Mechanisms underlying differential utilization of carbohydrates from diverse structures and sources in Nile tilapia (Oreochromis niloticus): Insights from glycolipid metabolism, protein deposition and liver health

The proper structure and source of dietary carbohydrates are vital for farmed fish health, yet their metabolic responses to varied carbohydrates are not clear. To explore fish metabolic responses to varied carbohydrates, Nile tilapia was fed with five different carbohydrate diets for eight weeks. The results showed that polysaccharides significantly enhanced the growth performance of fish (P < 0.05). The fish fed with polysaccharides showed lower serum glucose, pyruvate and lactate, while their liver glucose uptake and catabolic capacity were significantly enhanced (P < 0.05). Furthermore, the PDK4-PDHE1α axis, which linked glycolysis to the TCA cycle, exhibited a stronger response to polysaccharides, particularly in the corn starch group. The total lipid of the whole fish, as well as the TG in the liver and serum, were significantly elevated in the fish fed with wheat and tapioca starch (P < 0.05). Additionally, the fish fed with wheat and tapioca starch exhibited a significant higher expression of PPARγ, DGAT and FAS (P < 0.05). The serum levels of AST and ALT, as well as the liver MDA content, significantly decreased in the fish fed with polysaccharides (P < 0.05). Meanwhile, the expression levels of pro-inflammatory (TNFα and IL12) and apoptosis (caspase8 and caspase9) -related genes in the liver were significantly downregulated, while the antioxidant capacity markedly increased (P < 0.05). The total protein of the whole fish, muscle protein content, and muscle fiber diameter of fish fed polysaccharides were significantly increased, particularly in the corn starch group (P < 0.05). Furthermore, the fish fed with polysaccharides exhibited stronger mTOR signaling response and higher protein synthesis capacity in the muscle, particularly in the corn starch group (P < 0.05). This study demonstrated that the PDK4-PDHE1α axis and mTOR exhibit a stronger response to polysaccharides, boosting carbohydrate breakdown and protein synthesis for growth. PPARγ responds more robustly to wheat and tapioca starch, facilitating the esterification of free fatty acids into TG, which reduced lipid toxicity and maintained liver health.