| A trial was conducted to investigate the effects of dietary coconut oil(CO)replacing fish oil(FO)on the growth performance,fatty acid composition and lipid metabolism and lipid molecular composition and distribution of groupers(Epinephelus coioides).In this study,defatted fish meal,wheat gluten,soybean meal,gelatin and casein were used as protein sources,and FO,soybean lecithin and CO were used as lipid sources to prepare five iso-lipidic(12%crude lipid)and iso-nitrogenous(50%crude protein)diets with the levels of FO replacement by CO at 0,25%,50%,75%and 100%,recorded as FO,25CO,50CO,75CO and 100CO,respectively.A total of 375healthy juvenile groupers(initial average weight:22.35±0.01 g)were randomly distributed into15 blue polypropylene tanks(500 L/tank)with 25 fish per tank.Groups of triplicate tanks were hand fed one of the diets to apparent satiation twice daily(6:00 and 17:00)under a natural photoperiod across a feeding period of 56 days.The main research results are as follows:1.Growth performance and tissue biochemical componentsThe results showed that the growth performance of groupers was significantly affected by the replacement level of FO with CO.As the increasing level of CO replacing FO,the weight gain rate(WG,R~2=0.587,P=0.012),specific growth rate(R~2=0.630,P=0.007)and feed efficiency(R~2=0.725,P=0.001)showed an open downward parabola,while hepatopancrea somatic index(R~2=0.474,P=0.040)showed an open upward parabola,reaching the maximum and minimum values at the 25%CO substitution level,respectively.With the increase in CO replacement level,serum total cholesterol content firstly increased and then decreased(R~2=0.802,P=0.002),while the activity of serum alanine aminotransferase(R~2=0.878,P<0.001)and the content of high density lipoprotein cholesterol(R~2=0.786,P=0.001)increased linearly.However,condition factor,feed intake,survival rate,tissue approximation component,the contents of serum triglyceride and low density lipoprotein cholesterol,and the activity of aspartate aminotransferase were not significantly affected by CO replacement level(P>0.05).If WG was used as the evaluation index,the quadratic regression analysis between WG and CO substitution level showed that groupers grew best when the proportion of CO to FO was 29.39%.2.Tissue fatty acid composition,liver tissue morphology and expression of genes related to liver lipid metabolismThe tissue fatty acid composition reflects that feed fatty acid composition.The dietary replacement level of FO with CO significantly affected the fatty acid profiles in liver and muscle of groupers,while the changing trends of fatty acid profiles in liver and muscle of fish were basically the same.With the increase of CO replacement level,the saturated fatty acid(SFA)content in liver and muscle tissues significantly increased,while the monounsaturated fatty acid(MUFA)content,polyunsaturated fatty acid(PUFA)content and n-3/n-6PUFA ratio showed a linear decrease(P<0.001).Compared with the control group,the replacement of 75%and 100%FO with CO resulted in localized vacuoles and nuclear shift in liver cells.The expression of liver fat synthesis genes(fas,acc,g6pd andδ6fad)were significantly up-regulated,while the expression of lipolysis genes(hsl and atgl)and elongase of very long-chain fatty acids genes(elovl 4 and elovl5)were significantly down-regulated with the increase of CO replacement level(P<0.05).The expression of fatty acidsβ-oxidation(cpt 1)gene was up-regulated firstly and then down-regulated(P<0.05),with the increase of CO level.The above results indicated that feeding high-level CO reduced fatty acid utilization by activating liver fatty acid synthesis pathway and inhibiting the expression of related fatty acid metabolic enzymes and transcription factors in fatty acid decomposition pathway,causing increased liver fat deposition and hepatic steatosis.3.Molecular composition of liver lipids and distribution of fatty acids at different glycerol backbone of lipid moleculesIn order to investigate the regulatory mechanism of dietary different CO inclusion levels on liver lipid molecules of groupers,the liver lipid molecules in FO,25CO and 100CO groups were determined by non-targeted lipidomics.4519 lipid molecules were detected,belonging to 44subclasses in the liver.After principal component analysis(PCA)and orthogonal partial least squares discriminantion analysis(OPLS-DA)screening,a total of 114 differential lipid molecules(DLMs)were identified in the 25CO and FO comparison groups,including 15 significantly up-regulated and 89 significantly down-regulated,among which,the main DLMs were triglycerides(TGs,89%),diglycerides(DGs,2%),phosphatidylcholine(PCs,2%)and ceramide(Cers,2%).A total of 187 DLMs were screened out in the 25CO and 100CO comparison groups,where 78 were significantly down-regulated and 109 were significantly up-regulated,with the major DLMs being TGs(51%),DGs(23%),phosphatidylethanolamine(PEs,9%)and PCs(5%).A total of 264 DLMs were identified in the 100CO and FO comparison groups,where 71 were significantly up-regulated and 193 were significantly down-regulated,with the predominant DLMs being TGs(70%),DGs(14%),PCs(5%)and PEs(4%).Key fatty acids such as eicosapentaenoic acid(EPA)and docosahexaenoic acid(DHA)in TGs were generally higher than those in DGs,PEs and PCs.At the same time,the amount of EPA and DHA bound at sn-3 site in TGs was significantly higher than that at sn-1 and sn-2 sites(P<0.05).The SFAs and MUFAs in the liver were mainly bound to the sn-1 site of lipid molecules,while PUFAs were mainly bound to sn-2 and sn-3 sites.The amount of EPA and DHA bound at the same site decreased significantly with the increase of the proportion of CO replacing FO(P<0.05).The above results indicated that,with the increasing proportion of CO replacing FO,the main DLMs in the liver was composed of 4 subclasses of TGs,DGs,PEs and PCs.With the increasing proportion of CO replacing FO,the fatty acid composition and content in TGs,DGs,PEs and PCs changed significantly,and LC-PUFAs such as EPA and DHA are preferred to be stored in TGs. |
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