Rifaximin Treatment in Bloating Predominant Functional Bowel Disorders

This randomized, double-blind, placebo-controlled trial will evaluate the effects of a 14-day course of rifaximin on bloating symptoms, gut microbiota, microbial metabolites, and intestinal gases in adults with Rome V irritable bowel syndrome with constipation (IBS-C) or functional abdominal bloating (FAB). Participants with predominant, bothersome bloating despite adequate bowel movement management will be randomly assigned in a 1:1 ratio to receive rifaximin 550 mg three times daily or matching placebo for 2 weeks. The primary endpoint is the proportion of participants with a bloating response, defined as at least a 1-point reduction from baseline on a 7-point Likert bloating scale at the end of treatment. Additional assessments will include other gastrointestinal symptoms, quality of life, psychological symptoms, gut microbiota diversity and composition, fecal short-chain fatty acids, plasma bile acids and gut-derived uremic toxins, and breath hydrogen and methane levels. Associations between biological changes and clinical response will also be examined.

Inclusion Criteria: * Adults aged 18 to 80 years. * Rome V diagnosis of irritable bowel syndrome with constipation, or functional abdominal bloating. * Bothersome bloating with baseline severity of at least 3 on a 7-point Likert scale after adequate constipation treatment, defined as stool frequency from at least 3 times/week to 3 times/day and Bristol Stool Form Scale type 3-5. * Colonoscopy, CT colonography, or barium enema performed if clinically indicated as part of standard evaluation for bowel symptoms. Exclusion Criteria: * History of major gastrointestinal surgery, except appendectomy or laparoscopic cholecystectomy. * Inflammatory bowel disease or other inflammatory gastrointestinal conditions. * Current use of, or inability to discontinue, medications that may affect intestinal microbiota or gas measurements, including antibiotics, proton pump inhibitors, probiotics, lactulose, NSAIDs, or metformin. * Current use of, or inability to discontinue, medications that may affect bloating symptoms, including simethicone, simethicone-containing antispasmodics, or antidiarrheal medications. * Underlying conditions known to affect intestinal microbiota composition, including cirrhosis, uncontrolled diabetes mellitus, end-stage renal disease, obesity, malignancy, or psychiatric disorders. * Opioid-induced constipation. * Known allergy to rifaximin.