Effects Of Dietary Fiber On The Growth Performance,Digestive And Absorptive Capacities,Intestinal Health,As Well As The Underlying Mechanisms In On-growing Grass Carp (Ctenopharyngodon Idella)

The research investigated the impact of different dietary fiber levels on growth performance,digestive and absorptive capacities,intestinal immune function and physical barrier?antioxidant capacity,tight junction,apoptosis?,as well as the related potential mechanisms in on-growing grass carp,and finally determined the dietary fiber requirements of on-growing grass carp?Ctenopharyngodon idella?.540 healthy grass carp?initial weight 123.00±0.70 g?were randomly divided into 6 treatments with 3 replicates of each,and respectively fed six semi-homogenous diets containing graded levels of dietary fiber?neutral detergent fiber NDF was 9.78,11.90,14.02,15.94,18.12 and 20.16%?for 60 days to investigate the effects of dietary fiber on the digestive and absorbtive capacities,intestinal antioxidant capacity and tight junctions of grass carp.After the growth experiment,24 fishes from each treatment were chose and injected intraperitoneally with Aeromonas hydrophila for 6 days to investigate the effects of dietary fiber on the incidence of enteritis,intestinal immune function and cell apoptosis.In addition,540 healthy grass carp with a body weight?215.81±2.12g?were randomly divided into 6 treatments with 3 replicates of each and fed the test diet containing 0.5%Cr₂O₃ for 7 days to explore the effect of dietary fiber on the apparent digestibility of on-growing grass carp.The results shown that,compared with the low fiber group?NDF=9.78%?and the high fiber group?NDF=20.16%?,the optimal level of dietary fiber significantly increased feed intake,the apparent protein digestibility,feed efficiency,percent weight gain?PWG?and specific growth rate of on-growing grass carp?P<0.05?,thus improving growth performance of grass carp;meanwhile,optimal dietary fiber significantly increasing activities of digestive enzymes?such as trypsin?in hepatopancreas and intestines,and brush border enzymes?such as?-glutamyltransferase?in anterior,middle,and posterior intestine of on-growing grass carp?P<0.05?,thus promoted the digestion and absorption of nutrients in grass carp.Further studies found that the appropriate level of dietary fiber significantly reduced the enteritis incidence of grass carp?P<0.05?and improved intestinal health.It may be associated with the enhancement of intestinal physical barrier and immune function.Appropriate level of dietary fiber?1?significantly increased the gene expressions and activities of antioxidase?such as superoxide dismutase?and glutathione content through modulating Nrf2 signaling pathway,while reduced contents of reactive oxygen species in the intestine of on-growing grass carp?P<0.05?,thus alleviating the intestinal oxidative damage of grass carp;?2?significantly up-regulated the gene expressions of anti-apoptotic factors?Bcl-2,IAP and Mcl-1??P<0.05?,while down-regulated gene expressions of caspase 3,8,9,Apaf-1,FasL and Bax in the intestine of on-growing grass carp?P<0.05?,inhibiting the cell apoptosis;?3?significantly up-regulated the gene expressions of tight junction proteins zonula occludens 1?ZO-1?,occludin,claudin-f and claudin-11 in each intestinal segment of on-growing grass carp through down-regulating the gene expression of myosin light chain kinase?MLCK??P<0.05?,thus maintaining the intercellular structural integrity of intestine in grass carp;?4?significantly increased the activities of lysozyme and acid phosphatase,the contents of complement C3,C4 and IgM in the intestine of on-growing grass carp?P<0.05?,and up-regulated the gene expressions of Mucin2,antimicrobial peptide LEAP-2B and anti-inflammatory cytokine TGF-?1,down-regulated the gene expressions of pro-inflammatory cytokines?TNF-?,IL-1?,IL-12p40 and IFN-?2?through regulating NF-?B signaling pathway in on-growing grass carp,thus relieving the intestinal inflammatory and enhancing the intestinal mucosal immune function of grass carp.In summary,optimal dietary fiber?NDF?improved the enteritis resistance ability,and digestive and absorptive capacities of on-growing grass carp through enhancing the intestinal physical barrier and immune function,finally increasing fish growth.According to the present experiment,the requirement of ditery fiber?NDF?for on-growing grass carp?123-467g?was determined to be 14.39%and 16.42%based on PWG and enteritis morbidity respectively.