Beyond Immunity: Functional Outcomes of Dietary Yeast and Seaweed β-Glucans in Adult Canine Nutrition

β-Glucans are functional compounds associated with health benefits; however, the physiological effects of different β-glucan sources in dog foods remain incompletely understood. This study evaluated and compared the effects of β-glucans derived from Euglena gracilis (seaweed) and Saccharomyces cerevisiae (yeast) on nutrient digestibility, fermentation products, faecal microbiota composition, serum lipid profile, and postprandial glucose and insulin responses in healthy adult dogs fed extruded diets. A single basal diet was used across four treatments: CON (control, no β-glucan), B-Y15 (0.115% yeast β-glucan), B-S15 (0.155% seaweed β-glucan) and B-S30 (0.310% seaweed β-glucan), formulated to provide daily intakes of 15 or 30 mg β-glucan/kg body weight/day. Thirty-two adult Beagle dogs were allocated to a randomised block design (n = 8 per treatment), with each experimental block consisting of a 42-day washout period followed by a 42-day treatment period. Apparent nutrient digestibility, faecal fermentation products, serum lipid profile, postprandial glucose and insulin responses, and faecal microbiota composition (16S rRNA gene sequencing) were evaluated. Data were analysed using ANOVA, Tukey's, and repeated measures (p < 0.05). Low levels of E. gracilis β-glucans improved apparent digestibility of dietary gross energy. Supplementation with either β-glucan sources did not significantly alter postprandial lipid parameters or glucose responses; however, a high dose of seaweed-derived β-glucan delayed the insulin peak. Higher inclusion levels of seaweed β-glucans were associated with alterations in overall faecal microbiota community structure, as indicated by beta-diversity analysis, without corresponding changes in species richness, evenness, or faecal fermentation products. In conclusion, β-glucans from yeast and seaweed exhibited distinct physiological effects in healthy adult dogs. Moderate inclusion of seaweed-derived β-glucans enhanced nutrient utilization, whereas higher inclusion levels influenced insulin kinetics and faecal microbiota community structure. These findings underscore the importance of β-glucan source and inclusion level when considering their application as functional ingredients in canine nutrition.