Background Clinical guidelines provide limited and conflicting recommendations regarding dietary fiber supplementation in irritable bowel syndrome (IBS). Nopal (Opuntia ficus‐indica) is a cactus plant fiber containing both insoluble and soluble fibers that may have therapeutic potential in IBS. Our aim was to evaluate the dose–response effect of extracted nopal fiber powder on symptoms in IBS. Methods We performed a 4‐arm, double‐blind, parallel, randomized controlled trial in 60 patients fulfilling Rome IV criteria for IBS. Patients were randomized and blindly allocated to receive either nopal fiber (10, 20, or 30 g/day) or placebo (30 g/day dextrose) for one week and to keep their usual diet. Symptom severity (Global Symptom Question, IBS‐SSS, Gastrointestinal Symptom Rating Scale), stool frequency and consistency (Bristol Stool Form Scale), breath hydrogen response, and stool short‐chain fatty acids (SCFA) were measured at baseline and follow‐up. Key results Significantly more patients reported adequate relief of symptoms after 20 g/day (87%, p = 0.008) and 30 g/day (80%, p = 0.025) of nopal fiber compared to placebo (33%). More patients receiving 20 g/day nopal fiber (67%) had a > 50% reduction in IBS‐SSS compared to placebo (20%, p = 0.027), whereas the 30 g/day dose induced more loose stools (p = 0.027). Response rates were similar among IBS subtypes. There were no differences in breath hydrogen or stool SCFA between groups. Conclusions and inferences Nopal fiber supplementation at doses of 20 and 30 g/day was associated with short‐term improvement in IBS symptoms, warranting a fully powered clinical trial of longer duration with symptomatic, physiological, and microbiological endpoints.

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