The Application of Bile Acids in Animal Nutrition Research

Bile acids are an important component of bile and play a crucial role in fat metabolism. In recent years, bile acids have become increasingly popular as feed additives in the animal husbandry industry. To better select and utilize bile acid products in production practices, this article introduces the composition, nutritional mechanisms, and how to effectively add and use bile acid products in feed.

1. Composition of Bile Acids

Bile acids are a series of steroid substances produced during the metabolism of cholesterol in animals. They are primarily excreted from the gallbladder with bile into the intestine and are acidic, hence referred to as bile acids. There are various types of bile acids, with fourteen distinct types isolated from different animal species.

The types and quantities of bile acids vary across animals, and their active components also differ significantly. For example, human bile acids mainly consist of cholic acid and chenodeoxycholic acid, while cattle bile acids are primarily cholic acid and deoxycholic acid. Chickens have very little chenodeoxycholic acid, and geese have the highest levels of chenodeoxycholic acid. In contrast, crustaceans like shrimp and crabs, which lack a gallbladder, do not secrete bile and thus do not have bile acids. Therefore, to use bile acids more effectively, it is essential to select appropriate bile acid products based on the animal species.

2. Mechanism of Action of Bile Acids

Bile acids have surfactant properties because of their amphipathic molecular structure: one end is alkyl and hydrophobic, while the other end contains hydroxyl and carboxyl groups, making it hydrophilic. Therefore, the most important function of bile acids in animals is the digestion of fats and fat-soluble substances (such as fat-soluble vitamins and cholesterol) in food.

During digestion, bile acids not only assist in the action of lipase but also enhance its activity. The fats in food are emulsified by bile acids and then digested by lipases. The digestion products are contained within bile acid micelles and absorbed by the intestinal villi in the small intestine. Lipase functions best at a pH of 8–9, while it is almost ineffective at pH 6–7. Since the pH at the beginning of the small intestine is 6–7, lipase itself does not work effectively there. However, when lipase combines with bile acids to form a complex, its properties change, allowing it to function at the pH of 6–7 in the small intestine. Additionally, during absorption, lipase not only performs a transport function but also increases the concentration of fats on the villi surface of the small intestine, promoting absorption.

Bile acids also enhance animal immunity and reduce the absorption of bacterial endotoxins. With the widespread use of antibiotics in animal farming, issues such as bacterial resistance and secondary contamination by antibiotics have received increasing attention. After antibiotics kill bacteria, they produce a large amount of endotoxins, which severely affect the health of animals, often causing acute liver nutritional deficiency syndrome that is difficult to prevent. A deficiency of bile acids can accelerate the absorption of endotoxins in the small intestine and cause severe gastric obstructions. If animals consume adequate amounts of deoxycholic acid, endotoxins can be effectively broken down, helping maintain the health of the animals.

Bile acids also serve as effective antibacterial agents. In the large intestine of animals, bile acids can inhibit the proliferation of harmful bacteria such as Escherichia coli, streptococci, and others. They also help prevent food from decaying and fermenting in the stomach, thus reducing the risk of bloating and abdominal distension.

Certain bile acids, such as dehydrocholic acid and ursodeoxycholic acid, can also stimulate liver cells to secrete large amounts of thin bile, increase bile volume, clear bile ducts, and eliminate bile stasis, thus promoting bile flow. These acids also have a certain effect in enhancing fat digestion and absorption.

3. Synthesis Pathways of Bile Acids

Bile acids are derived from cholesterol and undergo three stages:

1. Synthesis of Primary Bile Acids: Cholesterol undergoes a series of metabolic reactions with the help of various enzymes, ultimately forming conjugated primary bile acids (cholic acid and chenodeoxycholic acid).
2. Synthesis of Secondary Bile Acids: After primary bile acids enter the small intestine via bile and participate in the digestion and absorption of lipids, most are reabsorbed into the portal vein blood circulation unchanged. About one-quarter of conjugated primary bile acids in the jejunum, ileum, and upper colon undergo bacterial enzyme-catalyzed modifications to form secondary bile acids, such as deoxycholic acid, lithocholic acid, and ursodeoxycholic acid.
3. Conjugation of Bile Acids: Bile acids synthesized in liver cells are conjugated with glycine or taurine, forming conjugated bile acids, which are then secreted into the bile ducts.

For young animals, bile acid synthesis is insufficient due to the underdevelopment of their digestive systems, which affects their ability to absorb lipids in the feed. In contrast, crustaceans like shrimp and crabs, which lack the ability to synthesize cholesterol and bile acids, need to obtain these substances from their feed to support normal growth. 

4. Conclusion

Reasonably utilizing bile acids can effectively improve animals' absorption of fats, fat-soluble vitamins, and cholesterol. It can enhance animal immunity, reduce the absorption of bacterial endotoxins, promote feed intake, improve feed efficiency, and reduce breeding costs. Additionally, bile acids can increase bile secretion, maintain bile duct patency, eliminate bile stasis, and protect the liver, ultimately improving animal production performance.