Dietary fibre: Essential nutrient for gut health

Tuesday, December 7, 2021


Dietary fibre: Essential nutrient for gut health


Usama Aftab, AB Vista Asia Pte Ltd., Singapore

 


Fibre represents fraction of diet not digested by the monogastric gut enzymes. It therefore does not contribute directly to the nutrient currency of the host animal. A diet void of dietary fibre, however, fails to support normal development and functioning of the gut and its microbiome, failure of which poses substantial consequences for the host animal's nutrition and metabolism. The current communication highlights the role of dietary fibre as an essential nutrient for gut-health; it emphasizes that strategies to exploit the full potential of dietary fibre are central to the problem of gut-health, especially when the use of AGP's are not permitted.


Dietary fibre – definition


Dietary fibre (DF) is sum of non-starch polysaccharides (NSP), resistant starch, oligosaccharides and lignin. The chemical analysis of DF is based on enzymatic breakdown of NSP to its constituent sugars which are then quantitatively measured. This however, provides limited information about the physiological properties and nutritional significance of DF in a given feedstuff. It has been recognized that attributes like chain length, degree of polymerization, solubility, ability to form viscous solution and fermentability serve more accurate predictors of the fate of DF in animal gut. It is the understanding of the physiological characteristics of DF which help classify if a given fibre was anti-nutrient, pro- nutrient, or merely an inert diluent.


It is also worth highlighting that the alternative, often more commonly employed, terms like crude fibre (CF) represents only a part of total DF e.g. the CF reflects 20-40% of DF in most common feed ingredients.


Fibre digestion in poultry


The digestibility of DF depends largely upon physiochemical properties of DF and the composition and activity of gut microbiota. The degradation of soluble DF e.g. soy alfa-galactosides, barley or oat beta-glucan, and soy pectin is more complete with a considerable part of this degradation taking place in the distal ileum. On the contrary, the fermentation of insoluble DF is far from complete with the major degradation taking place in caecal and in large-intestinal compartments. Using typical corn-soy diets, experiments with adult cockerels and young broiler chickens have shown almost complete digestibility of alfa-galactosides (raffinose, stachyose, and verbascose) while the apparent total tract digestibility of NSP was negligible. These estimates are consistent with the suggestion that NSP fermentation contributes little to the energy currency of chicken, perhaps around 3-4% of AME in adult chicken; these estimates are significantly lower than those projected for pigs who by virtue of being better equipped for fermenting dietary NSP can derive up to 25% of their maintenance energy need from absorbed SCFA. It is the relative capacity to utilize NSP which explains why pigs are able to extract more energy, especially from oilseed meals, compared with chickens.


Fibre and gut-health


DF can help promote improved gut-health in two possible ways; 1) direct effects, 2) indirect effects i.e. via modulation of gut-microbiome. The direct effects are largely associated with the insoluble fraction of DF while the indirect effects are mediated by soluble-, and fermentable-fractions. A brief account of these is presented in the following.


1. Fibre and gut development and functioning


DF mediates gut development and functioning in poultry via its effect on gizzard development and digesta rate of passage. The insoluble fibre, especially when provided in coarse form, has been known to promote development of a larger, more functional gizzard which is thought to improvenutrient digestibility. In summary, this effect has been thought to relate to: 1) a more developed gizzard would hold digesta for longer resulting in a more complete gastric digestion, 2) a developed gizzard would cause an increased CCK production, increased pancreatic secretions and promote reflux of the intestinal content resulting in improved intestinal digestion, 3) a larger gizzard would act as regulator of feed intake as well as the controlling the rate and amount of digesta travelling down the intestine. This would reduce the likelihood for 'starch-overload' in the small intestine – a phenomenon thought to explain poor starch digestibility at high feed intake, and finally 4) a reduced pH which is a characteristic of a more functional gizzard helps prevent feed borne pathogens entering the small intestine; it is this latter proposition which makes stimulation of a functional gizzard one of the key strategies as part of the so-called antibiotic-free program. Typical broiler diets based on corn and soybean meal maybe limiting in effective DF and may be benefitted by supplemental insoluble fibre source at the rate of 2-3% of diet.


2. Fibre and gut microbiome


DF passes down the tract largely undigested where it is fermented to a degree characteristic of the physiochemical properties of the DF and the microbiome present in the distal gut. Dietary interventions help boost capacity of monogastric animals to utilize DF more efficiently and can benefit the technical performance of poultry and pigs. Some of the common strategies which could help improve fibre utilization include the use of NSP enzymes and incorporation of fermentable fibre. More recently, evidence has been presented to suggest that provision of xylo-oligosaccharides (XOS) may increase the relative abundance of a gut-microbiome having increased capacity to ferment DF. Contrary to the wide-held view, these studies suggest that NSP degrading capacity of the gut-microbiome could contribute significantly towards total-tract degradation of the DF in chicken's gut. Recent studies at AB Vista support synergistic effect of endo-xylanase and XOS in promoting DF fermentation leading to improvements in growth performance, livability, and in those of the most common indices of the gut-health (Figure 1).

 

 

Figure 1 Common markers of gut-health


The beneficial effect of these interventions may not necessarily relate to noticeable increases in the 'digestibility' of DF per se since only small changes in the fermentation profile could have large implications for the host animal. One notable example of these effects is the presence of specific L-cells in caecal and large-intestinal epithelium which are sensitive to small changes in the profile of short chain fatty acids, especially butyrate, effectively translating these changes into systemic effects influencing gut-health, immune status, nutrient uptake and post-absorptive metabolism of the host animal. These studies bring new insight into fibre nutrition which extends beyond classical notions of 'fibre digestion' and the 'localized' effects of SCFA.

 


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Article made possible through the contribution of Usama Aftab and AB Vista Asia Pte Ltd., Singapore