Underlying mechanisms of high-carbohydrate diet-induced growth inhibition and metabolic dysregulation in grass carp (Ctenopharyngodon idella)

Carbohydrate is a common energy substance in aquaculture feed. Most studies show that the ability of fish to use carbohydrate is limited. This experiment aims to explore the effects of different carbohydrate levels on the growth mechanism of medium-sized grass carp medium-sized ( Ctenopharyngodon idellus ). 450 medium-sized grass carp with an initial weight of 45 ± 0.12 g were randomly divided into five groups, with three replicate cages per group and 30 fish per replicate. They were fed five different levels of carbohydrate diet: Group A (28.84 %), Group B (33.74 %), Group C (38.64 %), Group D (43.54 %), and Group E (48.44 %), with adjusted inert filler (cellulose) levels to ensure that each group's diet was isonitrogenous for 8 weeks.The results shows that as carbohydrate levels increase, the relative expression levels of weight gain rate (WGR), specific growth rate (SGR), feed coefficient (FCR), protein conversion efficiency (PER), and growth hormone ( gh ) genes in the pituitary gland, as well as growth hormone receptor ( ghr ) and insulin-like growth factor 1/2 ( igf-1,2 ) genes in the liver, first increased and then decreased; The gene expression of somatostatin receptors 1,2,3,5 ( sstr1,2,3,5 ) in the pituitary gland and somatostatin precursors ( pss1,2,3 ) in the hypothalamus first decrease and then increase. At the metabolic level, the visceral body index (VSL), liver body index (HSI), intraperitoneal fat index (IPF), and liver crude fat content of grass carp increase, while serum triglycerides (TG), total cholesterol (T-CHO), and low-density lipoprotein cholesterol (LDL-C) significantly increase; The relative expression levels of genes involved in glycolysis and lipid metabolism ( pfk, acc, fas, hsl, cpt-1 ) in the liver gradually increase. Overall, the carbohydrate tolerance range of medium-sized grass carp is 34.47 % −40.41 %, and excessive tolerance can seriously affect their growth and liver health; Somatostatin (SS) in fish can regulate carbohydrate and glucose lipid metabolism, and its specific molecular mechanisms and signaling pathways need further in-depth research. • The tolerance level of medium-sized grass carp to carbohydrates should be between 34.47 % and 40.41 %. • Moderately elevated carbohydrate levels enhance hepatic glucose and lipid metabolism in grass carp, whereas their limited carbohydrate tolerance leads to hepatic steatosis and liver damage upon excessive intake, indirectly impairing their growth. • Excessive carbohydrates upregulate the expression of somatostatin precursor and receptor genes, thereby suppressing growth hormone secretion and its receptor gene expression in grass carp, which directly impairs their growth. • Low dose somatostatin (SS-14) may improve the negative effects caused by high sugar diet by regulating the sugar metabolism process in fish.