Page 74 - Small Animal Clinical Nutrition 5th Edition

74 Small Animal Clinical Nutrition

number of bacteria in the colon to improve host health”
VetBooks.ir (Gibson and Roberfroid, 1995). (See below.) The definition
implies that prebiotic fibers are resistant to hydrolysis by
mammalian enzymes, are able to support the growth of ben-
eficial bacteria and do not support the growth of potential
pathogens, although some prebiotic fibers do support growth
of putative pathogens to a limited degree.
Fiber concentration, ionic concentration, pH, particle size
and the hydrophobic and hydrophilic properties of polysaccha-
ride structures affect the viscosity of fibers in water. An increase
in viscosity in the GI tract can slow nutrient absorption, reduce
postprandial glycemia, slow gastric emptying, delay mouth to
cecum transit and reduce interactions of food particles with
digestive enzymes and epithelial surfaces. The fermentation
rate of a fiber interacts with its water-holding capacity and vis-
Figure 5-13. Relative degree of fermentation of various dietary
cosity to affect the degree of fecal bulking (volume). For mod-
fiber sources in the gastrointestinal tract of dogs and cats. At the
extremes are pectin and gums, which are rapidly fermentable, and erately and slowly fermentable fibers, the degree of fecal bulk-
cellulose, which is slowly fermentable. Other fiber sources such as ing is related to water-holding capacity. Slowly fermented fibers
soy hulls and beet pulp are intermediate and termed moderately (e.g., cellulose) are the most effective stool bulking agents
fermentable. because they retain their structure longer and are thus able to
bind water. For rapidly fermentable fibers, the increased bound
water reduces fecal bulk. In fact, most fermentable fibers have
Other Classifications of Fiber laxative effects and may produce diarrhea if fed at high levels.
In addition to classifying fibers by their structure as described An increase in fecal bulk has been advocated for the treatment
above, fibers have been classified by their rate of fermentation, and prevention of irritable bowel syndrome, constipation and
digestible and indigestible fractions, solubility in water, water- other GI disorders (Chapters 48 to 65).
holding capacity and viscosity (Figure 5-12). The different
ways of classifying fibers allow the important physiologic func- Function
tions and diverse effects of fiber in animals to be highlighted The primary function and benefit of adequate dietary fiber are
and understood. to increase bulk and water in the intestinal contents (Leib,
Fiber sources can be described as rapidly to slowly fer- 1990; Twedt, 1993; Gurr and Asp, 1994). Fiber appears to
mentable (Figure 5-13). Rapidly fermented fibers produce shorten intestinal transit rate in dogs with normal or slow
more short-chain fatty acids and gases in a shorter period of transit time and prolong it in dogs with rapid transit rate
time compared to fiber sources that ferment more slowly (Burrows et al, 1982). Together, these factors help promote
(Sunvold et al, 1994, 1994a). Slowly fermentable fiber sources and regulate normal bowel function. In addition, the typical
used in pet foods contain primarily celluloses and hemicellulos- end products of microbial fermentation of fiber (acetate, pro-
es including purified cellulose and peanut hulls. Citrus and pionate and butyrate) are important in maintaining colonic
apple pectins and most gums are rapidly fermented. Fiber health. Fiber decreases luminal pH through production of
sources that contain mixtures of pectins,hemicelluloses and cel- short-chain fatty acids and increases the population of anaer-
luloses (e.g., rice bran, oat bran, wheat bran, soy fibers, soy hulls obic flora. The antibacterial properties of short-chain fatty
and beet pulp) are moderately fermentable.The rate and extent acids may decrease pathogenic intestinal bacteria, increase
of fiber fermentation are important distinguishing characteris- resistance of the gut to colonization by pathogenic bacteria
tics when discussing physiologic functions of fiber. As the fer- and may be important in prevention of and recovery from
mentation rate of fiber increases,GI transit time decreases,fecal intestinal disorders and cancer (Twedt, 1993; Gurr and Asp,
bulk decreases and fecal bile acid excretion increases. 1994; Burrows et al, 1982; Salter et al, 1993). Dietary fibers
Fiber is also classified according to solubility, or the ability classified as prebiotics also increase beneficial bacteria that
of a fiber to disperse in water. Most rapidly fermentable fibers protect the GI tract against colonization by pathogens.
such as pectins and gums are “soluble.” Lignin and the slowly Prebiotic fibers may also reduce fecal odor by modifying fecal
fermentable fibers such as cellulose and most hemicelluloses concentrations of metabolites and, via their carbohydrate
are “insoluble.” All fibers hold water to some degree; howev- residues, improve immune function by influencing gut-asso-
er, the soluble fibers have a greater water-holding capacity and ciated immune cells. (See below.)
may form gels and viscous solutions within the GI tract. A Colonocytes preferentially use butyrate, an end product of
new classification of fiber relates to the ability of certain fiber fermentation, as their energy source rather than glucose
microorganisms in the gut to use fiber as food. Fibers are clas- or amino acids (Roediger, 1982). In addition, short-chain
sified as “prebiotic fibers,” which are defined as “non- fatty acids facilitate absorption of sodium, chloride and water
digestible food ingredients that selectively stimulate a limited in the colon. The gut microflora produce an array of com-