Effects of adding mTOR activators to low-fishmeal diets on growth, antioxidant capacity, and intestinal health of spotted seabass(Lateolabrax maculatus)

An eight week feeding trial was conducted to investigate the effects of activating the mammalian target of rapamycin (mTOR) signalling pathway on growth performance, antioxidant capacity, and intestinal health of spotted seabass after reducing dietary fishmeal content. The control group was fed a diet with 25% fishmeal content (FM25). The experimental groups were fed diets where 20% and 40% of the fishmeal was replaced with complex protein, referred to as FM20 and FM15, respectively. Additionally, 0.1% oligopeptide complex to activate the mTOR pathway, resulting in groups FM20s and FM15s, making a total of five isonitrogenous and isocaloric diets. The results showed that weight gain rate (WGR) and specific growth rate (SGR) in FM20 and FM15 groups were significantly reduced compared with the FM25 group. After mTOR activation, the growth performance in FM20s group was restored to the level of FM25 group, with significantly improved feed efficiency ratio (FER) and protein efficiency ratio (PER). The liver histological analysis revealed that reducing dietary fishmeal content significantly induced hepatic lipid deposition and impaired liver antioxidant function, as evidenced by significantly decreased total antioxidant capacity (T-AOC), superoxide dismutase (SOD), and glutathione S-transferase (GSH-ST) activities, accompanied by significantly elevated malondialdehyde (MDA) content. The mTOR activation significantly alleviated hepatic lipid deposition and restored liver antioxidant capacity in FM20s group. The intestinal histological analysis demonstrated that intestinal villus height and expression levels of barrier function-related genes (zo-1, claudin-b, occludin) were significantly reduced in FM15 group. The mTOR activation elevated villus height and gene expression in FM20s but not in FM15s. Furthermore, expression levels of pro-inflammatory cytokines (tnf-α, il-8, il-1β) in FM20s group were significantly decreased, while the expression levels of anti-inflammatory cytokines (il-4, il-10, tgf-β1) were significantly increased. In conclusion, activating mTOR at 20% dietary fishmeal level effectively alleviates growth inhibition, liver oxidative damage, and intestinal inflammation cause by reduced fishmeal content, providing a novel strategy for reducing fishmeal inclusion in aquafeeds.