In recent years, antibiotics have been widely used in animal husbandry to prevent animal diseases and improve production performance. The use of antibiotics is an effective means to develop intensive and large-scale production. However, due to the excessive use of antibiotics, serious drug dependency has emerged, leading to bacterial resistance and antibiotic residues, which seriously threaten the health of humans and animals. Therefore, research on novel antibiotic alternatives has become a research hotspot in recent years. Developing green, efficient, and safe antibiotic alternatives is a primary task for the development of animal husbandry in China and globally. Tributyrin (TB) is an esterification product of butyric acid and glycerol, with a long half-life and no toxic side effects. It can be decomposed by pancreatic lipase in the animal intestine to produce 3 molecules of butyric acid and 1 molecule of glycerol, which are then transported by the blood to various tissues and organs in the body to exert their effects. Adding TB to the diet can improve animal growth performance, promote intestinal development, enhance immunity, and ensure the health of the animal body. This paper reviews the physicochemical properties, metabolic pathways, physiological functions, mechanisms of action, and applications of TB in animal production, aiming to provide theoretical references for the regulation of intestinal health and research in animal production using TB as a novel feed additive.
1. Characteristics and Metabolic Pathways of TB
1.1 Physicochemical Characteristics of TB
TB, also known as glycerol tributyrate, is the product of the esterification of 3 molecules of butyric acid and 1 molecule of glycerol. It is a typical butyric acid derivative, belonging to short-chain fatty acid esters, with the molecular formula C15H26O6 and a relative molecular mass of 302.36. The molecular structure is shown in Figure 1. TB is generally a colorless oily liquid with a slight fatty aroma and a slightly bitter taste. It is extremely difficult to dissolve in water but easily soluble in ethanol, chloroform, and ether, with a melting point of -75 degree and a boiling point of 305-310 degree , making its physicochemical properties relatively stable.
1.2 Metabolic Pathways of TB
TB, being a type of lipid, has metabolic pathways closely related to those of lipid metabolism and absorption mechanisms. During the digestion of lipids in the animal body, lipase has the strongest activity on triglycerides, followed by diglycerides, and the weakest on monoglycerides, especially 2-monoglycerides. In the process of fat absorption, the monoglyceride pathway plays a dominant role, with 2-monoglycerides being the main absorption route because pancreatic lipase specifically hydrolyzes the 1,3 positions of triglycerides to produce 2-monoglycerides and free fatty acids. Both 1-monoglycerides and 2-monoglycerides can be completely absorbed by intestinal epithelial cells and enter the lymph, with some entering the bloodstream to exert their effects. There are enzymes in the microsomes that further esterify monoglycerides into triglycerides, with a preference for using 2-monoglycerides.
TB is a typical short-chain fatty acid derivative. Since the content of lipase in the animal's mouth and stomach is very low and TB is not easily decomposed by gastric acid, its digestion and absorption mainly occur in the intestine. Through the action of pancreatic lipase and hindgut microorganisms in the intestine, TB is gradually and slowly decomposed into glycerol, butyric acid, and a very small amount of monobutyrin. The produced glycerol can permeate the cell membrane and enter the cell to participate in metabolism, while the absorption and transport of butyric acid mainly occur through two pathways: firstly, providing energy for intestinal cells. The intestine preferentially selects butyric acid to provide energy. After entering the cell, butyric acid is decomposed into acetyl-CoA through -oxidation, entering the tricarboxylic acid cycle to supply energy for the body, or forming ketone bodies in the liver, which are oxidized to supply energy for extrahepatic tissues. Secondly, synthesizing triglycerides within the body. When the body no longer needs fatty acids to provide energy, butyric acid, glycerol, and monobutyrin re-synthesize triglycerides stored in adipocytes.
2. Physiological Functions and Mechanisms of Action of TB
2.1 Energy Provision
Butyric acid is an important nutrient for intestinal cells, with colonic cells primarily relying on butyric acid as their energy source. TB, decomposed by pancreatic lipase in the intestine, produces butyric acid, providing energy for intestinal mucosal cells, with up to 75% of the energy for the intestine coming from butyric acid.
2.2 Regulation of Intestinal Barrier Function
2.3 Regulation of Intestinal Microbiota Balance
After entering the intestine, TB produces butyrate through the action of pancreatic lipase. Butyrate can reduce the acidity of the digestive tract, enter bacteria, and decompose to produce hydrogen ions and butyrate ions, leading to intracellular acidification. This causes a large number of acid-intolerant harmful bacteria such as Escherichia coli and Salmonella to die off, while acid-tolerant beneficial bacteria such as Lactobacillus and Bifidobacterium proliferate in large numbers, maintaining the balance of the intestinal microbiota in animals.
2.4 Regulation of Immune Function
TB has a certain regulatory effect on the immune system of animals, participating in the regulation of the body's immune system. TB decomposes in the intestine to produce butyrate, which exerts anti-inflammatory effects by inhibiting the activation of the nuclear transcription factor-κB (NF-κB) pathway, reducing the gene expression of pro-inflammatory cytokines such as tumor necrosis factor- (TNF- ), interleukins (IL) IL-1 , IL-2, IL-6, IL-8, and IL-12. TB can improve the immune function of animals through different pathways, enhancing the activity of immune substances, promoting the proliferation of regulatory T cells (Tregs), increasing the expression of anti-inflammatory cytokines, reducing the expression of pro-inflammatory cytokines, and enhancing antioxidant capacity to boost the immunity of animals.
2.5 Regulation of Metabolism
Numerous studies have shown that short-chain fatty acids play an important role in energy homeostasis and lipid metabolism by stimulating hormonal and neural signals in various tissues, regulating the metabolism of animals. TB, decomposed by pancreatic lipase in the intestine to produce butyrate, participates in the regulation of metabolic health in the body.
3 Application of TB in Animal Production
As a precursor of butyrate, TB is an excellent butyrate supplement with stable physicochemical properties, safe and non-toxic, addressing the issues of butyrate's unpleasant odor and volatility, and the difficulty of direct supplementation. It has broad application prospects in the field of animal nutrition. As a feed additive, TB can directly act on the digestive tract of animals, providing energy for the intestine, improving intestinal health, and regulating the growth performance and health status of animals.
3.1 Improving Growth Performance
Adding TB to feed has been widely applied in various animal production scenarios. Supplementing appropriate amounts of TB in the diet can increase the average daily gain of experimental animals, reduce the feed-to-gain ratio, and improve the growth performance of animals, with recommended addition rates ranging from 0.075% to 0.25%.
3.2 Improving Intestinal Health
TB can play a positive role in the health of the animal intestine by improving intestinal morphology, regulating the balance of intestinal microbiota, and enhancing intestinal barrier and antioxidant capacities. Adding TB can significantly increase the activity of serum glutathione peroxidase (GSH-Px), trypsin, and pepsin in grass carp, improving intestinal digestive and antioxidant capacities. As an energy source for the intestine, TB can effectively improve and repair intestinal morphology, enhance digestive and absorptive capacities, promote the proliferation of beneficial bacteria, improve the microbiota structure, alleviate oxidative stress responses, promote intestinal development, and ensure the health of the body.
3.3 Replacing Antibiotic Use
Currently, there are relatively few reports on replacing antibiotics with TB both domestically and internationally. Adding TB to the diet has similar growth performance effects to adding antibiotics in weaned piglets, and TB and antibiotics have synergistic effects. TB can stimulate the appetite of LPS-challenged piglets, alleviate intestinal damage and growth retardation by regulating inflammatory factors, expressing ileal fibroblast growth factor 19 (FGE19), and fermenting intestinal acetate. Long-term use of antibiotics can alter the structure of the intestinal microbiota, affecting the expression of anion exchangers, butyrate transport receptors, and tight junction proteins, leading to intestinal damage. Adding TB can alleviate antibiotic-induced intestinal damage and ensure intestinal health. However, current research on TB as an antibiotic replacement is still relatively limited, and further studies are needed for broader application.
4 Conclusion
(Excerpt from: Acta Animal Nutrition 2020, 32 (12): 5547-5555)
Anthony Liang- Hangzhou Well Sunshine Biotech Co.,LTD, May, 2024
