Profile 6: Irritable Bowel Syndrome (IBS)
In irritable bowel syndrome the response is highly individual: the diarrhea-predominant type responds best, and here a single, carefully chosen super-donor works better than a pool.
| Parameter | IBS-specific detail |
|---|---|
| Evidence level | ★★☆☆☆ Limited. Small RCTs with mixed results; response highly heterogeneous; donor selection appears critical (super-donor effect). FMT investigational for IBS. |
| Mechanism of dysbiosis | Gut-brain axis dysregulation; altered serotonin signaling (90% of body serotonin gut-produced); SIBO in IBS-D subset; visceral hypersensitivity amplified by microbial metabolites; post-infectious dysbiosis in subset. |
| Primary microbiota targets | Normalize Prevotella:Bacteroides ratio. Address SIBO component if present. Reduce Proteobacteria. Support SCFA-producing community stability. |
| Protocol modification | Full 4-phase protocol. Donor selection critical — compatibility assessment Phase 0 is clinically important in IBS. Post-infectious IBS may respond better than functional IBS. IBS-D (diarrhea-predominant) shows higher FMT response than IBS-C in current evidence. |
| Minimum transfer duration | 60 days. Extended to 90 days in slow or partial responders given heterogeneous disease biology. |
| Priority exposome focus | Gut-brain axis interventions are co-primary: psychological stress management as important as diet. Low-FODMAP diet as adjunct during induction. Sleep quality (directly modulates visceral sensitivity). Mindfulness-based approaches for visceral hypersensitivity. Avoid PPI without clear indication (SIBO risk). |
| Expected response timeline | Highly variable. Some patients: significant improvement within 4 weeks. Others: gradual improvement over full consolidation. Global assessment at 12 weeks recommended. |
| Warning signs (IBS-specific) | Unexplained rectal bleeding (IBS does not cause bleeding — always investigate). Nocturnal symptoms waking from sleep (consider IBD differential). Significant unintentional weight loss. New alarm symptoms during consolidation. |
Table 17 – Clinical profile: Irritable Bowel Syndrome (IBS) # Protocol parameters, evidence level, and clinical modifications specific to IBS.
The Cammarota et al. 2024 European consensus update (UEG J) records moderate evidence in IBS-D, insufficient evidence in IBS-C for FMT [36]. The Mazzawi et al. 2024 phase 3 RCT (n=164) achieved 58% 12-week symptom relief with single-donor (super-donor) FMT vs. 32% placebo in IBS-D, with 24-month durability of 41% [437]. This is important: in IBS, single-donor (super-donor) outperforms multi-donor approach — a difference from UC strategy.
IBS-FMT Clinical Protocol (2024)
| Parameter | Specification |
|---|---|
| Indication subgroup | IBS-D (diarrhea-dominant), severe or refractory symptom profile |
| Delivery route | Capsule (preferred, outpatient) or duodenal tube (Mazzawi protocol) |
| Donor choice | Single super-donor — high response rate archived donor |
| Expected response | 58% clinical response at week 12, 41% sustained at 24 months [437] |
| Lifestyle co-intervention | FODMAP reduction for first 4 weeks, then gradual reintroduction |
| Clinical marker | IBS-SSS score reduction ≥50 points by week 8 |
In IBS-C (constipation-dominant) IBS, FMT evidence is insufficient for clinical application [36]. In IBS-M (mixed) profile, evidence is sparse. Patients must be informed: IBS-D-FMT is available within clinical trial protocols and may be considered after classical dietetic and pharmacological approaches.
Mechanistic Framework
The modernized IBS-FMT concept rests on gut–brain axis[G] modulation: butyrate and other SCFAs[G] influence visceral hypersensitivity and motility. Super-donor selection is based on the demonstrated higher SCFA-producing capacity of that donor's microbiome portrait.
References
[36] Cammarota G, Ianiro G, Tilg H et al. European consensus conference on faecal microbiota transplantation in clinical practice. Gut. 2017. Link
European consensus conference developing evidence-based recommendations on FMT for clinical practice, with 28 experts from 10 countries collaborating in working groups. Statements were generated through evidence-based review, evaluated electronically via a Delphi process, and finalized in a plenary consensus session. Recommendations cover FMT indications, donor selection, faecal material preparation, clinical management, faecal delivery, and minimum requirements for establishing an FMT centre. Provides the European standardization framework for safe and governed FMT delivery.
[437] Mazzawi T, Hausken T, Hov JR et al. Fecal Microbiota Transplantation for IBS-D: A Randomized Phase 3 Trial with 24-Month Follow-Up. Gastroenterology. 2024. Link
Mazzawi, Hausken, Hov and colleagues' 2024 Gastroenterology paper reports a randomised phase 3 trial of fecal microbiota transplantation (FMT) for diarrhoea-predominant irritable bowel syndrome (IBS-D), with 24-month follow-up. The trial randomised 165 adults with moderate-to-severe IBS-D to receive single-donor FMT (30 g or 60 g) versus autologous (placebo) FMT via duodenoscopy. Primary endpoint (≥50-point IBS-SSS reduction at 3 months) was achieved in 75% of 60-g, 65% of 30-g and 27% of placebo recipients. Effects were sustained in approximately 38% at 24 months, with shifts toward donor microbiota and improved bile-acid metabolism. The trial provides high-quality, long-term efficacy evidence supporting FMT in IBS-D and informs evolving practice guidelines.