Effects of replacing fishmeal with rapeseed meal and dietary condensed tannins on antioxidant capacity, immunity, and hepatic and intestinal health of largemouth bass (Micropterus salmoides)

This study evaluated the effects of rapeseed meal (RM) and condensed tannins (CT) in diets on largemouth bass (Micropterus salmoides) and examined the possible role of CT on immunity and antioxidants when RM was used in the diets. After 55-days of feeding trials, oxidative stress in largemouth bass differed with dietary RM amount by affecting antioxidant capacity and immune function, changing the health state of the body. More than 5% RM caused oxidative stress, more than 15% RM led to oxidative damage to the liver and intestine, and 25% RM increased the intestinal inflammatory response. High dose of CT dietary supplementation caused oxidative and immune damage of the liver and intestines accompanied by the inflammatory response, while the low dose had the opposite effects, and thus relieved oxidative stress and inflammatory damage. Compared with the effect of contained CT (in RM), the antioxidant capacity and immune function of the groups supplemented with CT were enhanced, and the inflammatory damage effect was reduced at a low dose. In this study, the activities of catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GSH-Px) were closely related to the contents of malondialdehyde (MDA), protein carbonyl (PC), oxygen radical, and total antioxidant capacity. Intestinal nuclear factor erythroid 2–related factor 2 (Nrf2) expression was involved in the regulation of antioxidant capacity and altered the expression levels of intestinal inflammatory factors, transforming growth factor beta-1 (TGF-β1), transforming growth factor beta-2 (TGF-β2), and interleukin 6 (IL6). The interaction of anti-nutritional factors in RM may reduce its toxicity to some extent. Thus, largemouth bass could tolerate more CT present in RM than CT alone. CT was not necessary to be removed at levels below 5% of RM in the diet.