Agriculture Reference
In-Depth Information
scFOS, MOS, GOS, xylooligosaccharides (XOS), arabinoxylooligosaccharides (AXOS),
isomaltooligosaccharides (IMO), GroBiotic
®
-A, Previda™ (containing galacto-gluco-
mannans from hemicellulose extract and a combination of oligosaccharides) and Chinese
medicines (for review, see Burr
et al
. 2005; Merrifield
et al
. 2010; Ringø
et al
. 2010; Dim-
itroglou
et al
. 2011). However, according to the criteria for prebiotic classification given by
Gibson
et al
. (2004), Gibson and Roberfroid (1995) and Roberfroid (2007), the evidence for
the prebiotic status of these products for fish is not sufficient and strictly speaking they should
not be classified as prebiotics. These criteria and subsequent prebiotic classification were
based on human nutrition and applications but the use, definition and criteria of prebiotics for
aquatic animals may be more complicated. For example, the anatomic structure of the GI tract
in fish displays large variation (refer to
Chapter 1
). There are fish species with and without a
discernible stomach, the number of pyloric caeca can vary from nil to 1000, the intestine can
be short or up to 20 times the body length, and some species have a hindgut (fermentation)
chamber. Furthermore, aquatic animals do not have a well-defined colon.
Based on the prebiotic studies carried out using aquatic animals we suggest the following
criteria for prebiotic classification: (1) resistance to hydrolysis by the host's enzymes and GI
absorption, (2) improvement in intestinal microbial balance, and (3) conferring host benefits
(such as improved disease resistance, non-specific immunity, gut morphology, growth, survival
and/or nutrient digestibility). However, one should bear in mind that not all 'prebiotics' have
fulfilled these criteria.
7.3.1 Prebiotics: biochemistry and host benefits
Most non-digestible oligosaccharides (NDOs) are composed of 3-10 sugar moieties with gly-
cosidic bonds in the beta-configuration, thus rendering them resistant to hydrolysis of the
endogenous enzymes of animals including fish which only recognize the alpha glycosidic
linkage typical in starch.
Prebiotic oligosaccharides are able to provide the necessary energy to selective species
of bacteria which are responsible for the production of short-chain organic acids. Indeed,
metabolic cross-feeding is a process whereby metabolites from one bacterial species are the
energy source for other bacteria with a resulting production of short-chain fatty acids (SCFAs),
primarily lactic, acetate, propionate and butyrate. The functional benefits of prebiotics may
induce SCFA production leading to modulation of blood lipid, GI/systemic immunomodula-
tion, energy sources resulting in intestinal cell proliferation, and improved intestinal barrier
function (Tungland and Meyer 2006). Reduction in pH aids mineral absorption and general
nutritional support. Their synergistic promotion of commensal and symbiotic bacteria in turn
provides competitive exclusion of pathogens, indirectly enhancing pathogen resistance, reduc-
ing toxic microbial metabolites and suppressing intestinal inflammation (Pedron and San-
sonetti 2008). These SCFAs are very important in the growth and physiology of intestinal
tissue and systemic metabolism in humans and animals, accounting for a large proportion of
the energy requirements of colonocytes. Little information is available for fish, but it has been
shown that fish enterocytes can absorb SCFAs (Mountfort
et al
. 2002). Acetate is a primary
fuel for skeletal muscle, heart and brain. In humans, the crypt cells of the absorptive epithe-
lium are stimulated by propionate. It has also been reported that it can reduce hepatic glucose
output and modulate cholesterol biosynthesis. Butyrate is the most common short-chain fatty
acid produced by intestinal microbiota and accounts for most of the oxygen requirements for
the gut microorganisms in fish. The fatty acid may up-regulate glutathione S-transferase and
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