Dietary Inulin Supplementation Modulates Fecal Microbiota and Vaccine Response in Cats—A Double-Blind Randomized Controlled Pilot Study

Twenty-six cats were fed either a control or an inulin-supplemented diet (0.6%) for six weeks with measurements of fecal short-chain fatty acids (SCFAs) and microbiota. In the second phase, cats were fed their respective diets for another six weeks and were then primo-vaccinated against the feline leukemia virus (FeLV) at days 0 and 21. Serum anti-FeLV IgG responses were monitored. Compared to the control group, the inulin group exhibited higher Firmicutes (+14%) and lower Bacteroidetes (−56%) and Prevotellaceae (−51%) after six weeks of supplementation. Compared to T0, SCFAs initially decreased at week 3 but subsequently increased at week 6 with inulin supplementation, leading to higher butyrate compared to the control group at week 6. A significant diet-by-time interaction was also observed for propionate and total SCFAs. Compared to the control group, the inulin group tended to show a higher serum anti-FeLV IgG response after the first vaccination (p = 0.09), with significant differences at days 5 and 11 post-vaccination. These preliminary results suggest that a six-week dietary supplementation with a low dose of inulin in cats can modify the fecal microbiota and its functional metabolites, potentially influencing the early immune response to vaccination.