Effects Of Glycerol Monolaurate On Lipid Metabolism,Intestinal Health And Flesh Quality Of Grass Carp Fed With High-Fat Diet

The use of high-fat diets(HFD)can save dietary protein and reduce feed expenses.However,prolonged HFD feeding can cause excessive lipid accumulation,induce liver and intestinal inflammation,damage the normal function of liver and intestine,and diminish the flesh quality of cultured fish,resulting in huge economic losses.Glycerol monolaurate(GML)is a medium-chain fatty acid glyceride synthesized by the esterification of lauric acid and glycerol.Recent research has revealed that GML has the functions of anti-bacteria,anti-inflammation,regulating lipid metabolism and flesh quality.However,the effects of GML application varies depending on the fish species,dietary fat levels,and GML doses.Whether GML can alleviate the adverse effects of HFD on grass carp has yet to be verified.Therefore,the purpose of this study was to investigate the effects of GML on lipid metabolism,intestinal health and flesh quality of grass carp fed with HFD,identify the appropriate doses of GML,and provide a reference for the application of GML in production.The main conclusions are as follows:Experiment 1: This study explores the improving impact of GML on lipid accumulation,liver and intestinal inflammation,and intestinal microbiota imbalance of grass carp caused by HFD.The regular diet and HFD containing 0,0.5,1,2,and 4 g/kg GML were used to feed grass carp for 11 weeks.The results showed that 1,2,and 4 g/kg GML could alleviate the increased triglyceride content in the liver caused by HFD,reduce the number of lipid droplets in the liver,and reduced the adipocyte size(P<0.05).GML significantly downregulated the expression of the lipid synthesis-related gene(acc1)in the liver,and 4g/kg GML markedly up-regulated the m RNA expression of lipolysis-related genes(atgl,hsl,and cpt1)in the liver,and 4 g/kg GML markedly up-regulated the m RNA expression of atgl and hsl in the intraperitoneal fat,thereby alleviating the lipid accumulation induced by HFD(P<0.05).In addition,GML inhibited the m RNA expression of pro-inflammatory factors(il-6),and 2 and 4 g/kg GML promoted the m RNA expression levels of anti-inflammatory factors(tgf-β1)in the liver,thus alleviating the inflammation of the liver caused by HFD(P<0.05).2 and 4 g/kg GML significantly inhibited the m RNA expression of pro-inflammatory factors(tlr4 and nf-κb p65),and 1,2 and 4 g/kg GML significantly up-regulated the m RNA expression levels of tight junction proteins(claudin-3 and occludin)in the intestine,thus alleviating the inflammation and injury of the intestine caused by HFD.Particularly,GML also normalized HFD-induced intestinal microbiota imbalance in grass carp,which was characterized by promoting the beneficial bacteria growth(e.g.,Cetobacterium)and inhibiting the harmful bacteria growth(e.g.,Erysipelatoclostridium)in the intestine.Experiment 2: This study explores the improving impact of GML on the flesh quality of grass carp fed with HFD.The results showed that 2 g/kg GML increased the crude protein and DHA content in the muscle,thereby enhancing the nutritional value of grass carp.Additionally,1 and 2 g/kg GML increased the crude protein level by reducing the expression of protein expression regulatory gene(4e-bp1)and up-regulating the expression of protein expression regulatory genes(akt1 and tor)(P<0.05).GML also reduced the fat content in the muscle by decreasing the expression of lipid synthesis-related genes(acc1)and increasing lipolysis-related genes(pparα)(P<0.05).In addition,GML significantly increased the hardness,springness,and chewiness of grass carp by promoting the expression of myogenic regulatory factors(myod and myog)and collagen synthesis gene(col1a1),promoting the formation of muscle fibers,reducing the diameter of muscle fibers,and increasing the collagen content and water holding capacity(P<0.05).Importantly,1,2 and 4 g/kg GML could increase the lauric acid content in the flesh of grass carp,alleviate the rapid proliferation of muscle spoilage bacteria caused by HFD,and significantly reduce the total volatile basic nitrogen(TVB-N)and malondialdehyde(MDA)content in the muscle(P<0.05),thereby prolonging the shelf life of grass carp.In summary,GML can alleviate HFD-induced lipid accumulation and inflammation,thereby promoting grass carp liver and intestine health.This positive effect may be due to the alleviation of GML in HFD-induced intestinal microbiota disorders.At the same time,GML can increase the content of flesh protein and DHA,promote the formation of muscle fibers and increase the content of flesh collagen,thereby improving the nutritional value and texture of grass carp.Interestingly,GML can increase the lauric acid content in muscle and inhibit the proliferation of spoilage bacteria in flesh,thereby prolonging the shelf-life of grass carp.