Effect of glycerol monolaurate on lipid metabolism 

and liver function of Lates calcarifer

Abstract: Lates calcarifer, known as blind mullet, is widely distributed from Ryukyu to the Indian Ocean. Because of its fast growth rate, no intermuscular spines, delicious taste, and high nutritional value, it is loved by farmers and consumers. It is one of the essential breeding fish species in the southern region of China. L. calcarifer belongs to fish of relatively strong vitality. It can adapt to water from freshwater to a salinity of 56 ‰, belonging to euryhaline fish. At present, it can be cultured in both seawater and freshwater. L. calcarifer commands a high price in the market for its delicious meat. It can be fed with artificial feed or lowcost fish. In 2021, the total production of Lates calcarifer in Guangdong Province was less than 50 000 t, accounting for more than 80% of the country’s total production, and the total production of it in Zhuhai accounted for more than 40 % of the country, where it has formed a more complete production, processing and sales industry chain.

Glycerol monolaurate (GML), also known as monolaurate glycerol, is a mediumchain fatty acid ester extracted from natural plants. It is a compound formed by lauric acid and glycerol, which can emulsify, preserve, and inhibit bacterial growth in the digestive tract. It is a food emulsifier with emulsifying and preservative functions and is widely used in food and cosmetic industries. The shape is generally oily or finely granular crystals, white or pale yellow. Studies have shown that mediumchain fatty acid monoglycerides (MCFA) with their short carbon chains are rapidly absorbed and undergo rapid oxidative metabolism in the animal to provide the body with energy more quickly. The mechanism is that GML as a mediumchain fatty acid ester can easily cross the intestinal mucosa and enter the blood and liver. Studies have shown that MCFA can not only assist in the treatment of lipid absorption disorders and malabsorption syndromes but also have a beneficial effect on gastroenteritis and diabetes. It is believed that GML could affect the body’s lipid metabolism by promoting the oxidation of fatty acids in the liver and reducing body fat by consuming adipose tissue. Unlike longchain fatty acids, mediumchain fatty acids such as lauric acid can significantly reduce body weight and body fat content in mice. In addition, GML can improve the absorption of nutrients in farmed fish, accelerate growth, increase feed conversion rate, and improve body damage. Highdensity fishing and massive use of highfat, highenergy feed make the water eutrophication serious, and farmed fish generally have excessive fat accumulation and fish body odor problems, affecting their health, meat taste, and nutritional value.

In this study, the effects of glycerol monolaurate on growth performance, serum immune parameters, metabolismrelated enzyme activities, and intestinal digestive enzyme activities were analyzed by feeding different doses (0 g·kg-1, 1 g·kg-1, 2 g·kg-1, 4 g·kg-1) to artificially farmed L. calcarifer to improve commercial value, achieve quality and efficiency improvement and promote the healthy and sustainable development of the industry. We fed 200 L. calcarifer  with body weight (442.727±54.423) g, body length(28.438±1.493) cm by basal diet with GML at 0 g·kg-1 (G0 group, as the control group), 1 g·kg-1 (G1 group), 2 g·kg-1 (G2 group) and 4 g·kg-1 (G4 group) for eight weeks. Results showed that the addition of different levels of GML to the feed significantly reduced the ratio of viscerosomatic index (VSI), hepatosomatic index (HIS), and intraperitoneal fat ratio (IPF) in the fish compared to the control group. The ratio of VSI, HSI, and IPF was the ratio of wet weight of the organism’s organs to the unit body mass, and the change of its value reflected the change of fat content in body. The reduction in its value was therefore beneficial for reducing the accumulation of fat in the fish and improving health. Histomorphological analysis of liver showed a significant reduction in vacuolisation in GML group compared to the control group. The analysis of liver lipid metabolizing enzyme activities and changes in serum liver function related indicators also indicated that GML feeding could improve liver function. In this study, the activity of liver lipid metabolizing enzymes including hepaticlipase (HL), lipoprteinlipase (LPL) and HL+LPL were significantly higher in the test group than those in the control group, while serum triglyceride (TG), total cholesterol (TC), and lowdensity lipoprotein cholesterol (LDLC) were lower than those in the control group. So, GML could significantly improve lipid deposition in the body and alleviate liver damage, but the specific regulatory mechanism was still not completely clear and needed further research and analysis. The activity of glutamicpyruvic transaminase (ALT) within the serum of the test group was significantly higher than that of the control group, while globulin (GLB) content changed in the opposite direction, aspartate aminotransferase (AST) activity was not significantly different between the groups. In addition, results of liver antioxidant capacity showed that after 8 weeks of feeding GML, the activities of total antioxidant capacity (TAOC), superoxide dismutase (SOD), catalase (CAT), and glutathione (GSH) content in the livers of G2 group were significantly higher and malondialdehyde (MDA) concentration was significantly lower compared to the control group. It indicated that GML could effectively relieve the stress state of fish, reduce liver tissue damage and improve the antiinflammatory ability of L. calcarifer. The diet supplemented with 2 g·kg-1 of GML significantly increased the activity of amylase (AMS), lipase (LPS), and protease (PRS) in the intestine of L. calcarifer, indicating that GML can penetrate deep into the gastrointestinal interior under the protection of ester bonds, making it effective in promoting the digestive function of fish intestine. Conclusively, the findings suggested that GML supplementation contributed to improving lipid metabolism, digestive ability, immunity, liver tissue morphology, physiological function, and regulating the health of Lates calcarifer body, and the optimal GML supplementation level was 2 g·kg-1.

Keywords: glycerol monolaurate; Lates calcarifer; lipid metabolism; liver function