S392 Gut Dysbiosis and Its Role in Gastrointestinal and Extra-Intestinal Disorders: A Systematic Review of Human Clinical Studies

Introduction: Several diseases have been linked to an imbalance in the types of microorganisms found in the intestines. Dysbiosis is also connected to additional health issues, such as metabolic syndrome, various autoimmune conditions, and neurodegenerative problems. A modified microbiome may play a role in boosting the immune system and causing widespread inflammation, leading to disease development. Methods: We systematically reviewed studies on gut microbiota changes in gastrointestinal (GI) and extraintestinal diseases. Searches of PubMed, Google Scholar, Cochrane Library, Web of Science, and Embase (2000–2025) identified case-control and cohort studies comparing the gut microbiome in patients vs healthy controls with inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), obesity, diabetes, rheumatoid arthritis, and neurological disorders. We examined microbial diversity, taxonomic shifts, and functional changes. Due to small sample sizes and varied methodologies, findings were synthesized qualitatively. Results: Our search yielded 17 studies. Most GI diseases involve altered gut microbiota; for example, IBS patients often have lower microbial diversity and fewer beneficial bacteria. IBD patients typically show reduced Firmicutes (e.g. Faecalibacterium) and increased Proteobacteria. Dysbiosis is also linked to non-GI conditions. In autoimmune diseases like rheumatoid arthritis, reduced Prevotella spp. is common. Patients with Parkinson’s or Alzheimer’s often show distinct microbiomes, with PD cohorts having fewer short-chain fatty-acid (SCFA)-producing bacteria (e.g., Prevotellaceae) and more pro-inflammatory microbes (e.g. Enterobacteriaceae). Similarly, MS patients show reduced beneficial SCFA-producing taxa and increased inflammatory microbes. Microbial metabolites may reduce brain inflammation, while misfolded gut proteins (like α-synuclein) may reach the brain via the vagus nerve. These findings suggest these diseases share a pattern of reduced diversity and increased pro-inflammatory microbiota. Conclusion: Gut dysbiosis is common across many GI and extra-intestinal diseases, marked by reduced beneficial bacteria and increased inflammatory microbes. While the exact cause remains unclear, these shifts may trigger local and systemic immune dysfunction. This supports using diet and probiotics as adjunct therapies for conditions like IBD, IBS, rheumatoid arthritis, and Parkinson’s. Larger studies are needed to clarify dysbiosis's role and the potential benefits of correcting it.