Page 76 - Small Animal Clinical Nutrition 5th Edition

76 Small Animal Clinical Nutrition

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Figure 5-14. Pathways of fiber fermentation. Fibers (e.g., cellulose, pectins, hemicelluloses), starches and sugars are metabolized by intes-
tinal microbes through various pathways to a common endpoint of pyruvate. Pyruvate is then further metabolized to yield energy for the
microbe and waste products: short-chain fatty acids (e.g., acetate, propionate, butyrate) and gases (e.g., carbon dioxide, hydrogen and
methane). Short-chain fatty acids are valuable substrates for the host animal that help maintain gastrointestinal tract health.

energy available. Smaller energy-containing compounds (e.g., Prebiotic Fibers, Probiotics and Synbiotics
the short-chain fatty acids acetate, propionate and butyrate), Prebiotic Fibers
and gases (carbon dioxide, hydrogen and methane) are micro- In addition to the traditional classifications for fiber such as
bial waste products; however, short-chain fatty acids are valu- digestibility, solubility and fermentability, an additional func-
able substrates for the host animal. Microbes contain enzymes tional classification for fiber has emerged in the last 10 years.
that can break the chemical bonds in different fibers. For exam- As mentioned above, fibers are classified as “prebiotic fibers,”
ple, microbial enzymes that digest cellulose are called cellulases which are defined as “non-digestible food ingredients that
and common pectin-digesting enzymes include pectin esterase selectively stimulate a limited number of bacteria in the colon
and pectic lyases (Robinson, 1987). Figure 5-14 shows the to improve host health” (Gibson and Roberfroid, 1995).
major microbial pathways for fermentation of fibers through Prebiotic fibers are resistant to hydrolysis by enzymes, are able
pyruvate to short-chain fatty acids. to support the growth of beneficial bacteria and do not sup-
Fermentation of fiber in the colon affects nitrogen metabo- port the growth of potential pathogens, although some prebi-
lism. Fiber fermentation by bacteria increases bacterial num- otic fibers do support limited growth of some pathogens.
bers. Bacteria use ammonia as a source of nitrogen to build These traditional and new classifications are not mutually
amino acids, proteins and DNA for reproduction. Small exclusive; prebiotic fibers can have different levels of fer-
amounts of urea diffuse from the bloodstream into the colon mentability or solubility.
where it is hydrolyzed to form ammonia (BNF, 1990; A prebiotic is currently known as a selectively fermented
Mortensen, 1992; Younes et al, 1995). ingredient that allows specific changes in the composition