Dietary valine affects growth performance, intestinal immune and antioxidant capacity in juvenile largemouth bass (Micropterus salmoides)

Abstract

This study investigated the effect of dietary valine on growth, whole-body composition, plasma parameters, and intestinal genes expression related to TLR-MyD88-NF-κB signaling pathways of the largemouth bass. An 8-week feeding trial was designed to evaluate the effects of dietary valine levels on the growth performance, intestinal immunity, and antioxidant capacity of largemouth bass. Graded valine levels were used to produce six practical diets: 1.77% (control), 2.02%, 2.26%, 2.49%, 2.71%, and 2.94%. The findings demonstrated that, up to a dietary valine level of 2.26%, the weight gain rate, final body weight, condition factor, and specific growth rate all increased and thereafter declined (P < 0.05). The feed conversion ratio had the opposite trend but with no significant difference (P > 0.05). Dietary valine had no significant effects on HSI (hepatosomatic index), VSI (visceral adipose index) or the whole-body composition of largemouth bass (P > 0.05). The 2.26% dietary valine level significantly increased intestinal antioxidant factor activity, including the activities of CAT (catalase) and SOD (superoxide dismutase) (P < 0.05). The fish fed 2.94% dietary valine had the highest level of GSH (glutathione) (P > 0.05) and the highest activity of GPx (glutathione peroxidase) (P < 0.05). The 2.71% dietary valine level significantly reduced the MDA (malondialdehyde) content compared with that of the control group (P < 0.05). The 2.26% dietary valine level dramatically reduced the mRNA levels of IL-8 (interleukin-8), TNF-α (tumor necrosis factor-α), IL-1β (interleukin 1β), NF-κB p65 (nuclear factor kappa B p65), IL-10(interleukin 10), keap1 (kelch-like ECH-associated protein 1), C3 (caspase 3), MyD88 (myeloid differentiation factor 88), TLR2 (Toll-like receptor 2), CAT, and GPx and increased the mRNA level of SOD in the largemouth bass intestine when compared to levels in the control group (P < 0.05). The Nrf2 (nuclear factor 2-related factor) mRNA level was also increased but was not significantly different (P > 0.05). The mRNA levels of TGF-β (transforming growth factor β) did not change significantly (P > 0.05). In conclusion, according to the FCR (feed conversion ratio)- and SGR (specific growth rate)- based quadratic regression analyses, the optimum requirements for valine are 2.43% (5.01% of dietary protein) and 2.37% (4.95% of dietary protein), and 2.26% dietary valine help to improve the immune and antioxidant capacity of the intestine in largemouth bass.