Is Fructooligosaccharides Benefit for Intestinal Health?

In aquaculture production, the irrational use of antibiotics can lead to bacterial resistance, damage the intestinal health and immunity of animals, and its residues in meat products and the environment ultimately affect human health. China has completely banned the addition of growth-promoting antibiotics to feed. Therefore, finding feed additives with the same effect as antibiotics to alleviate the decline in livestock and poultry production performance and the increase in mortality has become a current research hotspot. The beneficial effect of probiotics on intestinal health has been confirmed, but due to the harsh storage conditions, the effect of their use in feed is unstable, and prebiotics are considered to be a good alternative. This article starts with the representative substance of prebiotics, oligofructose, and reviews its role in maintaining the intestinal health of livestock and poultry by describing its physicochemical properties and physiological effects.

1. Physicochemical properties of oligofructose

Oligofructose has the molecular formula G-F-Fn (G-glucose, F-fructose, n=1-3), and is a general term for a class of carbohydrates formed by the β-1,2 glycosidic bond of sucrose molecules with several D-fructose (Fanaro et al., 2010). It is resistant to high temperatures, has good solubility, and is stable in neutral solutions at 120 °C. However, its stability decreases with increasing acidity and temperature. Its sweetness is slightly lower than that of sucrose. Between 0 and 70 °C, the viscosity of fructooligosaccharides is similar to that of high-fructose corn syrup, and decreases with increasing temperature. It also inhibits starch aging, is not easily colored, is alkali-resistant, retains water, has poor moisture absorption, and has good mold resistance. These physical and chemical properties make fructooligosaccharides suitable for use as excellent feed additives (Vandana et al., 2015).

2 Physiological effects of fructooligosaccharides

The animal intestine is home to a large number of microorganisms. According to the relationship between microorganisms and their hosts, the microorganisms in the intestine can be divided into commensal bacteria, conditionally pathogenic bacteria and pathogenic bacteria. Under normal circumstances, the bacterial flora maintains a balance that is beneficial to the host, but under intensive farming conditions, conditionally pathogenic bacteria can sometimes become invasive, and the number of pathogenic bacteria can exceed normal levels, disrupting this balance and leading to bacterial diseases (Zhang R.J., 2003). Fructooligosaccharides can prevent the colonization of exogenous pathogens in the intestine and the overgrowth of endogenous conditional pathogens, thereby maintaining the intestinal microecological balance.

This effect is mainly due to the bacteriostatic effect of the short-chain fatty acids produced by Bifidobacteria (acetic acid: lactic acid ratio of 3:2). The β-1,2 glycosidic bond of oligofructose cannot be hydrolyzed by the various digestive enzymes secreted by animals, so most of the oligofructose can pass through the stomach and small intestine without problem. In the large intestine, beneficial bacteria such as lactobacilli and bifidobacteria can secrete enzymes to break the β-1,2 glycosidic bond and proliferate, while harmful bacteria cannot secrete these enzymes. After the beneficial bacteria proliferate, they form a film on the intestinal mucosa, making it difficult for pathogenic bacteria to colonize. At the same time, the beneficial bacteria can also ferment a large amount of the fatty acids such as acetic acid, propionic acid and lactic acid produced by fructooligosaccharides, which increases the acidity of the intestine and directly inhibits the growth of pathogenic bacteria (Munoz et al., 2012; Rober- froid et al., 2010; Callaway et al., 2009).

3 Fructooligosaccharides and intestinal health

There is no clear definition of animal intestinal health, which generally includes efficient food digestion and nutrient absorption, a normal and stable intestinal flora environment, a good immune status and an overall healthy state. It is mainly determined by three factors: feed composition, intestinal barrier and microbiota (Pluske et al., 2018; Celi et al., 2017; Kogut and Arsenault, 2016; Bischoff, 2011).

3.1 Fructooligosaccharides and the intestinal flora

Fructooligosaccharides selectively stimulate the growth of bifidobacteria and lactobacilli, while inhibiting the proliferation of harmful bacteria such as Escherichia coli, Clostridium perfringens and Streptococcus (Howard et al., 1995; Fishbein et al., 1988; Mitsuoka et al., 1987). Currently, most analyses of the intestinal microbial flora are based on methods that culture specific bacteria. In pigs, 4% oligofructose added to piglet feed increased the number of bifidobacteria in the intestine (Modesto et al., 2009). Mikkelsen et al. (2003) analyzed the feces of weaned piglets after adding 4% oligofructose, and the results showed that it had little effect on the physical and chemical properties, and had no significant effect on other microorganisms except for stimulating the growth of yeast. However, they did not analyze the microflora. Feeding weaned piglets with 5% oligofructose increased the number of bifidobacteria and reduced the number of Escherichia coli in the proximal colon (Gebbink et al., 1999). Feeding broiler chickens with 0.4% oligofructose for 14 days did not change the total number of bacteria in the cecum, but selectively increased the number of bifidobacteria and bacteroides (Cao et al., 2005); adding 0.2%, 0.4%, and 0.8% oligofructose to broilers, of which 0.4% oligofructose can significantly promote the growth of bifidobacteria and lactobacilli in the contents of the small intestine and cecum and inhibit the proliferation of Escherichia coli (Xu et al., 2003).

With the rapid development of quantitative fluorescence, denaturing gradient gel electrophoresis and high-throughput sequencing technologies, it provides technical support for in-depth research on intestinal microbial diversity. Analysis of the effects of 0.5% oligofructose on the ileal microflora of broilers using denaturing gradient gel electrophoresis showed that the microflora changed (Geier et al., 2009). Adding 0.25% oligofructose to the feed of broilers and analysing the intestinal microflora using quantitative fluorescent PCR showed that it increased the number of lactobacilli and reduced the number of Clostridium perfringens and Escherichia coli (Kim et al., 2011). In cattle, Chen Fengmei et al. (2020) used 16S rRNA gene sequencing and basic microbial community Alpha diversity analysis to study the effect of daily supplementation of 5 g oligofructose combined with probiotics on the intestine of lactating calves. The results showed that it had a significant effect on the abundance of intestinal microorganisms, with a significant increase in the relative abundance of the genus Bacteroides and a significant decrease in the relative abundance of Clostridium sporogenes, which in turn significantly reduces the occurrence of diarrhea.

3.2 Fructooligosaccharides and intestinal fermentation

Fructooligosaccharides are fermented by beneficial bacteria in the hindgut to produce lactic acid and acetic acid, propionic acid, butyric acid and other short-chain fatty acids, as well as carbon dioxide and methane. Most of the organic acids can be absorbed, which means that fructooligosaccharides can also indirectly provide part of the energy for the host. Organic acids also make potentially toxic ammonia ions protonated to form NH4+, and lactic acid increases the acidity of the cecum, which is beneficial for the growth of lactic acid bacteria and bifidobacteria. These bacteria can also use ammonia as a nitrogen source, thereby reducing ammonia in the intestine and blood. Feeding male turkeys different levels of oligofructose (0.5%, 1%, 2%) caused the pH of the contents of the ileum and cecum to decrease with increasing oligofructose, while the fatty acid content was not affected, but the accumulation of fatty acids in the ileum increased (Jus'kiewicz et al., 2007). Adding 8 g/d and 24 g/d oligofructose to the diet of foals significantly increased the total volatile volatile fatty acids, acetic acid, butyric acid, propionic acid and lactic acid in the feces (Berg et al., 2005); oligofructose can increase the content of short-chain fatty acids in pig feces and mouse intestines (Anthony et al., 2009; Ly Sun Yung et al., 2003); adding 5% oligofructose to the mouse feed can significantly increase the content of propionic acid and butyric acid, but has no effect on total volatile 020).

3.3 Fructooligosaccharides and the intestinal barrier

Prebiotics such as fructooligosaccharides can regulate the morphology, structure and function of the intestinal mucosa, such as the height of intestinal villi, the depth of crypts, the thickness of the epithelial layer, the number of epithelial cells, the number of mitotic cells, mucous cells and goblet cells. These intestinal changes can promote the digestion and absorption of nutrients. The mechanism of these changes may come from the following aspects: (1) fermentable sugars can affect the intestinal glucagon level to regulate the proliferation of crypt cells in the small intestine ( Gee et al., 1996); (2) prebiotics promote polyamine synthesis, which can positively regulate the growth and development of the intestinal and colonic mucosa (Delzenne et al., 2000); (3) prebiotics can increase digestive enzyme activity, which may promote the growth and transformation of the small intestinal mucosa (Yang et al., 2005); (4) an increase in the synthesis of short chain fatty acid synthesis also promotes the proliferation of large and small intestinal cells, which can improve the digestion and absorption of nutrients by the brush border by regulating the height of intestinal villi and the depth of crypts.

Adding 0.25% oligofructose to pig feed can increase the height of the villi in the proximal small intestine and the ratio of villus height to crypt depth (Shim, 2005); 0.4% and 0.6% oligofructose can increase the height of the villi in the porcine jejunum and the ratio of villus height to crypt depth (Xu et al., 2002); oligofructose can increase the proliferation rate of mucosal epithelial cells in the cecum and colon of suckling piglets and prevent atrophy of mucosal epithelial cells (Howard et al., 1995); adding 0.4% oligofructose to the diet was found to significantly increase the villus height and the ratio of villus height to crypt depth in the jejunum and ileum of chicks, while the crypt depth was significantly reduced. However, adding 0.8% oligofructose did not achieve this effect (Xu et al., 2003); 0.5% oligofructose can significantly improve the villus height, crypt depth and epithelial thickness of the broiler ileum, but has no effect on the duodenum and jejunum (Shang et al., 2014); the addition of 0.4% and 0.6% oligofructose significantly increased the crypt depth and and cell density increased significantly (Zhan et al., 2003); adding 4 g/d and 6 g/d oligofructose to the feed of early weaned calves can increase intestinal villus height and the thickness of the intestinal mucosa (Tai et al., 2009).

3.4 Oligofructose and immunity

The animal gut is where the intestinal microorganisms, foreign antigens and the immune system interact. The effects of prebiotics such as oligofructose on the immune function of livestock are not yet fully understood. Possible mechanisms include: (1) a direct relationship with the effects of lactic acid bacteria or their products on immune cells; (2) fermentation products of short-chain fatty acids can stimulate an increase in the number of T lymphocytes in the gastrointestinal tract; (3) by regulating the production of mucins to increase the number of goblet cells (Schley and Field, 2009). Prebiotics can regulate the function and type of intestinal lymphoid tissue, especially the Peyer's patches, secondary lymphoid follicles and cells in the peripheral circulation (Samal et al., 2009). In sows fed 10 g/d of fructooligosaccharides, there was a trend towards an increase in transforming growth factor β1 in colostrum, which led to an increase in IgA concentration, an increase in interferon γ concentration secreted by Peyer's patches and mesenteric lymphocytes, and an increase in IgA secreted by Peyer's patch cells. Interferon γ is associated with activated T lymphocytes (Cindy et al., 2014). Adding 0.5% oligosaccharide in broiler feed can reduce the proportion of B lymphocyte in the cecal tonsil and increase the blood IgM and IgG content (Janardhana et al., 2009).

4 Summary

At present, research on the application of oligofructose in animal feed mainly focuses on pigs and chickens, and there are not many reports on its application in other livestock and poultry such as cattle, sheep, and rabbits. Basic and mechanistic research on them is also lacking. Therefore, it is necessary to further study the application effect and mechanism of oligofructose in livestock and poultry feed. The further development of molecular biology techniques such as high-throughput sequencing has introduced new ideas into the study of oligofructose.

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