Background: A diet low in fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs) is increasingly recommended for patients with irritable bowel syndrome (IBS). We aimed to investigate the effects of a blinded low-FODMAP vs high-fructo-oligosaccharides (FOS) diet on symptoms, immune activation, gut microbiota composition, and short-chain fatty acids (SCFAs). Methods: Twenty patients with diarrhea-predominant or mixed IBS were instructed to follow a lo-FODMAP diet ( LFD) throughout a 9-week study period. After 3 weeks, they were randomized and double--blindly assigned to receive a supplement of either FOS (FODMAP) or maltodextrin (placebo) for the next 10 days, followed by a 3-week washout period before crossover. Irritable bowel syndrome severity scoring system (IBS-SSS) was used to evaluate symptoms. Cytokines ( interleukin [IL]- 6, IL- 8, and tumor necrosis factor alpha) were analyzed in blood samples, and gut microbiota composition ( 16S rRNA) and SCFAs were analyzed in fecal samples. Key Results: Irritable bowel syndrome symptoms consistently improved after 3 weeks of LFD, and significantly more participants reported symptom relief in response to placebo ( 80%) than FOS ( 30%). Serum levels of proinflammatory IL- 6 and IL- 8, as well as levels of fecal bacteria ( Actinobacteria, Bifidobacterium, and Faecalibacterium prausnitzii), total SCFAs, and n- butyric acid, decreased significantly on the LFD as compared to baseline. Ten days of FOS supplementation increased the level of these bacteria, whereas levels of cytokines and SCFAs remained unchanged. Conclusions and Inferences: Our findings support the efficacy of a LFD in alleviating IBS symptoms, and show changes in inflammatory cytokines, microbiota profile, and SCFAs, which may have consequences for gut health.