Dietary supplement with dihydroquercetin and arabinogalactan affects growth performance, intracellular protease activities and muscle-specific gene expression in bacterially infected Oncorhynchus mykiss

This feeding trial aimed to evaluate the effect of a natural dietary supplement with proposed immunostimulant and related biological activities on growth performance and muscle growth markers in rainbow trout, Oncorhynchus mykiss. Fish grown in cages were fed either a commercial diet (control) or a diet supplemented with dihydroquercetin and arabinogalactan at a dosage of 25 and 50 mg/kg of feed, respectively, during the summer-autumn season. Unplanned infectious disease did not interfere with the scheme of the experiment since both groups were infected and then treated with antibiotics. Significant differences in growth parameters between the control and experimental groups were shown only in a few cases; however, their body weight and length, Fulton's condition factor, and relative growth rate tended to be higher in the fish fed the diet with supplements. Bacterial infection influenced muscle growth mechanisms and protein turnover through partial suppression of myosin heavy chain (MyHC) expression and proteasome activity. In fish fed the experimental diet, MyHC expression was higher throughout the experiment and was more readily restored during the post-infection period. In the experimental group, the system of protein quality control involving proteasome activity was less perturbed by infection. Moreover, the suppression of the protein-degrading capacity of calpains found in the muscles of fish fed the experimental diet probably demonstrates a mechanism of growth acceleration during the post-infection period. Therefore, in infected rainbow trout, the experimental diet diminished the effects of the disease and substantially promoted postinfection repair, improving survival and infection tolerance and affecting growth mechanisms in fish.