| Global fish production constantly increases and aquaculture has become a vital contributor to the food supply.However,the rapid increase in the price of traditional protein sources in aquafeed components has led to a gradual increase in feed costs and limites the sustainable development of aquaculture.Currently,application of high level of carbohydrates was regarded as an efficient way to promote “protein sparing effects”in aquafeed,but it may inhibit the growth of aquatic animals,cause metabolic disorders and subsequently affect the quality of aquatic products.Nowadays,facing the high demand and requirements of consumers for aquatic products,how to improve the economic efficiency of aquaculture while ensuring the health status of aquatic animals and the quality of aquatic products is an urgent problem.As a class of beneficial substances,prebiotics have been widely used in aquaculture and have played an important role in growth promotion and immune function enhancement,but whether prebiotics can alleviate the metabolic disorders in fish caused by high-carbohydrate diet and affect the flesh quality remains unclear.In order to solve these questions,the present study used Nile tilapia（Oreochromis niloticus）as a research model to explore the effect of two prebiotics,Galacto-oligosaccharides（GOS）and Xylooligosaccharides（XOS）,on the growth,metabolism,flesh quality and intestinal microbiota of Nile tilapia fed with high-carbohydrate diet.In this study,Nile tilapia were divided into four dietary treatment groups for an 8-week nutritional trial,including control group（CK,30% carbohydrate diet）,high-carbohydrate group（HC,42%carbohydrate diet）,high-carbohydrate supplemented with GOS group（HCG,10g/Kg GOS）and high-carbohydrate supplemented with XOS group（HCX,10g/Kg XOS）.The results of the present study are as follows:1.Different types of probiotics have different effects on growth and metabolism of Nile tilapia fed with high-carbohydrate dietThe results showed that HC group and HCG group did not significantly affect the growth performance of Nile tilapia compared with CK group,while the addition of XOS to the high-carbohydrate diet（HCX group）significantly increased the weight gain rate and promoted the growth of tilapia.In addition,the high-carbohydrate diet significantly increased the viscerosomatic index（VAI）and the content of crude lipid,serum and liver triglyceride（TG）,serum total cholesterol（T-CHO）,and serum nonesterified fatty acid（NEFA）in tilapia compared with CK diet,exhibiting excessive lipid accumulation.At the same time,the gene expression analysis showed that the expression levels of lipid synthesis-related genes（accα and fas）were significantly increased.Compared with HC group,there was no significant change in lipid metabolism-related indicators in the HCG group,while the addition of XOS to the highcarbohydrate diet inhibited the expression of lipid synthesis-related genes（accα and fas）and activated β-oxidation-related genes（cptα）to alleviate the lipid metabolic disorders.The addition of XOS significantly reduced the VAI,crude lipid content,serum and liver TG content,serum T-CHO content,and serum NEFA content compared with the HC group.Thus,XOS could promote the growth performance of Nile tilapia fed with a high-carbohydrate diet and alleviate excessive lipid accumulation caused by highcarbohydrate diet,while GOS did not have a similar effect.2.Prebiotics can improve muscle flavor and nutritional value and alleviate muscle texture changes caused by high-carbohydrate dietThe regulation of flesh quality of aquatic products by prebiotics is also the main target of this study,and we evaluate the flesh quality of tilapia in terms of nutritional value and muscle texture.The results showed that the high-carbohydrate diet did not significantly affect muscle amino acid composition compared with the CK group,while the addition of GOS significantly increased the content of delicious amino acids（DAAs）,essential amino acids（EAAs）and total amino acids（TAAs）in muscle,showing an increase in muscle nutritional and flavor value.No significant change was observed with XOS-supplementation.In addition,the high-carbohydrate diet significantly reduced the content of docosahexaenoic acid（DHA）and polyunsaturated fatty acids（PUFA）in muscle compared with the CK group,but the addition of either prebiotic did not reverse this change.Analysis of muscle texture indicated that the high-carbohydrate diet changed the muscle texture,as evidenced by a decrease in hardness,adhesiveness and chewiness and an increase in springiness and cohesiveness compared with the CK diet,while both prebiotics alleviated the changes in muscle texture by modulating muscle fiber properties.Further analysis of molecular indicators showed that HC group inhibited muscle fiber growth by activating the expression of negative regulators of muscle growth（capn1 and foxo1）,whereas the two prebiotics activated the muscle growth and development regulator（myhc）as well as inhibited the negative regulators（capn1 and foxo1）.3.Prebiotics mitigate the adverse effects of high-carbohydrate diet on tilapia through intestinal microbiota and their metabolitesIt is commonly believed that prebiotics are utilized by the intestinal microbiota and have an impact on the microbiota composition.In order to investigate the mechanism of prebiotics in this study,we determined the intestinal microbiota composition of tilapia in different groups.The results showed that the intestinal microbiota composition of tilapia was significantly altered fed with different diets.In addition,the content of short-chain fatty acids（SCFAs）was altered.The content of propionic acid was significantly higher in the HCX group,and the content of butyric acid was significantly higher in the HCG and HCX groups.Correlation analysis showed a close relationship between the metabolic characteristics and the intestinal microbiota composition.Subsequent experiments in vitro confirmed that propionic acid alleviated the accumulation of lipid droplets in zebrafish hepatocytes and butyric acid activated the expression of genes related to muscle growth and development in tilapia primary myoblasts.Therefore,propionic acid is thought to be responsible for the reduced lipid accumulation in HCX group,while the ability of both prebiotics to restore muscle texture may be attributed to butyric acid.In this study,we investigated whether prebiotics could alleviate the adverse effects of high-carbohydrate diet in Nile tilapia,and systematically evaluated the effects of prebiotics on growth,metabolism and flesh quality of tilapia fed with highcarbohydrate diet,which provided valuable information for the application of prebiotics.This study innovatively demonstrated the role of two prebiotics,GOS and XOS,as aquafeed additives in regulating flesh quality,and provided valuable information for the “gut-muscle axis” of fish. |
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