Effects Of Dietary Carbohydrate And Lipid Levels On The Growth,Body Composition,Peripheral Glucose Metabolism And Glucose Tolerance Of Genetically Improved Farmed Tilapia Oreochromis Niloticus

In order to study the preference for glucose or lipid as dietary energy source in the omnivorous Nile tilapia Oreochromis niloticus,two feeding trials were conducted in this thesis:(1)effects of dietary lipid to starch ratios in practical diets on the growth,peripheral glucose metabolism and glucose homeostasis;(2)a comparison of the effects of high-carbohydrate and high-lipid diets reception on the growth,peripheral glucose metabolism and glucose homeostasis.Experiment 1: Effects of dietary lipid to starch ratios in practical diets on the growth,peripheral glucose metabolism and glucose homeostasis of O.niloticusThe present study was performed to ascertain whether glucose metabolism and glucose tolerance could be regulated by dietary lipid to starch ratios in the omnivorous O.niloticus.Three isonitrogenous(ca 34.5% protein)and isocaloric(ca 14.5 k J/g)practical diets were formulated,and being designated as diets L6S23(5.55% lipid and22.5% starch),L9S18(8.77% lipid and 18.1% starch)and L12S13(12.0% lipid and 13.8%starch).Genetically improved farmed tilapia(GIFT)juveniles of similar size(initial mean bodyweight: 23.0 g/fish)were allocated to 12 rectangular tanks(250 L)with 20 fish per tank,and were fed experimental diets to apparent satiation for 8 weeks.At the end of the trial,approximately 24 h after the last feeding,fish in each tank were bulk-weighed and counted.Nine fish per tank were sampled to analyze the whole-body composition(3 fish),biometric parameters and blood chemistry(3 fish)and the expression of glucose-metabolic genes,the enzyme activity and the glycogen content(3fish).Thirty-six of the remaining fish per treatment were subjected to an acute glucose tolerance test.The results showed that the weight gain,feed and protein efficiency ratios of tilapia were not affected by different treatments(P > 0.05).Intraperitoneal fat ratio and plasma glucose levels of the L9S18 and L12S13 fish were higher than those of the L6S23 fish(P < 0.05).In the liver,the m RNA levels of representative genes involved in glycolysis(glucokinase)and glucose release(glucose-6-phosphatase catalytic subunit a2)were simultaneously up-regulated in the L12S13 fish as compared with the L6S23 fish,indicating that a futile cycle between glucose andglucose-6-phosphate was associated with elevated dietary lipid to starch ratios.In the white muscle,the m RNA levels of glucose transporter 1a(glut1a),glut4,hexokinase 1b,phosphofructokinase muscle type a(pfkma),pfkmb and glycogen synthase 1(gys1)were 0.44-,0.71-,0.58-,0.51-,0.72-and 0.53-fold lower in the L12S13 fish than in the L6S23 fish,suggesting that muscular glucose transport and utilization were depressed with elevated dietary lipidto starch ratios.Although it took similar time(3 h)to restore plasma glucose for all treatments,plasma glucose was lower in the L6S23 fish than in the L12S13 fish during 1-3 h after the glucose injection(P < 0.05).Additionally,plasma glucose of the L9S18 and L12S13 fish decreased continually along the injection time(during 7-10 h)as compared with the L6S23 fish,which further proved that the regulatory mechanisms of glucose homeostasis were impaired in tilapia that were fed elevated lipid to starch ratios.Experimen 2: A comparasion of the effects of high-carbohydrate and high-fat diets reception on the growth,peripheral glucose metabolism and glucose homeostasis of O.niloticusIn this experiment,three isonitrogenous(ca 30.4% protein)diets were prepared.The control diet(CON)was formulated to contain 6.57% lipid and 24.2% starch.Compared with the CON diet,the high-carbohydrate(HCD,6.91% fat and 33.8% starch)and high-lipid diets were formulated with an increment of 10% starch and 10% lipid,respectively.GIFT juveniles of similar size(initial mean bodyweight: 32.2 g/fish)were allocated to 12 rectangular tanks(250 L)with 20 fish per tank,and were fed experimental diets to apparent satiation for 8 weeks.At the end of the trial,approximately 24 h after the last feeding,fish in each tank were bulk-weighed and counted.Nine fish per tank were sampled to analyze the whole-body composition(3fish),biometric parameters and blood chemistry(3 fish)and the expression of glucose-metabolic genes and the glycogen content(3 fish).There is no significant difference in feed utilization of different treatment groups(P > 0.05),but the growth performance of the HFD fish was significantly lower than those of the other treatments(P < 0.05).The intraperitoneal fat ratio and plasma bile acid and glucose levels of the HFD fish were higher than those of the CON and HCD fish(P < 0.05).There was no significant difference in plasma advanced glycation end products concentration among different treatments(P > 0.05).Compared with the CON and HCD fish,muscle glycogen concentration of the HFD fish was significantly lower(P < 0.05).Compared with the CON fish,the m RNA levels of the representative genes involved with hepatic glycolysis(gck,pfkma)and gluconeogenesis(g6pca2)were simultaneously up-regulated in the HFD fish(P < 0.05),indicating that a futile cycle between glucose and glucose-6-phosphate was associated with high level lipid diet reception in the liver.Compared with the CON fish,the m RNA levels of representative genes involved with glycolysis(pfkma)and lipogenesis(fas)in the white muscle of the HCD fish were significantly up-regulated(P < 0.05),while the m RNA levels of representative genes involved with glucose transport(glut4)and utilization(gys1)in the white muscle of the HFD fish were markedly down-regulated(P < 0.05).These results further proved that high-fat diets reception rather than high carbohydrate reception impaired the glucose homeostasis of the omnivorous tilapia.The above experiments showed that high-fat diets reception rather than high carbohydrate diets recption impaired the glucose homeostasis and glucose tolerance of tilapia whether the diets were isoenergetic or not.Taken together,it was speculated that tilapia prefered glucose rather than lipid as their energy source.