Introduction
Dietary fiber plays an important role, through the interaction among the diet, endogenous enzymes, the mucosa, and the symbiotic microbiota. All of these factors are considered important for nutrient assimilation and are key components for optimal gut health.
This summary is a translated excerpt from the article in allaboutfeed.net by Prof. Knud Erik Bach Knudsen, University of Copenhagen, entitled “Fibre’s role in animal nutrition and intestinal health,” which discusses the role of indigestible fiber as an important component in the digestive system of monogastric animals. Dietary fiber cannot be digested by endogenous enzymes but affects digestion and absorption and also serves as a prebiotic beneficial to the beneficial microbiota in the gut. The summarized parts of the article are briefly presented below.
What is dietary fiber?
Dietary fiber (DF) is an important component of a small number of plants used as feed ingredients for pigs. Fiber resists digestion by enzymes produced in the small intestine, thus providing bulk to the intestinal lumen and becoming the main substrate for bacterial fermentation, especially in the large intestine. Due to the physicochemical properties of dietary fiber, it affects both the microbiota and the mucosa at all points of the gastrointestinal tract.
Dietary fiber, in the recent definition, includes non-starch polysaccharides (NSP), resistant starch (RS), non-digestible oligosaccharides (NDO), and non-carbohydrate polyphenolic ether lignin. The main polysaccharides of NSP are cellulose and various non-cellulosic polysaccharides (NCP), β-glucan, arabinoxylan, xylan, xyloglucan, and pectic substances. DF content varies widely among feed ingredients; relative to cereals, it is generally lower in legumes and protein-rich crops and often higher in by-products from cereals and the food-processing industry (Figure 1).
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Figure 1: Dietary fiber concentration (g/kg dry matter) in different feed ingredients |
There is also wide variation in the composition of NCP among feed ingredients; in cereals, arabinoxylan and β-glucan are high, and β-glucan accounts for the relatively high soluble NCP in barley and oats compared with wheat and maize. In legumes, protein crops, and fibrous materials, pectin and xyloglucan are the main NCP, and pectins account for the relatively high soluble NCP in these ingredients. RS levels are higher in legumes than in cereals. In general, legumes also have high NDO levels among feed ingredients. Therefore, there is considerable potential to formulate diets with different fiber compositions.
A common feature of all dietary fiber sources is their ability to swell, retain water in the cell wall matrix, and increase viscosity when placed in water (Figure 2). However, while all dietary fiber sources swell and retain water, viscous properties depend on the type and chemical nature of the polysaccharides making up the fiber fraction of the diet. For example, sugar beet pulp mainly increases the water-holding capacity of digesta, whereas its viscosity-increasing property is low. In contrast, β-glucan is released to a greater extent from the cell wall matrix and increases viscosity in the lumen.
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Figure 2: Water-holding properties expressed as swelling capacity, solubility, and water-holding capacity. The first stage in the dissolution of macromolecules is swelling, in which water penetrates the macromolecules until they are fully swollen and dispersed. Most polysaccharides form viscous solutions if dissolved in water. Viscosity depends on chemical structure, molecular weight of the macromolecule, and concentration. |
Effects on digestion and fermentation
The gastrointestinal tract consists of different compartments such as the mouth, stomach, small intestine, and large intestine, and accessory organs such as the liver and pancreas are also involved in digestion and absorption. All of these parts work together with peripheral organs through a broad set of receptors (stretch, tactile, chemical) that monitor digestion and absorption via hormonal and neural feedback signals. In this way, changes in nutrient levels in the blood are minimized and the supply of nutrients to different organs is regulated and optimized. Dietary fiber is an important dietary factor that primarily affects digestion and absorption, thereby regulating and supplying nutrients to organs and tissues not in direct contact with the digestive tract.
Dietary fiber affects digestion and absorption at all different points of the gastrointestinal tract as well as secretions from the pancreas and liver (Figure 3). Increasing dietary fiber increases the weight and contents of the gastrointestinal tract and leads to a higher flow of nutrients at all points. Soluble dietary fiber increases viscosity in the lumen, thereby prolonging gastric emptying and causing gastric interference as well as small-intestinal digestion by hindering contact between substrate and digestive enzymes and slowing the movement of hydrolysis products during digestion. Both soluble and insoluble dietary fiber provide substrate for bacterial fermentation in the large intestine. Soluble dietary fiber is readily fermentable; most of it is degraded in the cecum and proximal colon, whereas insoluble dietary fiber is degraded more distally and its components are not completely broken down.
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Figure 3: Effects of dietary fiber on physiological processes related to transit time, digestion and absorption in different segments of the gastrointestinal tract, and fecal output in energy utilization. |
The main metabolites formed as a result of bacterial fermentation are short-chain fatty acids (SCFA; mainly acetate, propionate, and butyrate), and the pH of the luminal contents is also lower. However, most of the short-chain fatty acids produced are rapidly absorbed and provide energy to the host, although the energy utilization of SCFA is almost lower than that of absorbed energy, for example glucose from the small intestine. The reason for this lower energy utilization of SCFA compared with glucose lies in the losses from fermentative gas production and lower SCFA utilization because of intermediary metabolism. Therefore, high-fiber diets generally have lower energy content than low-fiber diets, primarily because of lower digestibility but also because of lower utilization of absorbed energy.
Effects on gut health
Prebiotics are non-digestible feed components that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon and thereby improving host health. The Firmicutes and Bacteroidetes phyla are present in most animal species; Enterobacteria spp., such as Escherichia coli, are considered unfavorable for gut health because they are often involved in intestinal disorders, whereas Lactobacillus spp., Bifidobacterium spp., and Roseburia spp. are generally considered more favorable because of their health-promoting properties.
While dietary fiber generally increases the flow of carbohydrates to the large intestine, thereby stimulating the activity of the entire bacterial population and thus lowering pH, only a few dietary fiber polysaccharides have the specific ability to stimulate beneficial bacterial groups. Both oligo- and polysaccharides are often more effective in modulating microbiota composition, and changes in bacterial populations have been observed when feeding inulin with varying chain lengths, fructans (a mixture of fructose oligo- and polysaccharides), and RS.
Arabinoxylan, a major fiber component of rye and wheat, has also been shown to affect microbiota composition by stimulating the growth of butyrate-producing microorganisms. Butyrate is an important metabolite for epithelial cells, providing energy and having regulatory functions in cell growth and differentiation. In this context, it is noteworthy that xylanase can be used to partially break down arabinoxylan into a mixture of oligomers in the distal small intestine. This may represent an underexplored potential for modulating metabolism and microbiota composition in the large intestine.
Comments and applications
Dietary fiber cannot be digested by endogenous enzymes in monogastric animals such as pigs and poultry, except for a very small and insignificant amount of soluble fiber that is digested by the intestinal microbiota, especially in the cecum of poultry. High-fiber diets generally have lower energy content than low-fiber diets and lower digestibility, as noted in the article. However, diets with too little fiber can cause constipation in animals. In antibiotic-free broiler production under current biosecurity methods, some fiber components are considered prebiotic feed for probiotic beneficial bacteria, helping improve digestion, health, and productivity.
In poultry production, it is necessary to formulate diets with an appropriate fiber level for each species, breed, production purpose, and age of the animals; this helps maintain good digestive physiology, improve nutrient digestion and absorption, and sustain beneficial microbiota and a balanced gut microbial ecosystem. For example, feed for colored-feather broilers contains only about 3.5–4% total dietary fiber, whereas diets for colored-feather breeder pullets may contain 4.5–5% fiber, etc.
Assoc. Prof. Bui Xuan Men, Vemedim Research and Development Center.
References
Knudsen KEB (2016) Fibre’s role in animal nutrition and intestinal health.
