Butyric acid is a short-chain fatty acid present in the intestinal tract that provides energy to intestinal epithelial cells and promotes their growth and development. Recent studies have shown that butyric acid also has strong bactericidal effects (Kwan et al., 1998), can affect protein synthesis (Kruh et al., 1982), gene expression (Miller et al., 1998) and cell differentiation (Pouillart et al., 1992), among other effects. However, the liquid form and volatility of butyric acid, as well as its peculiar olfactory odor, render it unfavorable for practical production applications. Glyceryl tributyrate, as a precursor of butyric acid, is easy to use, safe and non-toxic, without odor, which not only solves the characteristics of butyric acid being liquid and easy to volatilize and difficult to be added, but also improves the disadvantage of the direct use of butyric acid with an unpleasant odor, and it has the ability to promote healthy development of the intestinal tracts of the livestock and poultry, enhance the immune ability of the body, and promote the digestion and absorption of nutrients and thus improve the performance of the animals’ production, and it is the better nutritious additive product at present It is a better nutritional additive product at present.
1 Physical and chemical properties of glyceryl tributyrate
Glyceryl tributyrate (Tributyrin) https://calidbio.com/product/caltrin-p/naturally exists in milk fat, composed of three molecules of butyric acid and one molecule of glycerol, chemical formula for C15H26O6, for the white almost oily liquid, almost no odor or slightly fat aroma. It is easily soluble in organic solvents such as ethanol and ether, and extremely difficult to dissolve in water. Zhan Yan et al. (2007) reviewed that glyceryl tributyrate by oral administration, plasma butyric acid concentration of up to 0.34mM, and can maintain greater than 0.1mM concentration of at least 0.5 ~ 4h, drug half-life of 40min.
2 Functions of glyceryl tributyrate
2.1 Energy supply for intestinal mucosal cells
Glyceryl tributyrate belongs to short-chain fatty acid esters, which are decomposed into butyric acid, glycerol monoester of butyric acid and glycerol under the action of intestinal lipase. Butyric acid is absorbed by the intestinal mucosal cells mainly in a non-ionic diffuse form and then directly transported to the liver through the portal venous system for further metabolism or used as energy consumption by the intestinal mucosal epithelial cells. Butyric acid in intestinal epithelial cells is converted to butyric acid coenzyme A by butyric acid coenzyme A synthetase, which does not need to bind to the carnitine molecule but permeates directly through the inner mitochondrial membrane and then rapidly generates acetyl coenzyme A for energy metabolism through a series of reactions such as the hydroxymethylglutaryl coenzyme A cycle. Butyric acid is the main respiratory fuel for colonic epithelial cells, and in vitro tests have shown that the colon preferentially burns butyric acid to supply energy to epithelial cells, followed by glucose, glutamine and other nutrients (Roediger WE, 1982), and that butyric acid provides more than 70% of the energy utilized by the colonic epithelial cells (Lijie YANG, 2006; Shuangming YUE et al., 2007).
2.2 Improvement of small intestine morphology and structure
The small intestine is the most important digestive site and the most important absorption site in the digestive tract. The functional unit of the small intestine is the villi, and the integrity of the morphological structure of the small intestine is usually indicated by the height of the intestinal villi as well as the depth of the crypts. Glycerol tributyrate has the effect of increasing the height of intestinal villi, decreasing the depth of crypts, and increasing the ratio of intestinal villus height to crypt depth in animals.
All experiments have shown that butyric acid significantly promotes the development of epithelial cells (Roediger W.H, 1980). The study of J. Hu (2004) showed that glyceryl tributyrate was beneficial in maintaining the intestinal mucosal morphology, and compared with the control group, the chorionic villus height of piglets supplemented with glyceryl tributyrate was increased by 9.46%, the crypt depth was significantly decreased by 36.12% (P<0.05), and the ratio of chorionic villus height to crypt depth was significantly increased by 73.16%.
2.3 Maintaining the cellular integrity of intestinal mucosa
The small intestinal mucosa consists of epithelium, lamina propria and mucosal muscular layer. The intestinal mucosa is mainly composed of epithelial cells and cup-shaped cells dispersed between them, with the role of digesting and decomposing food and transferring nutrients. Therefore, the structural integrity of the mucosal epithelial cells is a guarantee for the execution of its digestive and absorptive functions (Prince et al., 2003).Sakata et al. (1987) concluded that butyric acid is the most effective of the short-chain fatty acids in promoting the proliferation of intestinal epithelial cells. The mechanism is that butyric acid activates mRNA protein synthesis in murine intestinal mucosal cells, leading to proliferation of intestinal villous cells and shallowing of crypts.R.E. Davis (1930) added excess glycerol tributyrate to turkey feed and found that there was an increase in the secretion of ketone bodies in the organism. Whereas ketone bodies and glutamine are the main energy-supplying substrates of the small intestine, they have a strong growth-promoting effect on the intestinal mucosa. Butyric acid can induce the secretion of trichostatin, which helps to improve the viscoelastic properties of the mucosal layer, reduces the replenishment of inflammatory cells, and participates in the maintenance and repair of the intestinal mucosa (Danfeng Xu et al., 2009).
2.4 Promoting water and sodium absorption
Cumming JH et al. (1995) concluded that butyric acid in the intestinal tract of animals can reduce the incidence of diarrhea in animals by stimulating the absorption of Na+ through Na+-H+ exchange and enhancing the absorption of intestinal water. Especially when there is no sufficient amount of carbohydrates in the colon, it will cause a decrease in the production of short-chain fatty acids, producing low sodium absorption of water in the colon, which can easily lead to diarrhea (Xu Qin, 1999).
2.5 Regulation of digestive flora
The digestive flora has a very complex effect on the morphology and structure of the gastrointestinal tract and the function of digestion and absorption. Under normal circumstances, the beneficial flora of the gastrointestinal tract of animals is absolutely dominant, which can inhibit the attachment and proliferation of harmful bacteria such as Escherichia coli in the intestinal tract, which is favorable to the physiological function of the intestinal tract.
The mechanism of regulation of intestinal flora by glycerol tributyrate is rarely reported, and it is generally believed that its decomposition of butyric acid and glycerol monobutyrate can inhibit the growth of pathogenic microorganisms such as Salmonella, Escherichia coli, and Clostridium difficile and increase the number of beneficial bacteria such as Lactobacillus.
The bactericidal mechanism of butyric acid is generally attributed to the action of organic acids, mainly in regulating the pH value in bacteria, disrupting cell membranes, and inhibiting the basal metabolic functions of bacteria (S Brul et al., 1999). It has been suggested that the carbon chain length, degree of saturation (Russell et al., 1992) and dissociation state (KA Presser et al., 1997) of organic acids, and the pH environment of the gastrointestinal tract (Nuria Canibe et al., 2002) have a greater influence on the bactericidal effect of organic acids. However, production practice has shown that the addition of high doses of organic acids to diets can reduce intestinal pH and kill harmful bacteria.
2.6 Suppression of intestinal inflammation
There are more studies on glyceryl butyrate for intestinal inflammatory diseases in human medicine, especially for the treatment of human colon cancer there is a large amount of experimental data. Studies have shown that butyric acid produced from the breakdown of glyceryl butyrate can act as an anti-inflammatory agent by affecting the expression of leukocyte adhesion molecules (Menzel T et al., 2004), and can also reduce local inflammation by inhibiting NF-κB and blocking IL-8 secretion. Medical research data illustrate the inhibition and better efficacy of glyceryl butyrate on intestinal inflammation, which is an important reason for choosing glyceryl tributyrate in pig feed production.
3 Application of glyceryl tributyrate in pig production
Regarding the application of glyceryl tributyrate in pig production, some enterprises have done more exploratory experiments according to the grease characteristics of glyceryl tributyrate, emulsification performance, regulation of the intestinal tract, such as adding 1~2kg 45% of glyceryl tributyrate to reduce the grease in the diet by 1~2%, and replacing whey powder by 2kg of 45% of glyceryl tributyrate, 2kg of acidifier and 16kg of glucose in equal amount. Replacement of whey powder and so on have achieved successful experience, but at present the domestic and foreign reports on this aspect of the literature is relatively small, mainly focusing on the improvement of intestinal function, the replacement of antibiotics and the effect of the complex of lactitol and prebiotics, and other aspects.
Hu J (2004) conducted a 4-week trial of adding glyceryl tributyrate to 21±1d weaned piglets. The results showed that in the second and third weeks of the trial, the average daily weight gain of glycerol tributyrate group was increased by 15.17% and 16.53% respectively; in the first, second, third and fourth weeks of the trial, the feed-to-meat ratio of glycerol tributyrate group was reduced by 13.11%, 9.34%, 8.54% and 10.96% respectively; and the diarrhea rate of glycerol tributyrate group was reduced by 40.99%, 40.99% and 40.99% in the first, second and third weeks of the trial, respectively. were reduced by 40.99%, 9.33%, and 20.59, respectively.Similar results were obtained in the study of Wang et al. (2009) as described above.
In an experiment with piglets weaned at 18 days of age, Hou et al. (2006) concluded that the addition of 5 g/kg of glycerol tributyrate was more effective than 10 g/kg of glutamine in improving intestinal morphology and structure, increasing the activity of intestinal disaccharidases, and decreasing feed consumption, and that the best results were obtained with a complex of 5 g/kg of glycerol tributyrate and 3 g/kg of lactitol.
Liu Tong et al. (2011) showed that the addition of more than 0.1% glyceryl butyrate to the diet of weaned piglets at the age of 40 days could increase the daily weight gain of weaned piglets up to 16.69%, reduce the feed-to-meat ratio up to 13.67%, and increase the feed remuneration, and improve the apparent digestibility of crude protein, crude fat, crude fiber and energy of the feed for weaned piglets.
The use of liquid glycerol tributyrate drinking water (20 L/T water for small pigs and 1.5 L/T water for medium and large pigs) instead of zinc oxide and mucilage bacillus has been reported to reduce the feed-to-weight ratio by 6.43%, mortality by 40% and the cost of gain by 2.14% on the Centeno farm in Spain. In another Santo farm in Spain, starting from 30 kg piglet rearing, the drug usage of tylosin and lincomycin was reduced by 58% in cost savings by drinking water containing 0.2% glycerol tributyrate (for small pigs) and 0.1% glycerol tributyrate (for medium and large pigs). More interestingly, at Granja Baeza en Vallelado farm, weaned piglets at 23 days of age reduced mortality by 15% and feed-to-weight ratio by 22% and increased daily weight gain by 5.2% compared to the addition of 3.2 kg/t of zinc oxide to the feed by drinking water from 1-17 days of age and feeding glyceryl butyrate from 18-32 days of age.
Glyceryl tributyrate has the functions of promoting the development of small intestinal villi, supplying energy to the intestinal mucosa rapidly, regulating intestinal micro-ecological balance, and inhibiting enteritis, which is being gradually applied to feed. However, the application of glyceryl tributyrate in China is still less reported, and there are still many problems that have not been widely recognized. The mechanism of action of glyceryl tributyrate on intestinal mucosa, the immunomodulatory ability of glyceryl tributyrate on the body, and the inhibitory ability of glyceryl tributyrate on inflammation are still to be further studied.