Advances in the biological activity of tannins and their application in animal production

Abstract 

Because tannins bind to proteins, polysaccharides, enzymes, metal ions, etc., they have long been used as an antinutritional factor. In recent years, research has found that tannins have strong physiological activities in antibacterial, antiviral, antioxidant and free radical scavenging. Suitable tannins in diets have positive effects on growth, immunity and reproductive performance of animals. The article provides a reference for the application of tannins in animal production by reviewing their research progress in the above areas.

keywords: tannins; anti-nutritional factors; antibacterial effects; antioxidant

Abstract Because tannins bind to proteins, polysaccharides, enzymes, metal ions, etc., they have long been used as an antinutritional factor. In recent years, research has found that tannins have strong physiological activities in antibacterial, antiviral, antioxidant and free radical scavenging. Suitable tannins in diets have positive effects on growth, immunity and reproductive performance of animals. The article provides a reference for the application of tannins in animal production by reviewing their research progress in the above areas.

Tannins (tannins) are secondary metabolites of plants and are natural polyphenol compounds with molecular weights of about 0.5 to 3ku and are widely found in a variety of feedstuffs such as legumes, sorghum seeds, and broad beans. Tannins interact with salivary proteins, glycoproteins, etc. to produce bitter and astringent taste, affect the palatability, reduce the amount of food consumed by animals, affect the digestion and absorption of proteins [1], complex with metal ions, affect the absorption and utilization of iron and other mineral elements [2], but also indirectly affect the activity of some enzymes containing metal elements, thus affecting the digestion and absorption of nutrients, and is therefore considered an antinutritional factor [3,4]. In recent years, in-depth research has found that tannins have unique biological effects such as antioxidant, free radical removal, antibacterial and antiviral. The appropriate amount of tannins in feed has a promoting effect on animal growth, immunity and nitrogen metabolism in the body, providing a new theoretical basis and research direction for the development of feed resources.

1 Physical and chemical properties

Tannins have a more complex chemical structure and can generally be divided into two broad categories: hydrolyzed tannins (HT) and condensed tannins (CT). The former is mainly polyphenols of polycrystalline orange acid ester, i.e., it is made from orange acid and its derivatives connected with polyols by ester bonds, which can be easily hydrolyzed into small molecule compounds under the action of dilute acid, dilute base or enzyme; the latter is a flavanol derivative, the second position of flavanol in the molecule is bound to catechol or benzotriol by C-C bond, which can easily undergo intermolecular condensation under the action of acid, base or enzyme to form large molecule substances. Because of its polyphenol hydroxyl structure, tannins are chemically active and therefore have good biological activities such as antioxidant, antibacterial and antiviral.

2 Biological activity

2.1 Antioxidant and free radical capture

Free radicals are products of the body’s metabolism and are directly related to the physiological health of the body. An excess of free radicals in the body will cause the body’s normal cell membrane structure, biological macromolecules and tissue oxidative damage, so that the body is in a state of oxidative stress, on the production performance of livestock and poultry, meat quality and other adverse effects [5,6]. Determination of the reducing power of ellagic acid revealed that the reducing power of the extract was comparable to that of vitamin C at a concentration of 1.0 mg/mL and 0.5 mg/mL, but the increase in reducing power was not significant when the concentration exceeded 5 mg/mL [7]. This suggests that ellagic acid tannins have a strong reducing power. It has been reported that tannins remove free radicals better than spirulina with cinnabar [8]. Grapeseed tannins have a strong scavenging ability against hydroxyl radicals and superoxide radicals, and their scavenging tannin concentration has a special saturation mechanism, while their antioxidant effect on lard and sesame oil is stronger than vitamin C [9]. Zhang Zong Yuan et al. (2011) found similar results through a recent study of tannins in alfalfa [10]. Glutathione peroxidase and superoxide dismutase are the two major enzyme systems of the body’s antioxidant defense system, which enhance the activity of the corresponding enzymes and help to improve the body’s resistance to oxidative stress. Liu Weihua showed that chestnut tannin (CT) significantly increased the activity of superoxide dismutase (SOD), Cu, Zn-SOD and GSH-Px in the serum of rabbits, and decreased the level of malondialdehyde (MDA), with a corresponding increase in the expression of its associated genes [11]. This suggests that tannins can not only directly terminate the free radical reaction chain by providing reactive hydrogen as well as its phenol hydroxyl groups, but also indirectly improve the antioxidant properties of the organism by enhancing the activity and expression of antioxidant enzymes.

2.2 Antibacterial effects

The inhibitory effects of tannins on bacteria and other microorganisms have been studied for a long time, and in summary, they are mainly achieved through the following mechanisms: (1) cross-linking with various enzymes, inhibiting the activity of exogenous enzymes and some key enzymes of the oxidative phosphorylation reaction, and affecting the enzyme-promoting reaction and thus metabolism. The formation of insoluble protein-tannin compounds with proteins through hydrogen bonding and hydrophobic bonding inhibits nitrogen metabolism by microorganisms and reduces protein levels in microbial fermentation substrates. (3) Methylation of tannins, which facilitates tannins to cross the cell membrane and exert antibacterial effects.

Li Xiaoping (2010) confirmed that ellagic acid extract had a good inhibitory effect on test bacteria such as E. coli, Salmonella and Bacillus subtilis, but weaker inhibitory effect on test mycobacteria, Aspergillus mites, Aspergillus niger, and test yeast [7]. This suggests that tannins also exhibit different effects on different types of microbial inhibition, and that Gram-positive bacteria are more sensitive to tannins than Gram-negative bacteria [12]. At the same time, the inhibitory effect of tannins from different sources on microorganisms varies and is related to the size of their molecular weight. It has also been reported that tannins also have an inhibitory effect on the protozoa of the rumen, and Moran et al. found that condensed tannins at concentrations of 400 mg/mL or higher inhibited the growth of many species of rumen bacteria, inhibited the development of infectious larvae, and reduced larval viability [13], the mechanism of which is unclear and may be related to the astringent nature of tannins.

2.3 Antiviral, anti-tumour effects

The use of tannins for antiviral research has been reported both domestically and internationally. The antiviral effect of crane grass, green tea, five-fold seeds and other herbs is more significant. Numerous studies have shown that the antiviral and anti-mutagenic mechanisms of action of tannins are largely related to their antibacterial properties. It mainly inhibits the synthesis of viral and cancer cell DNA and interferes with protein synthesis and metabolism to block the cell cycle and induce apoptosis [14,15]. It can also inhibit various inflammatory pathways including NF-κB pathway to inhibit tumor vessel formation, inhibit cell proliferation and promote apoptosis [16]. It was found that the galanganoyl of orange tannins is closely related to the anti-HSV-1 activity of the compounds, and the introduction of hydrophobic groups in the compounds promotes the inhibitory activity of HSV-1 virus [17], and also has a more significant effect on hepatitis B and C viruses [18,19]. The results of in vitro and in vivo tests showed that glucose sulfate pentavalerate could significantly increase the feed conversion rate of weaned piglets and reduce the degradation of intestinal protein microorganisms by inhibiting viral adsorption of tannins in vitro [21], and when the tannin content in feed was 2%, it could significantly increase the average daily weight gain and body height of mature sika deer [22]. The results of the Schiavone study on broilers showed that the addition of chestnut tannin (CT) to the diet significantly increased ADG and ADF I in broilers by 0.2% CT for the first two weeks of the trial, but not after two weeks [24]. Similar results were obtained in Liu Huawei’s study, and CT significantly increased ADG (P <0.05) but decreased FCR (P <0.05) in meat rabbits at both room temperature and high temperature. In a further study, it was found that CT significantly increased the activity of α-amylase and trypsin in duodenal and jejunal coeliacs and α-amylase in ileal coeliacs of meat rabbits, with no significant effect on lipase activity, probably due to the fact that CT promotes the secretion of pancreatic fluid from the small intestine [11]. Tannins, in combination with proteins, enzymes and vitamins, have an impact on the digestion and absorption of nutrients. Studies have shown that tannins can reduce rumen gas production and avoid rumen flatulence; they can also protect against peri-rumen proteins and increase protein utilization, but the tannin content in feed is not too high [25,26]. feed if its content to CD+ cells, thus achieving a therapeutic effect on HIV [20]. This may be associated with too high an integration of tannin-inhibiting viruses, which then affects the growth performance of the animal. Leigh

The enzyme is associated with reverse transcriptase activity. This suggests that tannins, as natural antiviral compounds, have good research promise in medicine.

3 Applications in livestock and poultry production

3.1 Impact on growth performance

Recent studies have shown that tannins can improve the growth performance of animals to some extent. It was reported that the effect of tannic acid on protein intake and protein digestibility in root voles was significantly reduced (P < 0.05) at 10% and 20% protein content, but not at 20% protein level [3]. This suggests that tannins have an inhibitory effect on the amount of food consumed and the digestibility of proteins in animals, and a side effect on the growth performance of root voles. It has also been shown that animals have a certain tolerance to tannins, possibly related to the detoxification mechanism of the organism. The effects of plant tannins on animal growth performance are therefore more complex and may be related to differences in tannin tolerance and the type and protein content of tannins in feed, which require further study.

3.2 Immunity and health

Tannins have good antioxidant and free radical scavenging ability, as well as antiviral and anti-disease effects, and can regulate the activity of NK cells, with a slight effect on cytokine secretion and immunoglobulin secretion. Thus, tannins have immune-boosting, anti-fatigue and other effects [27]. Tannin polyphenols can also work synergistically with vitamin E and vitamin C to prevent free radicals (-O2, -OH, -ROO, etc.) from causing oxidative damage to lipids, proteins and nucleic acids, causing damage to normal cells and tissues in the body, leading to a decline in immunity and the development of diseases. Secondly, tannins can exert antiviral and anti-tumor effects by regulating the cell cycle and the expression of inflammatory cytokines through the regulation of NF-κB and other pathways, thus enhancing the immune effect of the organism. It was found that the addition of tannins to the feed of small carp resulted in increased levels of leukocytes, plasma proteins, albumin and globulin in the blood of small carp and decreased levels of hemoglobin [28]. This suggests that tannins can stimulate an immune response in the animal organism. Tannin compounds extracted from the leaves of the wild peony family Phytophthora were able to enhance the ability of the intestinal mucosa to resist aggressive factors, and NO and nonprotein S H-based compounds were observed to be involved in the prevention and mitigation of intestinal ulcers [29]. Malik et al. also found that tannins extracted from oleaginous seeds have antidiarrheal and antispasmodic effects, and the regulatory mechanism may be through the dual blocking effect of Ca2+ channels and alkaloid receptors [30], suggesting that tannins have a certain immune-promoting effect on animal organism.

3.3 Effects on nitrogen metabolism in the body

In ruminants, due to the fermentation of rumen microbes, most of the protein in the feed reaches the rumen and is broken down into oligopeptides, amino acids, ammonia, etc., with only a small portion passing through the rumen and not being degraded. However, a large amount of degraded produced ammonia (20% to 35%) is absorbed in the rumen wall and excreted with the urine, resulting in a large protein loss [31]. Therefore, it is particularly important to protect the protein and increase the utilization of the protein by increasing the amount of perovskite. Numerous reports have shown that tannins block protein degradation in the rumen and increase peri-ruminal protein, which in turn increases protein utilization [32].

Tannins form insoluble complexes with proteins mainly through hydrogen bonding, but tannin-protein complexes are readily separated in the wrinkled stomach (pH<3.5) or intestine (pH>7), thus releasing proteins [33], which are somewhat protective against rumen protein. It has been reported that moderate amounts of condensed tannins can reduce the rate of degradation of rumen feed protein and improve the posterior rumen protein supply to increase protein retention [34]. Zhang Xiaoqing (2005) made similar findings through his study of red beangrass condensed tannins and also facilitated the maintenance of stability of dynamic changes in the concentration of total nitrogen, ammonia nitrogen and urea nitrogen and protein nitrogen in rumen fluid [35]. M i n et al. reported that when sheep were switched from perennial rye grass and white clover to c o m iculatus (32 g/kg of condensed tannins in dry matter) as the main diet, significant decreases in protein-degrading bacteria such as Clostridium proteoelastieum B316, Eubacteriumsp C12b, Streptoeoee us bovis B315 and Butyrivibrio fibrisolvens C2lla were observed in lmL rumen proteolytic bacteria [36]. Similar results were obtained by Yingliang Yuan et al [37]. Thus, tannins act as a proteoprotective agent and have a good boost in rumen protein metabolism. However, its study of nitrogen metabolism in non-ruminants requires further research.

3.4 Other

It has been reported that ewes fed L.corniculatus have increased ovulation rates by 22% compared to those fed rye grass and alfalfa. Xin et al. (2011) showed that tannins increased plasma testosterone and estradiol levels in plateau rabbits and root voles, but had no significant effect on hypothalamic gonadotropin-releasing hormone levels [38]. This indicates that tannic acid has a positive effect on the sexual maturity, ovulation rate and reproductive hormone levels of animals, and has a positive effect on the reproductive performance of animals, but the mechanism is still unclear.

4 Outlook

Tannins and other secondary metabolites are second only to cellulose, lignin, etc. in nature, but due to their anti-nutritional effects, many resources are unexploited and a lot of feed resources are wasted. Due to its unique biological activities such as antioxidant, antiviral, scavenging free radicals and anti-mutation, it has great application prospects in animal growth, immunity and reproduction and has become a research hotspot in agriculture, medicine and chemistry in recent years. Further study of the effects of tannins on animals and their mechanism of action, and make full use of tannin-containing feed resources, has great production and application value.

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单宁的生物学活性及其在动物生产中的应用进展