Effect of a Low FODMAP Diet in IBS and Functional Constipation with Bloating: a Randomized Double-Blind Trial

Irritable Bowel Syndrome (IBS) and Functional Constipation (FC) are chronic Disorders of Gut-Brain Interaction (DGBI) characterized by altered motility, visceral hypersensitivity, dysbiosis, and central nervous system processing abnormalities. Bloating is a challenging and distressing symptom that often persists despite standard pharmacological treatments.

The low FODMAP diet has been widely studied as a dietary approach to alleviate IBS symptoms. FODMAPs (Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols) are poorly absorbed short-chain carbohydrates that increase osmolarity, promote intestinal gas production, and alter gut microbiota, leading to bloating and discomfort. Although studies support the effectiveness of the low FODMAP diet, existing research lacks controlled, blinded trials assessing its impact on bloating-specific symptoms and related physiological markers.

This study addresses these gaps by using a double-blind, controlled design with rigorously standardized meals containing well-defined FODMAP content. It will assess clinical outcomes, gut microbiota composition, gas production, and metabolomic changes to elucidate the mechanisms underlying the diet's effects.

Study Design This is a randomized, double-blind, controlled trial conducted at a single center. Participants will be assigned to receive either a low FODMAP or a high FODMAP diet for two weeks.

Participants:

Adults ≥18 years diagnosed with IBS or FC with persistent bloating based on Rome IV criteria.

Intervention:

Standardized meal plans containing either low FODMAP or high FODMAP content.

Randomization:

Computer-generated block randomization (block of 4)

Blinding:

Investigators, study staff, and participants will remain blinded to the dietary allocation.

Primary Outcome:

≥30% reduction in bloating severity on the VAS scale.

Secondary Outcomes:

Changes in global IBS symptoms (IBS-SSS) or constipation symptoms (PAC-Sym) Abdominal pain (VAS) stool frequency, and consistency (Bristol Stool Form Scale). Gut microbiota diversity and composition (16S rRNA sequencing). Intestinal gas production (hydrogen/methane breath test). Metabolomic analysis (short-chain fatty acids, urine histamine, azelaic acid). Use of rescue medications for symptom relief.

Study Procedures

Baseline Assessment:

Demographic data, medical history, and dietary habits (7-day food diary). Baseline bloating severity, stool consistency, and quality of life (IBS-QoL, PAC-QoL).

Microbiota, breath test, and metabolomic sampling.

Intervention Phase (2 Weeks):

Participants consume standardized low or high FODMAP meals (3 main meals + 1 snack/day).

Daily recording of symptoms, bowel movements, and use of rescue medications.

Follow-up Assessments:

Week 1: Phone follow-up to assess adherence and symptom progression. Week 2: Final clinical and laboratory assessments (identical to baseline). Data collection for statistical analysis comparing treatment effects.

Statistical Analysis Primary endpoint (bloating reduction) analyzed using Chi-square/Fisher's exact test.

Secondary outcomes analyzed with paired t-tests, Mann-Whitney U tests, and logistic regression.

Microbiota diversity assessed using Shannon index, beta diversity, and PERMANOVA testing.

Significance and Impact This study will provide high-quality evidence on the effectiveness of the low FODMAP diet for bloating-predominant IBS and FC, addressing critical knowledge gaps. By integrating clinical, microbiota, and metabolomic data, it will enhance understanding of diet-microbiota interactions and their role in symptom modulation.

Inclusion criteria

• Age ≥18 years
• Diagnosed with Irritable Bowel Syndrome (IBS) or Functional Constipation (FC) based on Rome IV criteria
• Persistent bloating affecting daily life, with a bloating VAS score ≥4
• Stable bowel habits with Bristol Stool Form Scale (BSFS) type 3-5
• History of prior treatment with antispasmodics and/or prokinetics (allowed if dose remains stable)
• Able to comply with dietary intervention and study procedures

Exclusion criteria

• Prior gastrointestinal surgery (except appendectomy or cholecystectomy)
• Use of antibiotics, proton pump inhibitors (PPIs), probiotics, NSAIDs, lactulose, or metformin within the past 4 weeks
• Presence of systemic diseases affecting gut microbiota (e.g., liver cirrhosis, uncontrolled diabetes, end-stage renal disease, obesity, cancer, psychiatric disorders)
• Inability to consume study meals or follow dietary restrictions