A short-term 2-week (2w) and long-term 8-week (8w) feeding trial was conducted to investigate the effects of low-starch (LS) and high-starch (HS) diets on the growth performance, metabolism and liver health of largemouth bass (Micropterus salmoides). Two isonitrogenous and isolipidic diets containing two levels of starch (LS, 9·06 %; HS, 13·56 %) were fed to largemouth bass. The results indicated that HS diet had no significant effects on specific growth rate during 2w, whereas significantly lowered specific growth rate at 8w. HS diet significantly increased hepatic glycolysis and gluconeogenesis at postprandial 24 h in 2w. The hepatosomatic index, plasma alkaline phosphatase, total bile acid (TBA) levels, and hepatic glycogen, TAG, total cholesterol, TBA, and NEFA contents were significantly increased in the HS group at 2w. Moreover, HS diet up-regulated fatty acid and TAG synthesis-related genes and down-regulated TAG hydrolysis andβ-oxidation-related genes. Therefore, the glucolipid metabolism disorders resulted in metabolic liver disease induced by HS diet at 2w. However, the up-regulation of bile acid synthesis, inflammation and energy metabolism-related genes in 2w indicated that largemouth bass was still in a state of ‘self-repair’ response. Interestingly, all the metabolic parameters were returned to homoeostasis, with up-regulation of intestinal glucose uptake and transport-related genes, even hepatic histopathological analysis showed no obvious abnormality in the HS group in 8w. In conclusion, HS feed induced short-term acute metabolic disorder, but long-term metabolic adaptation to HS diet was related to repairing metabolism disorders via improving inflammatory responses, bile acid synthesis and energy metabolism. These results strongly indicated that the largemouth bass owned certain adaptability to HS diet.

An 8-week feeding trial was conducted to investigate the effects of high-starch diets and the supplementation of an olive extract (OE) on the growth performance, liver health and lipid metabolism of largemouth bass (Micropterus salmoides). Four isonitrogenous and isolipidic diets were prepared: two basal diets containing low (9.0%) and high (14.4%) levels of starch (named as LS and HS), and 0.125% OE was supplemented to each basal diet (named LSOE and HSOE). The results show that high-starch diets had significant negative effects on growth performance, with lower FR, SGR and higher FCR, whereas OE significantly lowered FCR, determined by two-way ANOVA analysis. High-starch diets induced oxidative stress, inflammatory response and liver function injury, with significant increases in the content of plasmatic AKP, AST, ALT, hepatic SOD and MDA, and up-regulation of hepatic TNFα, IL1β, and TGFβ1 gene expression. In addition, a high-starch diet decreased the phosphorylation of AMPK and upregulated the expression of SREBP, together with higher hepatic liver lipid and HSI. The oxidative stress and lipid metabolism disorders indicate metabolic liver disease (MLD) of largemouth bass fed high-starch diets. Feeding on OE-supplemented diets increased the hepatic antioxidant capacity by decreasing the content of MDA and SOD. Fish fed the HSOE diet had an activated phosphorylation of JNK and decreased expression of pro-inflammatory IL1β compared with those fed the HS diet, which strongly indicated that the degree of inflammatory responses was reduced after OE supplementation. Interestingly, this study demonstrated that OE regulates hepatic lipid metabolism in fish by inhibiting the expression of hepatic lipogenesis genes (ACC1 and FASN) and promoting lipolysis (ATGL) and β-oxidation (CPT1α) to prevent TG accumulation. In conclusion, high-starch feed induced oxidative stress and lipid metabolic disorder of largemouth bass, while supplementation with OE improved its antioxidant capacity, anti-inflammatory responses and lipid metabolism. However, hepatic histopathological results suggested that OE supplementation could not completely repair the MLD caused by the high level of starch in largemouth bass.

A 10-week growth trial was conducted to investigate the effects of a phytogenic feed additive (PFA) containing olive by-products and green tea extracts supplemented to a reduced fishmeal/high soybean meal diet on the growth performance, hepatic antioxidant capacity, lipid metabolism, and liver health of largemouth bass (Micropterus salmoides). Three experimental diets were tested: (1) a control high fishmeal (40%) and low soybean meal (15.57%) diet (named HFM), (2) a reduced fishmeal (30%) and high soybean meal (30.97%) diet (named HSB), and (3) a HSB diet supplemented with the PFA at 500 mg/kg (named HSB+P). Each diet was assigned to four replicate tanks, each containing 30 largemouth bass (initial body weight, IBW = 48.33 ± 0.01 g). The results showed that increasing the soybean meal content in the diet did not negatively affect growth performance, whereas supplementation with PFA significantly increased weight gain and specific growth rate of largemouth bass compared to both HFM and HSB groups. Reducing fishmeal and increasing soybean meal in the diet caused oxidative stress with a higher content of ROS in the liver. However, the hepatic antioxidant capacity was enhanced, with reduced ROS and increased GSH-Px levels in the HSB+P group. Moreover, the decrease of plasma TG, LDL-C, and LDL-C/TC, and downregulation of lipogenesis and cholesterol synthesis gene expression in liver, indicated that supplementation with the PFA improved fish lipid metabolism. Protein retention efficiency was also significantly increased in largemouth bass fed the diet with PFA supplementation, which regulated (enhanced) AKT-mTOR phosphorylation. These results clearly indicated that a PFA containing olive by-product and green tea extracts can positively improve growth performance, protein retention efficiency, antioxidant capacity, and lipid metabolism of largemouth bass fed a reduced fishmeal/high soybean meal diet.

A 10-week growth study was conducted to evaluate the effect of a natural gas fermentation bacterial meal (FeedKind^®, FK) as a fishmeal (FM) alternative in largemouth bass (Micropterus salmoides) (48.0 ± 0.03 g). Four isonitrogenous and isoenergetic diets were formulated including one commercial control (C, 42% FM) and three experimental diets with gradient FK of 3% (FK3, 29%FM), 6% (FK6, 26%FM) and 9% (FK9, 23%FM), respectively. FK-fed groups showed significantly higher SR than that of C group. The WGR and SGR of fish fed FK3 and FK6 were significantly higher than those of FK9, but not statistical different from the C group. FK-fed groups showed higher apparent digestibility coefficients of dry matter and nutrients. Further, FK-fed groups increased the ratio of SOD/MDA in the plasma and liver, and the upregulation of intestinal Keap1 and downregulation of HIF1α was found in FK3. Furthermore, FK-fed groups showed higher microbial richness and diversity. Pearson correlation analysis found that antioxidant relevant biomarkers were negatively correlated with the relative abundance of certain potential beneficial bacteria. In conclusion, supplemented up to 3–6% FK replacing FM in a low FM diet of largemouth bass could increase growth, survival rate, antioxidant capacity, and improve gut microbiota.