A comparison between high carbohydrate and high lipid diets reception on the growth, feed utilization and glucose homeostasis of genetically improved farmed tilapia Oreochromis niloticus

Abstract

This study was performed to compare the effects of high carbohydrate and high lipid diets reception on the growth, feed utilization and glucose homeostasis of genetically improved farmed tilapia Oreochromis niloticus. Three isonitrogenous (ca 30.2% protein) diets were formulated with different levels of starch and lipid: the control (CON, 24.2% starch and 6.57% lipid), high carbohydrate diet (HCD, 33.8% starch and 6.91% lipid), and high lipid diet (HLD, 24.0% starch and 16.5% lipid). Tilapia juveniles (initial mean body weight: 32.2 ± 0.2 g/fish) were randomly divided into 12 rectangular tanks (250 L) with 20 fish per tank, and fed the diets to apparent satiation for 8 weeks. The results showed that mean body weight was lower in the HLD fish (183 g/fish) than in the CON (212 g/fish) and HCD fish (206 g/fish). Feeding ratio and feed efficiency ratio were not affected by different treatments. Both dietary starch and lipid provision in excess exhibited poor protein-sparing effect, as protein efficiency ratio and protein productive value were not affected while whole-body protein percentages were lower in the HCD and HLD fish than in the CON fish. High lipid diets reception exerted a greater impact on proximate composition of the whole fish and intraperitoneal fat deposition, while high carbohydrate diets reception exerted a greater impact on liver size through promoting glycogen deposition. Plasma glucose level sharply increased in the HLD fish (7.17 mmol/L) as compared with the CON fish (4.66 mmol/L). Evidences from the molecular level revealed that hyperglycemia of the HLD fish was probably resulted from hepatic glucose output (represented by mRNA levels of glucose transporter 2 and glucose-6-phosphatase catalytic subunit a2) through stimulating gluconeogenesis (the expression of phosphoenolpyruvate carboxykinase 2) rather than glycogenolysis. In addition, high lipid diets reception suppressed hepatic de novo lipogenesis (mRNA level of fatty acid synthase (fas)) along with muscular glucose transport (mRNA level of glucose transporter 4) and glycogenesis (the expression of glycogen synthase 1), which aggravated the hyperglycemia of the HLD fish. In contrast, high carbohydrate diets reception did not affect the expression of representative genes involved with glycolysis and gluconeogenesis, but sharply upregulated the mRNA abundances of lipogenic acetyl-CoA carboxylase α and fas in the liver of tilapia. Except lipid, glycogen concentration also increased in the liver of the HCD fish as compared with the CON fish. Thus, plasma glucose slightly increased in the HCD fish (5.58 mmol/L) as compared with the CON fish. The overall results of this study suggested that tilapia possessed a better capability to tolerate high carbohydrate diets (33.8%) than high lipid diets (16.5%) based on growth performance and the maintenance of glucose homeostasis. This conclusion might have limitations, because the energy density of diets HCD and HLD was different. Further studies are still warranted to compare the glucose homeostasis of tilapia in response to high carbohydrate and high lipid diets reception on an isoenergetic basis.