Effect of dietary leucine on the growth parameters and expression of antioxidant, immune, and inflammatory genes in the head kidney of Labeo rohita fingerlings

- An appropriate level of leucine administration improved the growth of L. rohita.
- Antioxidant activities could be regulated at the transcriptional level after leucine supplementation.
- Leucine up-regulated Nrf2, NKEF-β, and TLR22 and down-regulated TNF-α, Keap1, and IL-1β expression.
- The optimal leucine level for L. rohita juveniles is 4.7 g kg−1 diet based on PWG.

The present investigation evaluated the effects of dietary leucine (Leu) on growth performance, head kidney antioxidant status, and gene expression in Labeo rohita juveniles. Fish were fed with six isonitrogenous diets containing graded levels of Leu at 0.75 (control), 1.7, 3.2, 4.6, 6.3, and 7.6 g Leu kg−1 of feed for 8 weeks. Compared with the control group, appropriate Leu supplementation significantly enhanced the percent weight gain (PWG), feed intake (FI), and protein efficiency ratio (PER) (P < 0.05) but decreased the plasma ammonia content (PAC) (P < 0.05). Similarly, optimal Leu supplementation stimulated head kidney glutathione (GSH) content, and superoxide dismutase (SOD) and glutathione peroxidase (GPx) activities as compared to the control group; however, a reverse trend was observed in malondialdehyde (MDA) content. Further, relative gene-expression levels of lysozyme, complement C3, β-microglobulin, immunoglobulin-M, SOD, GPx, nuclear factor erythroid 2-related factor 2 (Nrf2), natural killer-cell enhancing factor β (NKEF-β), and toll-like receptor-22 (TLR22) in the head kidney were enhanced (P < 0.05) at leucine levels of 4.6 g kg−1 of feed. Conversely, the mRNA levels of tumour necrosis factor-α (TNF-α), Kelch-like-ECH-associated protein 1 (Keap1), and interleukin 1β (IL-1β) in head kidney were down-regulated by Leu supplementation. Collectively, our results revealed that appropriate Leu supplementation improved fish growth and antioxidant capacity, and regulated the mRNA levels of related signalling molecules in L. rohita juveniles. Based on the quadratic regression analysis of PWG, PER, and PAC, the optimum dietary leucine requirements of L. rohita juveniles were estimated to be 4.7, 4.5, and 4.8 g kg−1 of feed.