| Non-starch polysaccharide(NSPs)is one of the most important components of plant protein sources in animal feed.It has anti-oxidation,immune enhancement and intestinal flora improvement effects at low content,while it has significant adverse effect at high content.Studies in terrestrial animals have shown that NSPs can reduce the cholesterol level,affect nutrient digestion and absorption,and water-soluble NSPs(SNSPs)is often more resistant to nutrition than water-insoluble NSPs(INSPs).As the resources of animal protein such as fish meal are becoming more and more intense and the scale of aquaculture is expanding,the amount of plant protein source in aquatic animal feed is increasing.As a result,the NSPs content of aquatic feed increased gradually.Pectin and cellulose are two kinds of SNSPs and INSPs with obvious metabolic properties in plant protein sources commonly used in aquatic animals‘diet formulas,respectively.However,the adverse effects of high dose NSPs on the growth performance and physiology of aquatic animals have not yet attracted enough attention.Yellow catfish(pelteobagrus fulvidraco)is one of the most important freshwater fish in China.In recent years,with the continuous increase of market demand,the breeding area in China is expanding,the latest figures in the 2019 China Fisheries Statistics Yearbook shows that the national breeding volume of yellow catfish is as high as 509,000 tons.At the same time,there are more and more diseases,especially fatty liver,green liver,hepatobiliary swelling and other hepatobiliary diseases,high morbidity,high mortality,and for the unknown causes of the diseases,aquaculture has brought huge losses.In the present study,two different water-soluble NSPs,high-dose pectin and cellulose,were compared to evaluate their adverse effects on growth performance and physiological metabolism of yellow catfish.Besides,the dose and time effects of pectin on the physiological effects of yellow catfish were investigated.Morever,the possible mechanism and inducible pathways of pectin-induced bile acids(BAs)metabolism disorders in yellow catfish were revealed.This study can provide a new perspective for the nutrition evaluation of plant feed,provide reference for feed formulation optimization,and also provide a direction for improving the nutritional welfare of aquatic animals.The main findings are as follows:1.High dosages of pectin and cellulose cause different degrees of damage to the livers and intestines of yellow catfish,Pelteobagrus fulvidraco.The aim of this study was to investigate the nutritional stress effect of high dose NSPs on fish and to determine the difference of the intensity of anti-nutritional effect of different NSPs on fish.A feeding trial was conducted to investigate the physiological effects of high dietary pectin and cellulose,two NSPs with different water solubilities,on the yellow catfish,Pelteobagrus fulvidraco.A diet with 300 g kg-1dextrin was used as the control(CON diet).In test diets,dextrin was replaced with 300 g kg-1 pectin(PEC diet),cellulose(CEL diet),or a mixture of pectin and cellulose(1:1,MPC diet).Each diet was randomly assigned to triplicate groups of juvenile yellow catfish(5.80±0.20 g).Fish were fed twice daily to apparent satiation for 8 weeks.The growth performance,apparent digestibility coefficients,hepatosomatic index,serum concentration of triglycerides,total cholesterol,total protein,albumin,seroglobulin and the activities of alkaline phosphatase and cholinesterase were significantly decreased in fish fed the PEC diet compared with those in fish fed the CON diet(P<0.05).The liver showed obvious adipose degeneration and fibrosis,and the fold height and width of the hindgut significantly decreased(P<0.05).Fish fed the MPC diet had a similar but weak response,and fish fed the CEL diet had the weakest response.However,the concentration of total bile acid in the serum of fish fed the MPC diet was 74.5 times higher than that of fish fed the CON diet,while total bile acid in the serum of fish fed the PEC and CEL diets was 4.9 and 1.2 times higher,respectively,than in fish fed the CON diet.The green liver symptom occurred in 11.67%and 10.0%of fish fed the CEL and MPC diets,respectively,while the liver was pale in fish fed the PEC diet.These results indicated that high dose pectin and cellulose can cause different degrees of structural and functional damage to the liver and intestine of yellow catfish.However,the damage caused by pectin is more serious than that caused by cellulose,and the tissue damage caused by high dose NSPs may be related to the BAs metabolic disorder of yellow catfish.2.Short-term stress due to dietary pectin induces cholestasis,and chronic stress induces hepatic steatosis and fibrosis in yellow catfish,Pelteobagrus fulvidraco.According to the first study,the liver and intestinal tissue damage of yellow catfish was the most serious,but the total BAs level in serum was lower than that of cellulose diet,suggesting that the effect of high dose pectin on BAs metabolism may vary with the action time and dose.In this study,the dose-effect and time-effect of pectin were investigated,and the regular pattern of anti-nutritional effects were revealed.Pectin was included in diets at 0(FM),10 g kg-1(PEC1),50 g kg-1(PEC5),150 g kg-1(PEC15),and 300 g kg-1(PEC30),respectively.Fish with an initial mean weight of 9.65±0.08 g were fed the above diets for 56 days.Interval sampling was conducted at 2,7,14,28,and 56 days in fish fed the PEC30 and FM diets.The results showed that in fish fed PEC30 diet,green liver syndrome occurred after 2 days and peaked after 7 days,when the total bile acids(TBA)in the serum and liver were 80 and 3 times higher,respectively,than in fish fed the FM diet.The green liver syndrome was not observed after 28 and 56 days,and the color of the bile became light after 28 days and colorless after 56 days.The expression of LXRa,CYP7A1,and NTCP was upregulated,and CYP8B1 was downregulated in the liver after 7 days,and all these genes were expressed weakly and had much lower expression levels than in fish fed the FM diet after 56 days(P<0.05).Obvious diffuse adipose degeneration and fibrosis of hepatocytes were observed after 56 days.After 56 days of feeding,the total bilirubin,direct bilirubin,triglyceride,total cholesterol,total protein,albumin and seroglobulin were decreased with increasing dietary pectin.However,indirect bilirubin and TBA were the highest in fish fed the PEC15 and PEC5 diets,respectively.In addition,green liver syndrome and cholestasis were also observed in fish fed PEC5 and PEC15 diets for 56 days.These results suggested that the nutritional stress of pectin was dose-and time-dependent.Short-term stress induced hypersynthesis of bile acids and then cholestasis,and chronic stress induced hepatic steatosis and fibrosis in yellow catfish.Soluble NSPs are likely the cause of fish hepatobiliary syndrome.3.The exploration of the bile acids disordered mechanism induced by dietary pectin and its inducing pathway in yellow catfish,Pelteobagrus fulvidraco.Previous studies have proved that high dietary pectin can cause the disorder of bile acid metabolism in Pelteobagrus fulvidraco,but its potential induction mechanism is not clear.This study consists of three parts:reabsorption of BAs binding by pectin,alteration of intestinal flora structure and function by pectin,analysis of differential target metabolites of cholestatic bile acids and exploration of improvement measures to reveal the potential mechanism of pectin-induced BAs metabolism disorder in Pelteobagrus fulvidraco.The results showed that diet conten high dose pectin combined with intestinal bile acids and induced hyper-synthesis of hepatic BAs,and after the first meal,the concentration of intestinal BAs increased and the reabsorption of BAs decreased in the intestine.In addition,pectin changes the structure of intestinal flora and decreases the concentration of bile saline hydrolase,which could be another inducing pathway for bile acid metabolism disorder.After 7 days,dietary pectin can cause liver bile acid hypersynthesis,intestinal bile acid absorption enhanced,resulting in liver cholestasis,hydrophobic bile acid content ratio increased,by supplementing glycine cholic acid can effectively alleviate this adverse effect.These results indicate that pectin can cause bile acid metabolism disorder of yellow catfish by direct entrapment and alteration of intestinal flora structure and function.In conclusion,the scientific problem of this study stems from the actual production of aquaculture.Furthermore,this study is expected to reveal the mechanism of NSPs on enterohepatic circulation of BAs in fish,and will provide a new sight for the prevention and treatment of hepatobiliary syndrome in fish.In addition,the research results are expected to serve the production of aquaculture,and have a broad application prospects. 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