When the liver of an animal is damaged, it can have detrimental effects on health, feed intake and performance. This can be prevented by balanced diets and the addition of bile acids.
The liver, which is the largest organ in the animal body, has many important functions. It plays a significant role in the regulation of blood circulation and hormones, nutrient metabolism, secretion of bile, detoxification and immunity. Here we explain the role of the liver, how the liver can become damaged in farm animals and ways to support liver health.
For starters, the liver can be seen as the metabolic centre of nutrients, as it plays a role in storing glycogen, decomposing glycogen into glucose, gluconeogenesis, and converting glucose into lipids to maintain the balance of glucose metabolism in the body (Figure 1). The liver is also an important organ for lipid metabolism and the hub of fat transportation. It plays an important role in the processes of digestion, absorption, synthesis, decomposition and transportation. It is also one of the main organs for the synthesis of fatty acids, cholesterol and phospholipids in the body. Moreover, in the process of lipid biotransformation, the liver can produce many important biologically active substances, such as bile acids, corticosteroids, sterol hormones, etc. When fat metabolism is disordered, fat cannot be transported out in time and it accumulates in the liver, creating fatty liver syndrome. Amino acids perform protein synthesis, deamination, transamination and other functions in the liver. The synthesised protein enters the blood circulation for the needs of the body’s organs and tissues, which is of great significance for maintaining the body’s protein metabolism. At the same time, the liver is the organ that stores the most vitamins A, E, K, B1, B2, B6, B12, pantothenic acid and folic acid. Once the liver gets diseased, there will be symptoms such as those caused by a lack of vitamins and slow growth.
Liver cells produce bile acid, which is transported to the gall bladder through the bile duct. The gall bladder concentrates and discharges bile. Bile enters the duodenum through the bile duct, which not only promotes the digestion and absorption of fat in the small intestine, but also continuously passes the body’s metabolic waste and foreign bodies into the intestine and excretes foreign bodies. Therefore, bile is both a digestive and an excretory fluid.
The liver is the body’s main detoxification organ. It reduces mycotoxins, drugs, endotoxins, and metabolites in the body. It reduces the toxicity of drugs and endotoxins through combination, oxidation, reduction, and hydrolysis, and even breaks them down into non-toxic compounds which are excreted through urine or bile. When it comes to immunity, the liver also plays an important role. This is because the liver has the largest phagocytic system of reticuloendothelial cells in the animal body. The endothelial layer of the hepatic sinusoid contains a large number of Kupffer cells, which have a strong phagocytic ability. As much as 99% of the bacteria in the portal blood are swallowed when passing through the hepatic sinus. At the same time, the liver is also an important place for the synthesis of immunoglobulins (IgG, IgA, IgM) and complement (C3, C4), and plays a vital role in the non-specific immunity of animals.
Due to the fierce competition among feed companies, in order to obtain a higher growth rate and lower feed conversion ratio (FCR), feed generally contains high fat (high energy) levels. In addition, to shorten the time to market, farmers sometimes increase the amount of feed. In this case, the burden of the liver of the farmed animals is greatly exceeded, which easily leads to an accumulation of fat in the liver. This leads to enlargement of the liver and the occurrence of fatty liver diseases, such as fish liver and ‘gall-bladder syndrome, fatty liver of laying hens, etc. In addition, anti-nutritional factors (free gossypol, glucosinolates, tannins, soybean protease inhibitors), mycotoxins (aflatoxin B1), heavy metals (lead, mercury) and other toxic and harmful substances in the feed can also cause damage to the liver, which can lead to fungal liver and toxin liver. The addition of antibiotics can also cause secondary damage to the liver of farm animals, leading to drug-induced liver disease. Once the liver and gall’bladder function is damaged, feed intake and growth will be reduced. Damage to the liver will also cause insufficient bile secretion, so that animals are unable to fully digest and absorb dietary fat, resulting in reduced feed utilisation, increased FCR, and increased production costs. In more severe cases, the immune system is weakened and the risk of developing diseases and even mortality increases.
To protect liver health, bile acid is indispensable. It is the most direct, most rapidly absorbed, and most effective functional feed additive. Bile acid is a steroid substance synthesised by the liver using cholesterol. It participates in the process of fat digestion and absorption, reducing fatty liver and the excretion of toxins. At the same time, bile acids, as an important ligand molecule, participate in the regulation of key gene expression in animals. Bile acids can activate farnesoid X receptor α (FXR-α) in liver cells, by inhibiting the expression of SREBP-1c, thus inhibiting the synthesis of lipids in the liver, promoting the synthesis of very low density lipoprotein (VLDL), and accelerating the transportation of fat from the liver, hence reducing fatty liver. FXR-α regulates the enterohepatic circulation and the biosynthesis of bile acids by controlling the expression of small dimer partners (SHP; NR0B2), playing a role in down-regulating liver fatty acid and triglyceride biosynthesis.
Against the background of aiming for natural, safe and healthy breeding practices for farm animal, bile acids have been gradually recognised by feed mills and breeding companies as a valuable tool to protect liver health and reduce liver diseases in farm animals.