Brewers’ Yeast ( <i>Saccharomyces cerevisiae</i> ) Purified Functional Feed Additives Mitigate Soybean Meal‐Induced Enteritis in Atlantic Salmon ( <i>Salmo salar</i> ) 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 (<i>Salmo salar</i>), 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 <i>β</i>-glucans (P<i>β</i>G [purified <i>β</i>-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% <i>β</i>-glucan (30-SBM+P<i>β</i>G) 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 (<i>casp3b</i>, <i>pcna</i>, and <i>hsp70</i>). The results showed that P<i>β</i>G 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 <i>casp3b</i> and <i>pcna</i> were significantly downregulated in the P<i>β</i>G 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 P<i>β</i>G and YE from 100% <i>Saccharomyces cerevisiae</i> can alleviate SBMIE in juvenile Atlantic salmon by supporting gut health and modulating cellular recovery processes.