Quercetin as a Nutritional Intervention: Enhancing Growth, Metabolism, and Immune Gene Modulation in <i>Labeo rohita</i>

The objective of the study was to evaluate the use of quercetin-supplemented diet as a growth, metabolism, and immunity promoter in <i>Labeo rohita</i>. Fish (8 ± 0.5 g) were randomly divided into five groups in triplicates (30 fish in each replicate) in circular tanks of 2000 L capacity. The fish were fed on a control diet (0% quercetin and four experimental diets supplemented with quercetin inclusion levels 0.2% (T1), 0.4% (T2), 0.8% (T3), and 1.6% (T4)). After the experimental period, the highest growth performance weight gain %, specific growth rate, protein energy ratio, and intestinal somatic index were recorded in the T4 group, followed by the T3 group. The lowest yield was recorded in the control group. However, the feed conversion ratio was significantly decreased with the 1.6 quercetin-supplemented group. The activity of digestive and metabolic enzymes of quercetin-fed groups was significantly different (<i>p</i> < 0.05) compared to the control group. In the treatment groups, there was a substantial increase (<i>p</i> < 0.05) in the relative expression levels of insulin-like growth factor-1 gene expression in the liver and muscle when quercetin supplementation was compared to the control group. The tumor necrosis factor-α, interleukin-1β, Toll-like receptor-22, and interferon-γ mRNA levels significantly (<i>p</i> < 0.05) increased with increasing quercetin levels, and highest expression was found in the T4 treatment group. Expression profiles of antioxidant-related genes also revealed significant difference (<i>p</i> < 0.05) between treatment and control groups. In the experimental challenge, the control group experienced higher mortality rates than the other groups following an intraperitoneal injection of <i>Aeromonas hydrophila</i>. Consequently, quercetin supplementation is recommended at a level of 1.6% to improve the growth performance, antioxidative capacity, and immune response of <i>L. rohita</i>.