<b>Dietary Fiber and Microbiota Interactions in Colonic Inflammation, and Vitamin D Activity and Absorption</b>

<p dir="ltr">Inflammatory bowel diseases (IBD), such as Ulcerative Colitis (UC) and Crohn’s disease (CD), are characterized by chronic intestinal inflammation, microbial dysbiosis, and impaired gut barrier function. These conditions result from a combination of genetic, environmental, and immune factors. Vitamin D is essential for maintaining gut barrier integrity and regulating immune responses. However, individuals with colitis have higher incidences of Vitamin D deficiency, and inflammation can disrupt its metabolism and bioavailability. Despite increased attention to dietary fibers for their ability to promote gut health, there is limited research on the impact of fibers and their effects on Vitamin D absorption.</p><p dir="ltr">This dissertation examines the role of colitis-associated microbial communities in colitis severity and explores the role of inflammation, microbiota, and dietary fibers on colonic VDR expression and activity, and assesses the impact of dietary fibers on Vitamin D absorption.</p><p dir="ltr">In the first study, germ-free IL-10 -/- mice were colonized with microbiota from fecal samples from healthy individuals, individuals with severe colitis, healthy mice, or DSS-induced colitis mice to assess colitis severity. In the second study, VDR expression and activity were measured in DSS-induced colitis and fecal microbial transplant mice models, with correlation analyses conducted to explore relationships between VDR activity, inflammation markers, and microbial composition. For the third study, dietary fibers (beta-glucan, cellulose, and psyllium husk) were mixed with whole milk and tritium-labeled 25-Hydroxyvitamin D3, subjected to an upper gastrointestinal digestion, and analyzed to determine the recovery yields of Vitamin D in the free phase as a measure of bioaccessibility.</p><p dir="ltr">The results of the study demonstrated that UC-associated microbiota, but not DSS-induced colitis-associated microbiota, increased pro-inflammatory cytokine expression and mucus thinning. Furthermore, the results showed that inflammation and colitis severity were correlated with VDR expression and activity, and the effects of the fiber diets were donor-dependent in influencing VDR expression and activity. Lastly, both beta-glucan and psyllium husk, but not cellulose, reduced Vitamin D in the free phase, suggesting that less Vitamin D was available for absorption.</p><p dir="ltr">In conclusion, these studies suggest that dysbiotic bacterial communities can increase colitis biomarkers, and human fecal microbiota transplantation (FMT) can be used to study colitis progression in IL-10 -/- mice. Positive correlations between inflammation, colitis severity, and VDR expression and activity suggest that resolving inflammation may help restore VDR function. Additionally, specific taxa may be associated with VDR dysfunction. Furthermore, this study suggests that Vitamin D bioaccessibility may be impacted by viscous dietary fibers, which warrants further research to determine whether these effects can impact Vitamin D status.</p><p><br></p>