose, galactose, phenylpropane associated in complex cross-linked structures.
mannose, xylose, arabinose and uronic acids joined together in Lignin is highly resistant to enzymatic (intestinal and bacteri-
different combinations and various linkages. Hemicelluloses are al) digestion and chemical degradation. Strong chemical bonds
closely associated with the cellulose in the cell walls of plants. between lignin and plant cell wall fibers, proteins and other
Most hemicelluloses are not water soluble because of their var- compounds make lignins unavailable during digestion.
ious structures and composition. Other plant polysaccharides can also be considered as fibers.
Pectin is a linear chain of galacturonic acid linked by α1,4- These include fructans (inulin), galactans, mannans, mucilages
glycosidic bonds found in cell walls and intercellular regions of and β-glucans with β1,3- and β1,2-glucose bonds. Amounts of
plants. The linear galacturonic chain of most pectins is inter- these fibers in foods are usually quite low.

Carbohydrate and Method Fiber solubility Total dietary Crude fiber
fiber fractions fiber analysis analysis

Fructans, galactans,
mannans, mucilages Rapidly fermentable
Soluble fiber
Pectin Total dietary fiber
Moderately fermentable
Hemicellulose
Insoluble fiber
Slowly fermentable
Cellulose
Crude fiber
Lignin Not digested or fermented

Resistant starch Moderately fermentable
Starch Enzymatically digested

Mono- and
disaccharides Absorbed

Figure 5-12. Physiochemical and analytical properties of dietary fiber components.