Effects of dietary autolyzed yeast and quercetin on growth performance, antioxidant and immunological parameters, and resistance to heat stress in rainbow trout

This study aimed to evaluate the effects of dietary autolyzed yeast and quercetin on the growth performance, biochemical parameters, and heat stress resistance of rainbow trout (Oncorhynchus mykiss). The fish were fed either a control diet (without supplements), diets supplemented with 300 mg/kg quercetin, 2 g/kg autolyzed yeast, or a combination of both for a duration of eight weeks. Following this period, the fish were subjected to elevated water temperatures (25 °C) for 96 h to assess their stress resistance. The results indicated that while quercetin did not significantly affect growth performance, autolyzed yeast did enhance it. The addition of either quercetin or autolyzed yeast to the diet significantly increased red blood cells, white blood cells, hematocrit levels, plasma immunoglobulin, total antioxidant capacity, and bactericidal activity. Furthermore, skin mucus alkaline phosphatase, protease, peroxidase activities, and splenic cytokine transcripts were also elevated, whereas hepatic malondialdehyde levels were decreased. Quercetin alone significantly increased the plasma lysozyme and hepatic glutathione levels but resulted in a decrease in the mean corpuscular hemoglobin level. Conversely, autolyzed yeast alone significantly increased the mean corpuscular volume and neutrophil counts, as well as plasma alternative complement levels, skin mucus lysozyme and immunoglobulin concentrations, and hepatic catalase activity; however, it also led to a significant reduction in blood lymphocyte counts. Both quercetin and autolyzed yeast contributed to reduced mortality rates following heat stress, with the highest survival rates observed when both supplements were included in the diet. In summary, incorporating dietary autolyzed yeast significantly improved the growth performance of rainbow trout, whereas quercetin did not provide the same benefits. Both supplements enhance innate immunity and increase antioxidant levels; however, autolyzed yeast has a more pronounced effect on immune-related factors, whereas quercetin is more effective as an antioxidant. Additionally, both supplements led to improved blood parameters and greater resistance to heat stress, with the best outcomes achieved when autolyzed yeast and quercetin were combined. © 2025 Elsevier B.V.