Brewers’ Yeast (Saccharomyces cerevisiae) Purified Functional Feed Additives Mitigate Soybean Meal-Induced Enteritis in Atlantic Salmon (Salmo salar) Parr

Soybean meal (SBM) is commonly used in aquafeeds due to its wide availability, reasonable protein content, and cost-effectiveness. However, high SBM inclusion levels in the diets of carnivorous fish, such as Atlantic salmon (Salmo salar), can cause soybean meal-induced enteritis (SBMIE), resulting in compromised gut health, reduced nutrient absorption, and impaired growth. An 8-week study was conducted to evaluate the potential of brewers’ yeast-derived functional feed additives (FFAs), specifically yeast cell wall β-glucans (PβG [purified β-glucan]) and yeast cytosolic extracts (YEs), to mitigate the adverse effects of SBMIE in Atlantic salmon parr. Fish were fed diets containing 30% SBM (30-SBM) with either 0.02% β-glucan (30-SBM+PβG) or YE at 1% (30-SBM+YE1) and 2.5% (30-SBM+YE2.5) inclusion levels and compared against a control diet without SBM (0-SBM). The study assessed growth performance, haematological parameters, distal intestinal morphology, and the distal intestinal gene expression levels of enteritis biomarkers (casp3b, pcna, and hsp70). The results showed that PβG and 1% YE supplementation significantly reduced the severity of SBMIE, with improvements in intestinal morphology, including reduced intraepithelial leukocytes (IELs) levels and goblet cell hyperplasia. Intestinal gene expression levels of casp3b and pcna were significantly downregulated in the PβG and YE fed fish relative to the 30-SBM fed fish, indicating reduced apoptosis and more controlled cell proliferation. However, the effects of 2.5% YE supplementation were less pronounced, indicating a dose-dependent response. These findings demonstrate that both PβG and YE from 100% Saccharomyces cerevisiae can alleviate SBMIE in juvenile Atlantic salmon by supporting gut health and modulating cellular recovery processes.