A Fiber- and Polyphenol-Enriched Diet Enhances Humoral Immunity, Reshapes Cecal Microbiota, and Improves Short-Chain...

Background/Objectives: Dietary fiber and polyphenols are recognized modulators of intestinal and immune homeostasis; however, evidence regarding their combined impact under physiological conditions remains limited. This study aimed to evaluate whether a diet enriched with fermentable fiber and polyphenols modulates mucosal and systemic immune biomarkers, as well as microbiota composition and function in healthy adult female rats. Methods: Wistar rats were fed either a reference diet (REF group) or a fiber- and polyphenol-enriched diet (FP group) for nine weeks. At the end of the intervention, plasma lipid profile, systemic and mucosal immune status (assessed by immunoglobulin (Ig) content in several compartments), cecal microbiota composition (determined by 16S rRNA sequencing), cecal short-chain fatty acids (SCFAs) and cecal Ig-coated bacteria, among other variables, were quantified. Results: The FP group exhibited a higher IgG concentration in plasma and elevated IgG2c levels in mucosal compartments compared with REF animals. The FP diet did not alter either intestinal morphology or hematologic and lipid variables; however, FP rats exhibited increased fecal moisture and reduced fecal pH. With regard to cecal microbiota, the FP group displayed higher microbial evenness, distinct β-diversity clustering, and shifts in the abundance of multiple genera. In addition, elevated cecal SCFA concentrations, particularly for acetate and propionate, were found in the FP group. Conclusions: Long-term intake of fermentable fiber and polyphenols promotes microbial fermentation and enhances humoral immunity without inducing structural or systemic physiological alterations. These findings support the role of plant-based foods in promoting immune and gut microbiota homeostasis under healthy conditions.