Dietary carbohydrates induce a greater protein sparing effect than lipids in golden pomfret (Trachinotus ovatus) by modifying hepatic metabolic patterns

In recent years, there has been a growing imperative to identify an economical alternative to protein, which is the primary energy source for aquatic commercial fish, owing to the cost of it has boomed continually. As is widely acknowledged, carbohydrate and lipid are two of the most cost-effective substitutes for protein in aquaculture due to their relatively low cost. However, which of them possess a better protein-sparing effect in low-protein diets for Trachinotus ovatus and the precise mechanism behind this difference remains elusive. An 8-week feeding trial was performed on 200 fish in floating cages for seawater to investigate whether carbohydrate or lipid offers a superior protein-sparing alternative in low-protein diets, alongside improved growth performance, antioxidant capacity, glycolipid metabolism, and protein synthesis in T. ovatus. The study encompassed one commercial feed diet (CF) and four low-protein diets: LP (a low protein diet with 36 % crude protein), HC (a high-carbohydrate diet containing 9 % cassava starch), HL (a high-lipid diet with 13 % crude lipid), and HCL (a high-carbohydrate-lipid diet incorporating 5 % cassava starch and 10.8 % crude lipid). The findings revealed that fish fed high carbohydrate diets exhibited significantly higher growth performance, feed utilization, and whole-body crude protein levels, along with a lower feed conversion ratio compared to those fed high lipid diets(HL and HCL groups). Furthermore, the findings pertaining to antioxidative enzyme activities and gene expression suggested that diets with high carbohydrate levels confer superior antioxidant capacity to the liver compared to diets low in protein. Conversely, diets high in lipid content may lead to excessive oxidative stress and inhibit catalase activity. More importantly, the HC group exhibited a substantial upregulation in the hepatic expression of gk, pepck, srebp1 and hsl, suggesting an activation of hepatic glucose and lipid catabolism. Conversely, the expression of g-6-p, igf, glp-1, fas, acc, agpat3, elovel5, pparα, lpl and cpt1 were markedly downregulated, implying a possible attenuation in anabolic processes. This pattern indicates that hepatic glucose and lipid metabolism may be skewed towards catabolism and away from anabolism in the HC group. Besides, the expression patterns of protein synthesis in muscle tissue suggest that a high carbohydrate diet inhibits both the PI3K-AKT and mTOR-S6K signaling pathways, potentially leading to the suppression of muscle growth. In conclusion, a suitable addition of carbohydrate in the diet of T. ovatus elicits a more pronounced protein-sparing effect compared to lipids, achieved through the modulation of hepatic metabolic patterns.