Abstract

The biggest progress of carbohydrate research in pig nutrition and health is the more clear classification of carbohydrate, which is not only based on its chemical structure, but also based on its physiological characteristics. In addition to being the main energy source, different types and structures of carbohydrates are beneficial to the nutrition and health functions of pigs. They are involved in promoting the growth performance and intestinal function of pigs, regulating the intestinal microbial community, and regulating the metabolism of lipids and glucose. The basic mechanism of carbohydrate is through its metabolites (short chain fatty acids [SCFAs]) and mainly through scfas-gpr43 / 41-pyy / GLP1, SCFAs amp / atp-ampk and scfas-ampk-g6pase / PEPCK pathways to regulate fat and glucose metabolism. New studies have evaluated the optimal combination of different types and structures of carbohydrates, which can improve growth performance and nutrient digestibility, promote intestinal function, and increase the abundance of butyrate producing bacteria in pigs. Overall, compelling evidence supports the view that carbohydrates play an important role in the nutritional and health functions of pigs. In addition, the determination of carbohydrate composition will have theoretical and practical value for the development of carbohydrate balance technology in pigs.

1. Preface

Polymeric carbohydrates, starch and non starch polysaccharides (NSP) are the main components of diets and the main energy sources of pigs, accounting for 60% - 70% of the total energy intake (Bach Knudsen). It is worth noting that the variety and structure of carbohydrates are very complex, which have different effects on pigs. Previous studies have shown that feeding with starch with different amylose to amylose (AM / AP) ratio has obvious physiological response to the growth performance of pigs (Doti et al., 2014; Vicente et al., 2008). Dietary fiber, mainly composed of NSP, is believed to reduce nutrient utilization and net energy value of monogastric animals (NOBLET and le, 2001). However, dietary fiber intake did not affect the growth performance of piglets (Han & Lee, 2005). More and more evidence shows that dietary fiber improves the intestinal morphology and barrier function of piglets, and reduces the incidence of diarrhea (Chen et al., 2015; Lndberg，2014； Wu et al., 2018). Therefore, it is urgent to study how to effectively utilize the complex carbohydrates in the diet, especially the feed rich in fiber. The structural and taxonomic characteristics of carbohydrates and their nutritional and health functions for pigs must be described and considered in feed formulations. NSP and resistant starch (RS) are the main non digestible carbohydrates (wey et al., 2011), while the intestinal microbiota ferments non digestible carbohydrates into short chain fatty acids (SCFAs); Turnbaugh et al., 2006). In addition, some oligosaccharides and polysaccharides are considered as probiotics of animals, which can be used to stimulate the proportion of Lactobacillus and Bifidobacterium in the intestine (Mikkelsen et al., 2004; M ø LBAK et al., 2007; Wellock et al., 2008). Oligosaccharide supplementation has been reported to improve the composition of intestinal microbiota (de Lange et al., 2010). In order to minimize the use of antimicrobial growth promoters in pig production, it is important to find other ways to achieve good animal health. There is an opportunity to add more variety of carbohydrates to pig feed. More and more evidence shows that the optimal combination of starch, NSP and MOS can promote growth performance and nutrient digestibility, increase the number of butyrate producing bacteria, and improve the lipid metabolism of weaned pigs to a certain extent (Zhou, Chen, et al., 2020; Zhou, Yu, et al., 2020). Therefore, the purpose of this paper is to review the current research on the key role of carbohydrate in promoting growth performance and intestinal function, regulating intestinal microbial community and metabolic health, and to explore the carbohydrate combination of pigs.

2. Classification of carbohydrates

Dietary carbohydrates can be classified according to their molecular size, degree of polymerization (DP), connection type (a or b) and composition of individual monomers (Cummings, Stephen, 2007). It is worth noting that the main classification of carbohydrates is based on their DP, such as monosaccharides or disaccharides (DP, 1-2), oligosaccharides (DP, 3-9) and polysaccharides (DP, ≥ 10), which are composed of starch, NSP and glycosidic bonds (Cummings, Stephen, 2007; Englyst et aL. , 2007; Table 1). Chemical analysis is necessary to understand the physiological and health effects of carbohydrates. With more comprehensive chemical identification of carbohydrates, it is possible to group them according to their health and physiological effects and to include them in the overall classification plan (englyst et al., 2007). Carbohydrates (monosaccharides, disaccharides, and most starches) that can be digested by host enzymes and absorbed in the small intestine are defined as digestible or available carbohydrates (Cummings, Stephen, 2007). Carbohydrates that are resistant to intestinal digestion, or poorly absorbed and metabolized, but may be degraded by microbial fermentation are considered resistant carbohydrates, such as most NSP, indigestible oligosaccharides and RS. Essentially, resistant carbohydrates are defined as indigestible or unusable, but provide a relatively more accurate description of the classification of carbohydrates (englyst et al., 2007).

3.1 growth performance

Starch is composed of two kinds of polysaccharides. Amylose (AM) is a kind of linear starch α（ 1-4) linked dextran, amylopectin (AP) is a α（ 1-4) linked dextran, containing about 5% dextran α（ 1-6) to form a branched molecule (tester et al., 2004). Due to different molecular configurations and structures, AP rich starches are easy to digest, while am rich starches are not easy to digest (Singh et al., 2010). Previous studies have shown that starch feeding with different AM / AP ratios has significant physiological responses to the growth performance of pigs (Doti et al., 2014; Vicente et al., 2008). Feed intake and feed efficiency of weaned pigs decreased with the increase of AM (regmi et al., 2011). However, emerging evidence reports that diets with higher am increase the average daily gain and feed efficiency of growing pigs (Li et al., 2017; Wang et al., 2019). In addition, some scientists reported that feeding different AM / AP ratios of starch did not affect the growth performance of weaned piglets (Gao et al., 2020A; Yang et al., 2015), while high AP diet increased the nutrient digestibility of weaned pigs (Gao et al., 2020A). Dietary fiber is a small part of food that comes from plants. A major problem is that higher dietary fiber is associated with lower nutrient utilization and lower net energy value (noble & Le, 2001). On the contrary, moderate fiber intake did not affect the growth performance of weaned pigs (Han & Lee, 2005; Zhang et al., 2013). The effects of dietary fiber on nutrient utilization and net energy value are affected by fiber characteristics, and different fiber sources may be very different (lndber, 2014). In weaned pigs, supplementation with pea fiber had a higher feed conversion rate than feeding corn fiber, soybean fiber and wheat bran fiber (Chen et al., 2014). Similarly, weaned piglets treated with corn bran and wheat bran showed higher feed efficiency and weight gain than those treated with soybean hull (Zhao et al., 2018). Interestingly, there was no difference in growth performance between the wheat bran fiber group and the inulin group (Hu et al., 2020). In addition, compared with the piglets in the cellulose group and xylan group, the supplementation was more effective β- Glucan impairs the growth performance of piglets (Wu et al., 2018). Oligosaccharides are low molecular weight carbohydrates, intermediate between sugars and polysaccharides (voragen, 1998). They have important physiological and physicochemical properties, including low calorific value and stimulating the growth of beneficial bacteria, so they can be used as dietary probiotics (Bauer et al., 2006; Mussatto and mancilha, 2007). Supplementation of chitosan oligosaccharide (COS) can improve the digestibility of nutrients, reduce the incidence of diarrhea and improve the intestinal morphology, thus improving the growth performance of weaned pigs (Zhou et al., 2012). In addition, diets supplemented with cos can improve the reproductive performance of sows (the number of live piglets) (Cheng et al., 2015; Wan et al., 2017) and growth performance of growing pigs (wontae et al., 2008). Supplementation of MOS and fructooligosaccharide can also improve the growth performance of pigs (Che et al., 2013; Duan et al., 2016; Wang et al., 2010; Wenner et al., 2013). These reports indicate that various carbohydrates have different effects on the growth performance of pigs (table 2a).

3.2 intestinal function

High am/ap ratio starch can improve intestinal health(tribyrincan be protect it for pig) by promoting intestinal morphology and up regulating intestinal function related to gene expression in weaning pigs (Han et al., 2012; Xiang et al., 2011). The ratio of villi height to villi height and recess depth of ileum and jejunum was higher when fed with high am diet, and the total apoptosis rate of small intestine was lower. At the same time, it also increased the expression of blocking genes in duodenum and jejunum, while in the high AP group, the activities of sucrose and maltase in jejunum of weaned pigs were increased (Gao et al., 2020b). Similarly, previous work found that am rich diets reduced pH and AP rich diets increased the total number of bacteria in the caecum of weaned pigs (Gao et al., 2020A). Dietary fiber is the key component that affects the intestinal development and function of pigs. The accumulated evidence shows that the dietary fiber improves the intestinal morphology and barrier function of weaned pigs, and reduces the incidence of diarrhea (Chen et al., 2015; Lndber，2014； Wu et al., 2018). Dietary fiber deficiency increases the susceptibility of pathogens and impairs the barrier function of colon mucosa (Desai et al., 2016), while feeding with highly insoluble fiber diet can prevent pathogens by increasing the length of villi in pigs (hedemann et al., 2006). The different types of fibers have different effects on the function of colon and ileum barrier. Wheat bran and pea fibers enhance gut barrier function by regulating TLR2 gene expression and improving intestinal microbial communities compared with corn and soybean fibers (Chen et al., 2015). Long term ingestion of pea fiber can regulate metabolism related gene or protein expression, thereby improving colon barrier and immune function (Che et al., 2014). Inulin in diet can avoid intestinal disturbance in weaned piglets by increasing intestinal permeability (Awad et al., 2013). It is worth noting that the combination of soluble (inulin) and insoluble fiber (cellulose) is more effective than alone, which can improve nutritional absorption and intestinal barrier function in weaned pigs (Chen et al., 2019). The effect of dietary fiber on intestinal mucosa depends on their components. A previous study found that xylan promoted the intestinal barrier function, as well as changes in bacterial spectrum and metabolites, and glucan promoted intestinal barrier function and mucosal health, but the supplementation of cellulose did not show similar effects in weaning pigs (Wu et al., 2018). Oligosaccharides can be used as carbon sources for the microorganisms in the upper gut instead of being digested and utilized. Fructose supplementation can increase the intestinal mucosa thickness, butyric acid production, the number of recessive cells and the proliferation of intestinal epithelial cells in weaned pigs (Tsukahara et al., 2003). Pectin oligosaccharides can improve intestinal barrier function and reduce intestinal damage caused by rotavirus in piglets (Mao et al., 2017). In addition, it has been found that cos can significantly promote the growth of intestinal mucosa and significantly increase the expression of blocking genes in piglets (WAN, Jiang, et al. in a comprehensive way, these indicate that different types of carbohydrate can improve intestinal function of piglets (table 2b).

Summary and Prospect

Carbohydrate is the main energy source of pigs, which is composed of various monosaccharides, disaccharides, oligosaccharides and polysaccharides. Terms based on physiological characteristics help to focus on the potential health functions of carbohydrates and improve the accuracy of carbohydrate classification. Different structures and types of carbohydrates have different effects on maintaining growth performance, promoting intestinal function and microbial balance, and regulating lipid and glucose metabolism. The possible mechanism of carbohydrate regulation of lipid and glucose metabolism is based on their metabolites (SCFAs), which are fermented by intestinal microbiota. Specifically, carbohydrate in diet may regulate glucose metabolism through scfas-gpr43 / 41-glp1 / PYY and ampk-g6pase / PEPCK pathways, and regulate lipid metabolism through scfas-gpr43 / 41 and amp / atp-ampk pathways. In addition, when different types of carbohydrates are in the best combination, the growth performance and health function of pigs may be improved.

It is worth noting that the potential functions of carbohydrate in protein and gene expression and metabolic regulation will be discovered by using high-throughput functional proteomics, genomics and metabonomics methods. Last but not least, the evaluation of different carbohydrate combinations is a prerequisite for the study of diverse carbohydrate diets in pig production.

Souce：Animal Science Journal