Dietary fiber refers to the combination of polysaccharide carbohydrates such as pectin, cellulose, hemicellulose and other non-carbohydrate components. Which is not digested by mammalian endogenous enzymes, but can be digested by pig hindgut (including terminal ileum, cecum, rectum and colon) bacteria fermented to produce acetic acid, propionic acid, butyric acid and other volatile fatty acids (Volatile Fatty Acid, VFA) a class of mixtures(
Buttriss and Stokes, 2008;
Slavin, 2013). Generally speaking, adding fiber to pig diets will reduce the use of energy and nutrients, which is conducive to the growth and development of pigs (
Wenk, 2001). Traditional pig feed contains large amounts of cereal crops (such as corn and wheat) and protein supplements (such as soybean meal) to provide energy and nutritional requirements for pigs. In recent years, this traditional feed demand and supply have changed and pig producers around the world are looking for low-cost alternatives from cereal products to reduce feed costs and improve pig production performance
(Woyengo et al., 2014). Current research shows that adding appropriate amounts of fiber to pig diets can improve productivity and enhance economic benefits while ensuring pig health. In recent years, dietary fiber has received extensive attention. As well, use amount of fiber diets instead of soybean meal will reduce the costs of diets.
The main reason is that dietary fiber can be fermented in pig intestines, improve the structure of intestinal microflora and reduce the occurrence of intestinal diseases. This can promote the healthy growth of pigs and also contribute to reducing the use of antibiotics and other antibacterial drugs on pigs (
Agyekum and Nyachoti 2017;
Woyengo et al., 2014) This article we show the efficacy of dietary fiber on pig intestinal microorganisms and the factors that affect the utilization of dietary fiber by pig intestinal microorganisms and provide a theoretical basis for the production and research of dietary fiber in pig breeding. As well, it will focus on the impact of dietary fiber on the digestive physiology of pigs.
Definition of dietary fiber
Carbohydrates can be divided into sugars, disaccharides, oligosaccharides, or polysaccharides (
i.e. starch and non-starch). Sugars, oligosaccharides and starch can be found inside plant cells. Non-starch polysaccharides and lignin are the main components of plant cell walls and are called dietary fiber. There are several definitions of “rational fiber”. However, all of these definitions have certain limitations, due to the composition of plant cell walls, which are variable and complex in terms of chemical and physical components and their metabolic effects, Based on the human digestive system
(Jensen et al., 2015), preliminarily defined dietary fiber as “the sum of lignin and cell wall polysaccharide-resistant enzyme-hydrolyzed polysaccharides.” Similarly, this definition of dietary fiber also applies to other monogastric animals, such as pigs.
Effect of dietary fiber on pig intestinal microbes
Dietary fiber is not easily digested in the swine pancreas and small intestine, but can be digested by the more active fiber-decomposing bacteria in the large intestine
(Filamentous succinogenes, Ruminococcus Albicans, Ruminococcus luteus, Vibrio butyrolyticum and Rumen (
Ravo bacteria). Also, other substrates are fermented and decomposed to produce large amounts of short-chain fatty acids (SCFAs) (
Deehan and Walter, 2016). SCFA is mainly produced by the glycolysis pathway and the pentose phosphate pathway
(Bindelle et al., 2008). As shown in (Fig 1). SCFA is mainly composed of acetic acid, propionic acid and butyric acid. It plays an important role in the metabolism of the host body, immune system and microbial proliferation. It is an important component of the animal body that maintains its growth and development
(Koh et al., 2016). Some studies showed the fermentation of dietary fiber in the large intestine to produce VFA can provide 5% to 20% of the energy requirements of the pigs (
Den Besten et al., 2013). As well, improve metabolic health status and enhancing the productivity of animal, the intestine environment and fermentation acids production
(Aschalew et al., 2020). Besides, there are certain nutrients in the dietary fiber that can be used by beneficial bacteria in the intestine without negative effect, which promotes the proliferation of beneficial bacteria in the intestine and makes it a dominant flora while inhibiting the growth the pathogenic microorganisms, to enhance the maintenance of intestinal micro ecological homeostasis(
Den Besten et al., 2013; Jha and Berrocoso, 2016).
The effect of dietary fiber on the number of pig intestinal flora
The microbial fermentation of dietary fiber can change the number of intestinal microorganisms and provide energy to maintain intestinal health
(Berrocoso et al., 2015), were observed in case of using the fiber supplementation led to increasing the number of bacteria, decreased the number of Lactobacillus in the large intestine rods in the cecum of pigs
(Wang et al., 2018). On the other hand, increase the
lactobacilli and
cellulobacteria in pig manure grown in the 5% wheat bran group were significantly higher than those in the corn-soybean meal diet group. Furthermore, konjac flour can significantly increase the number of intestinal lactic acid bacteria in the second reproductive cycle of the sow and reduce the number of
E. coli and
Clostridium (Laitat et al., 2015). As well, the diet supplemented with 24% sugar beet pulp can significantly increase the number of
bifidobacteria and
lactobacilli in growing pig intestines and reduce the number of
E. coli In case of compared with the basic diet group
(Laitat et al., 2015). So, the possible reasons for the ability fiber to change the number of microorganisms in pig intestines depend on the developed of the villi, pH values and metabolic processes in the GIT, thereby improving absorption of nutrients as a result of the larger intestinal surface areas
(Ohanaka et al., 2018). On the other hand, Fiber can be used as a substrate for some microorganisms and natural antioxidant (
Riar and Paul, 2017). Furthermore, competitively inhibit the growth and development of other microorganisms. As well, it has a protective effect against various diseases such as constipation, diverticular disease, large bowel cancer, diabetes, coronary heart disease and obesity and gall stones (
Rana and Dahiya, 2019). On the other hand, the acidic environment produced by microbial fermentation of fibers inhibits the proliferation of some pathogenic bacteriaand the similarities and differences of soluble fiber and insoluble fiber on pig intestinal microbes. So, through the pathways and mechanisms specific fibers affect intestinal microbes.
Effect of dietary fiber on the diversity of pig intestinal flora in genera
The higher intestinal microbial diversity, the more stable the intestinal flora structure and the healthier the intestinal tract. Some studies showed the changes of microbial intestinal microbes and their cytokines in piglets by 16SrRNA applicant sequencing and found that piglets fed 5% corn bran had a higher Shannon index compared to the control group, indicating that Corn bran can increase the bacterial diversity of piglet intestines
(Liu et al., 2018). Some studies shown the effect of fermented corn stalks on the performance of fattening pigs, they found in case of increased in fiber content, the Shannon index of ceca microbes in pigs increased first and then decreased, but both were higher than the control group
(Woyengo et al., 2014). Thus, it was confirmed that dietary fiber can promote the microbial diversity of pig intestines. However, the results show wheat bran diet will stimulate the growth of
ileal lactic acid bacteria in pigs and produce a variety of antibacterial compounds, inhibit the growth of other microorganisms. On the other hand, reduce the diversity of pig intestinal microbes (
Ivarsson, 2012). So, we suggestion use the wheat bran in the pig diets. Furthermore, it can lead to increase the micro bacteria amount in the GIT.
The effect of dietary fiber on the metabolites intestinal microbes of pig
The pig intestinal has bacteria, fungi, viruses and other microorganisms, whereas, the bacteria are the predominant flora in pig intestines. There are fibrin lytic bacteria that can ferment and decompose fiber, such as, filamentous
succinogenes, ruminococcus Albicans, rumenococcus aureus and
Prevotella ruminant, which can promote digestion and absorption for dietary fiber in the pig. The microorganisms in pig intestines have a big role for hydrolyzing the dietary fiber to produce VFA that help the animal on inhibiting the growth and development of pathogenic bacteria, regulate the structure of intestinal flora and thus maintain intestinal health
(Montagne et al., 2003). In simulating the colon’s micro-ecological environment in the laboratory found adding beet residue to the diet, led to different content of acetic acid in pig colon, from 57.54% to 61.74%, while the butter acid content increased. Remarkably from 10.00% to 7.97%
(Che et al., 2019; Luo et al., 2018; von Heimendahl et al., 2010). However, other study found the acetic acid, butter acid and the SCFA total in the fibrous cecum of growing pigs feeding on 5% of insulin were lower than those in the control group and their fecal content was higher than in the control group. This may be due to the lower content of degenerating bacteria of the fibers and, the activities of micro bacteria in the GIT.
Factors effect of dietary fiber on intestinal microbes
Fiber level
The content of microbial and ATP in the stomach of pigs in the high-fiber diet is higher than in the low-fiber diet group (
Jensen and Jorgensen, 1994). That indicates the high-fiber diet group had higher microbial activity in the gastrointestinal tract than the low-fiber diet group. That is evidence that dietary fibre levels have an improved effect on pig intestinal microbial activity
(Sun et al., 2015). For example, some studies found that 5%, 10% and 15% alfalfa meal can significantly increase the concentration of SCFA in growing pig colonic chyme and increase with the increase of fiber levels, but no significant effect on colonic chyme microbial. Besides, other studies found the piglet faces fed with sugar beet pulp (highly soluble fibre) found the content of
E. coli in piglet intestines didn’t change significantly in case of increase fiber
(Yan et al., 2017). In case of add different proportions of xylan to the diet of weaned piglets. The number of caeca
E. coli, bifidobacteria and
lactobacilli in weaned piglets and the types and concentrations of their microbial fermentation products have not changed significantly.
Fiber sources
Different fiber sources have different composition and proportion of pig intestinal microorganisms and their fermentation metabolites
(Jonathan et al., 2012). However, the diets of different fibre sources showed very different dominant bands, which also confirmed that the intestinal microbial changes were related to fiber sources. Some studies have shown that wheat bran fibre can increase the number of
bifidobacteria and
lactic acid bacteria in pig intestines
(Zhang et al., 2018). As well, soybean and pea fibre can stimulate the reduction of the number of
E. coli in the intestine. however, pea fibre can increase the number of lactobacilli and bifidobacteria in the intestine
(Chen et al., 2014). Furthermore, the Lactobacilli in pig feces of wheat bran fiber group were significantly higher than the soybean hull fiber group
(Jonathan et al., 2012).
The effect of dietary fiber on the digestive physiology of pigs
The dietary fibre is the main source of plant ingredients ingested by pigs. It plays a specific role in all digestive processes and indirectly affects the intermediate metabolic process. Long-term feeding of high-level fibre diets can change the structure and physiological characteristics of the digestive tract in the pig. Soluble fibre (SF) is an important substrate for microbial fermentation, which can enhance the fermentation strength of intestinal microbes from the ileum to the colon, thereby producing a large amount of short-chain fatty acids (SCFAs) and metabolizing intermediates the process produces a specific effect
(Jensen et al., 2015). However, the level source and composition of dietary fiber will affect the rate of digestion of pig feed. On the other hand, the physiological and chemical characteristics ofdietary fiber determine its effect on feed digestion and nutrient absorption to a certain extent. In addition, the effect on the digestion process of pigs. So, the main role of cellulose and insoluble lignin dietary fiber is to reduce the transit time of excreta and reduce nutrient digestibility in the process of gastric emptying. The role of satiety and biological activity is compared to dietary fibre highly soluble. which can prolong satiety to increase microbial activity and reduce microbial transport time in the pig intestine (
Agyekum and Nyachoti, 2017) as shown in (Table 1).
Effects of dietary fiber on nutrient absorption
The increased of fiber content in pig diets may have a negative effect on the digestibility of pig nutrients (
Agyekum and Nyachoti, 2017). Due to the increase in SF Adding the viscosity of the pig’s intestinal chime reduces the contact between digestive enzymes and substrates. Promotes the formation of an immobile water layer on the surface of the intestine, thereby creating a physical barrier and it reduces the absorption of nutrients by pigs
(Sun et al., 2015). It has been reported the purified SF derived from guar gum, sugar beet pulp, oat, wheat and barley, those diets enhance pig intestinal chyme viscosity, which in turn reduces the digestive absorption of nutrients by pigs
(Owusu-Asiedu et al., 2006). On the other hand, insoluble fiber (ISF) reduces the contact time between digestive tract enzymes and chyme by reducing the time that takes chyme to pass through the digestive tract of pigs
(Owusu-Asiedu et al., 2006). For example, excessive ISF and cellulose derived from wheat bran and corn bran reduce the time that chyme passes through the digestive tract. Thereby reducing the digestion and absorption of nutrients by pigs
(Oliveira et al., 2017; Owusu-Asiedu et al., 2006). In addition, studies have shown the dietary fiber can cause an increase in the loss of endogenous nutrients in pig intestines, which in turn reduces the absorption of nutrients by pigs (
Jha and Berrocoso, 2016;
Sun et al., 2015; Wu et al., 2018). Other study shown feeding high-fiber diets to fattening pigs will increase the shedding of bile, mucin and epithelial cells (
Low, 1989). However, mucin and epithelial cells are rich in nitrogen and amino acids. As their shedding decreases, it reduces the digestion and absorption of amino acids in pigs.
The role of dietary fiber in gastric emptying
The studies show the dietary fiber is closely related to gastric emptying rate and digestion and absorption of nutrient
(Cisse et al., 2017). In case of, fattening pigs will reduce feed intake due to slow gastric emptying, there by inhibiting growth. On the other hand, sows slow down their gastric emptying rate during pregnancy, which can both reduce feed intake to control body conditions and enhance their pregnancy, Satiety and reducing stereotyped behaviors due to feeding restriction and hunger. As well, improving reproductive and nutritional performance of animals
(Sapkota et al., 2016; Wanders et al., 2013) Some studies have revealed the effects of different fiber types and sources on the gastric emptying rate. For example, studied the addition of oat SF to pigs and found that the experimental group showed gastric emptying one hour after feeding compared with the control group, Slowing the trend
(Johansen et al., 1996). Other studies on the effects of different dietary fibers on the gastric emptying rate of pregnant sows found that compared with the control group. The wheat bran group (high ISF) pigs and the control group reached the peak of gastric content after 0.5 hour, while sugar beet the residue group (high SF) reached its peak after two hour at the same time. Besides, it was found that the gastric content emptying rate of the sugar beet residue group was significantly lower than that of wheat bran and the control group. So, the source and type of dietary fiber has role on the rate of gastric emptying.
Effect on digestive tract organs and intestinal morphology
Adding a certain proportion of fiber to the pig diet lead to increasing the load on the pig digestive tract organs, promotes digestive enzymes and, pancreatic juice due to the low energy content of fibre raw materials. As well, the pig’s gastrointestinal tract will adaptive changes to maintain its own nutritional needs, such as increased feed intake to supplement energy value and intestinal growth
(Agyekum et al., 2012). In some studies on the pig shown the group that (fed with 30% DDGS diet) and the control group (fed with basic corn-soybean meal diet) pigs and found that the experimental group’s liver, spleen, pancreas and heart. The weight and the total length of the colon and rectum are larger than the basic diet group. As well, compared the growth of pigs fed a 30% DDGS diet with corn-soybean meal diets and found the weight of the heart, lung, liver, kidney and spleen of the DDGS group was greater than the basic diet group
(Coble et al., 2018). However, high-fibre diets do have an effect on the digestive organs of pigs. The complete intestinal morphology is the basis for pigs to digest and utilize nutrients. It mainly includes the length of intestines, villi and the depth of crypts. Intestinal villi mainly decompose and absorb nutrients through secreted enzymes. As well, the shallower crypts indicate the increased cell maturation rate and, secreting active substances such as digestive enzymes. however, in case of increase of fibre levels in pigdiets, it is easy to lead to change the intestinal morphology, such as intestinal villi length and crypt depth
(Gao et al., 2015). Besides, The mechanism of fibres is fermented by microorganisms in the intestine and produce SCFAs, which provide enough energy to intestinal cell differentiation and proliferation, then promote the proliferation, development of villi and crypts
(Wu et al., 2018).
Impact on digestive tract pH
The digestive tract pH of pigs is mainly regulated by nerves and hormones. But in case of increases the age of pigs the digestive tract pH is susceptible to various factors. Some studies have shown the pigs that fed 5% sodium carboxymethyl cellulose instead of part of the corn-soybean meal diet had lower pH than the basic corn-soybean meal diet and the difference was significant
(Gao et al., 2015). So, the effect of different types of fiber on the gastric pH of pregnant sows and, depend on increase the (highly- insoluble Fiber), lead to the pH value in the stomach reaches the highest value. Furthermore, stimulate the secretion of hydrochloric acid, which in turn causes the pH value in the stomach to decrease it.
Effects on microbes and mucosa of the digestive tract
The maintenance of intestinal health is complex and depends on the synergy between diets symbiotic microbiota and mucosa
(Zhou et al., 2017). So, diets are essential to maintain the balance between the intestinal tract microbiome and intestinal environment. On the other hand, the dietary fibre can be fermented and decomposed by the end of intestine in pig and, producing large amounts of SCFA such as acetate, propionate and butyrate, besides, these SCFAs can stimulate mucosal mucin and small molecule peptide levels and enhance the defence function of the mucosa. As well, the effect of high-resistance starch diets on the end products of the fermentation of protein and mucin secreted by pig colons high-resistance starch can significantly increase intestinal mucosal mucin levels. Furthermore, SCFA can selectively inhibit the growth of pathogenic bacteria such as Salmonella,
Escherichia coli and Clostridium in an acidic environment, regulate the structure of intestinal flora and maintain intestinal balance
(Kurian et al., 2012).
The role of dietary fiber on digestive juice
Increase the fiber content in the diet has effect on digestive enzymes activity. Most scholars believe the dietary fiber can reduce the activity of intestinal amylase, lipase, pancreatic and chymotrypsin. besides, the fiber intake, types and type of enzymes in the GIT
(Jepson et al., 2005). another study found that the activity of lipase, trypsin and chymotrypsin in pigs fed a 7.5% pectin diet was lower than the basic diet by inserting T-tubes into pigs
(Mosenthin et al., 1994). This depends on SF (soluble fibre) sticky and easily coats the chyme to form a complex, which hinders the contact between the enzyme and the substrate. thereby reducing the digestive enzyme activity (
Agyekum and Nyachoti, 2017). ISF accelerates the flow rate of chyme in the digestive intestines of pigs. Also reduces the activity of digestive enzymes (
Agyekum and Nyachoti, 2017). promote the proliferation of pig intestinal microorganisms the secretion of digestive enzymes (
Agyekum and Nyachoti, 2017;
Yan et al., 2013). As well, dietary fiber has an effect on digestive tract pH. Besides, the acid environment has an effect on the activity of digestive enzymes and the digestive juices produced by the decomposition of fibers by microorganisms.
