Postbiotics are emerging as potential functional ingredients for companion animal diets. This study aimed to determine if a Saccharomyces cerevisiae-based postbiotic can alter cytokine and stress responses to exercise and transport stress in adult Labrador Retrievers. Dogs received 15 g ground corn germ (Control, n = 12), 7.5 g postbiotic (Low, n = 12), or 15 g postbiotic (High, n = 12), daily for 63 days. Exercise was twice weekly for 7 weeks, and a single transport per dog occurred in week 8. Fecal inflammatory biomarkers, serum chemistries, and complete blood counts were assessed at the beginning and end of the study. Serum cytokines were quantified before and 18–20 h after the first and last exercise runs. Gait analysis was assessed before and 24 h after the first and final runs. Saliva cortisol was measured before and after transportation. Treatment did not affect blood chemistries, gait, fecal biomarkers, or saliva cortisol (p ≥ 0.19). Eosinophils increased slightly in Controls (p = 0.01), though remained below 0.80 × 109 cells/L. Most cytokines were unaffected by treatment (p ≥ 0.15), but there were minor changes in circulating monocyte chemoattractant protein-1 (p = 0.01) and IL-8 over time at the initial run (p = 0.03) and IL-10 in males (p = 0.02) in the Low dose dogs. The High dose decreased Blautia (p = 0.04) slightly and tended to decrease Fusobacterium abundances (p = 0.07). The Low dose tended to increase Clostridium hiranonis (p = 0.07) slightly. The tested S. cerevisiae postbiotic produced small changes in immune function and gut microbial species in dogs.
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