Effects Of Dietary Carbohydrate To Protein Ratio On Growth,Body Composition,Peripheral Tissue Glucose Metabolism And Glucose Tolerance Of Tilapia

As the cheapest energy substance,carbohydrate has been widely added to fish feed to exert its protein saving effect.The carbohydrate utilization ability of fish is closely related to its feeding habits.The carbohydrate fish's starch utilization ability is much lower than that of omnivorous and herbivorous fish,but the molecular mechanism of the difference in starch utilization ability of fish with different feeding habits remains unclear.Therefore,in this experiment,the GIFT tilapia(Oreochromis niloticus)with strong starch utilization ability was taken as the research object.In practical feed and semi-purified feed,the adaptation mechanism of tilapia to high starch feed was explored.The potential mechanism for fish to use starch efficiently.1.Effects of dietary carbohydrate to protein ratio on growth,body composition,peripheral tissue glucose metabolism and glucose tolerance of tilapia.Prepare three isolipidic practical diets : P38S13(crude protein,38.2%;starch,13.1%),P34S23(crude protein,34.5%;starch,22.5%),P30S32(crude protein,30.5%;starch,32.3 %).Feed the juvenile tilapia with an initial body weight of approximately 22 g for 8 weeks.The results showed that weight gain,specific growth rate and food intake of tilapia significantly decreased,while protein efficiency and protein production value markedly increased,but the feed efficiency was not affected with the increase of dietary starch to nitrogen ratio.With the increasing dietary starch level,intraperitoneal fat ratio,whole-body lipid,liver lipid and muscle lipid content of tilapia increased significantly.Glycogen content of the liver and muscle also markedly increased with the increase of the starch to protein ratio.In the liver,the expression of glucose transporter 2(glut2),muscle type a phosphofructokinase(pfkma),glycogen synthase2(gys2),acetylcoenzyme A carboxylase a(acca)and fatty acyl synthase(fas)were up-regulated,but the expression of phosphoenolpyruvate carboxykinase2(pck2)and glucose-6-phosphatase a 2(g6pca2)were inhibited with increasing starch intake.In the white muscle,up-regulated mRNA levels of glut4,pfkma,pfkmb,gys1,accb and fas were also observed with the increase of starch to protein ratio.With increasing starch intake,plasma glucose level of tilapia significantly increased,but the clearance rate of acute glucose load were also markedly improved.Taken together,it was concluded that tilapia adapted well to high carbohydrate diets through promoting peripheral glucose storage and utilization thus enhanced glucose tolerance and spared dietary protein,although high carbohydrate intake could reduce its growth performance.2.Effects of dietary carbohydrate to protein ratio and metformin on tilapia growth,body composition and live glucose metabolismIn Trial 1,tilapia seemed to adapt well to practical feed starch levels from 13% to the upper limit of 32%.To further explore the potential effect of higher levels of starch intake on the mechanism of tilapia carbohydrate metabolism and the protective effect of the hypoglycemic drug metformin.In this experiment,corn starch was used as the carbohydrate source,and three groups of semi-purified P40S5(40.4% crude protein and 4.62% starch),P30S25(30.4% crude protein and 24.2% starch)and P20S50(19.9% crude protein and 48.6% starch)were prepared feed.In addition,the Metformin treatment P20S50 M was prepared,that is,0.33% metformin hydrochloride(50 mg/d /kg)was added to the feed of the P20S50.The results showed that the high glucose group(P20S50)significantly inhibited the growth of tilapia.The VSI,HSI,IPF,crude fat,plasma triglyceride and cholesterol levels of tilapia in the high glucose group(P20S50)were significantly higher than those in the P40S5 group(P<0.05).However,the increase of feed carbohydrate level had no significant effect on tilapia blood glucose level(P>0.05).0.33% Metformin significantly reduced the level of plasma glycosylation end products and bile acid content in the high glucose group(P<0.05).TUNEL method was used to detect liver tissue and found that high glucose induced apoptosis.At the molecular level,high glucose significantly promoted the expression of genes related to glycolysis(gck and pfkma),lipogenesis(acca and fas)in the liver of the fish(P<0.05);inhibited gluconeogenesis(g6pca1 and g6pca2)Gene expression(gys1)and amino acid metabolism(gpt1,got1 and got2)related gene expression(P<0.05).The addition of 0.33% metformin significantly inhibited the expression of gck,acca and fas in the liver of high glucose group(P<0.05).To sum up,high-carbohydrate feed significantly reduces the growth performance of GIFT tilapia(25% starch group is the best),causing fish body fat accumulation and even liver damage,but tilapia can increase the oxidation of starch in the body to a certain extent.Decompose and transform storage,adapt to the negative effects of high carbohydrate.Although the addition of 0.33% metformin can not change the growth inhibition of high glucose on fish,it still has a certain improvement effect on glucose and lipid metabolism and plasma indicators.The omnivorous tilapia adjusts the body's oxidative decomposition and conversion of glucose for storage,which alleviates a series of negative effects of high carbohydrate-nitrogen ratio feed on the body's growth and health.It also has a certain reference value to make good use of the protein saving effect of carbohydrate.