Starch to protein ratios in practical diets for genetically improved farmed Nile tilapia Oreochromis niloticus: Effects on growth, body composition, peripheral glucose metabolism and glucose tolerance

Abstract

The present study was performed to evaluate the effects of dietary starch to protein ratios on growth, feed utilization, body composition, peripheral glucose metabolism, and glucose tolerance of genetically improved farmed Nile tilapia (GIFT) Oreochromis niloticus. Three iso-lipidic (5.54%) practical diets were formulated with increased starch levels at the cost of protein provision, being designated as diets LSP (13.1% starch and 38.2% protein), MSP (22.5% starch and 34.5% protein), and HSP (32.3% starch and 30.5% protein). Two hundred and forty tilapia juveniles (initial mean body weight: 23 g/fish) were maintained in 250 L rectangular tanks at 20 fish per tank and were fed experimental diets to apparent satiation for 8 weeks. Approximately 24 h after the last feeding, total fish weight of each tank was measured, and the fish number was counted. Thirty-six fish per treatment (9 per tank) were sampled to analyze the body composition, blood biochemistry, biometric indices, glucose metabolic enzymatic activities, and gene expression of the liver and white muscle. Remaining fish (36 per treatment) were subjected to a glucose tolerance test. The results showed that the mean body weight of the LSP fish (145 g/fish) was higher than that of the other treatments (130–133 g/fish). Protein efficiency ratio and protein production value were improved in tilapia that were fed diets with increased starch level from 13.1% to 32.3%. The lipid percentages and glycogen concentrations in the liver and white muscle were elevated when dietary starch level was increased from 13.1% to 32.3%. Compared with the LSP fish, the expression of representative genes involved in glucose transport (glucose transporter 2, glut2), glycolysis (muscle type a of phosphofructokinase, pfkma), glycogenesis (glycogen synthase 2, gys2), and lipogenesis (acetyl-coenzyme A carboxylase α, accα; fatty acyl synthase, fas) were upregulated in the liver of the MSP fish. The mRNA levels of hepatic gluconeogenic phosphoenolpyruvate carboxykinase 2 (pck2) and glucose-6-phosphatase a2 were 0.68- and 0.37-fold lower in the HSP fish than in the LSP fish. In the white muscle, the mRNA levels of glut4, pfkma, pfkmb, gys1 and accβ were 1.59-, 1.66-, 1.55-, 1.38- and 2.01-fold higher in the HSP fish than in the LSP fish. Activity of PFK in the liver and the muscle along with hepatic PCK activity followed similar trends to the expression of their corresponding encoding genes. Though the MSP and HSP fish obtained higher basal plasma glucose levels than the LSP fish, they took less time to restore plasma glucose after a glucose load. Thus, it was concluded that tilapia metabolically adapted well to an increase in the dietary starch level from 13.1% to 32.3% by promoting peripheral glucose storage and utilization, subsequently enhancing glucose tolerance and improving protein retention, although elevated levels of starch reduced their growth.