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Overlap with functional dyspepsia

TL;DR — Functional dyspepsia is scoped in this knowledge base as an overlap of IBS, not as a separate condition, because the two co-occur in roughly a third to a half of cases and the overlap behaves as a severity marker. Pooling 38 studies (17,993 FD and 15,883 IBS patients), the prevalence of FD in IBS is 34.6% (95% CI 28.2–41.4) and of IBS in FD 37.5% (31.5–43.8), with the Rome IV figures at least 10 percentage points higher than Rome III (FD in IBS 42.6%, IBS in FD 50.4%) (Andreev 2022, PMID 36286762). In a UK cohort of 807 Rome IV IBS patients followed for 12 months, 55.3% also met Rome IV criteria for FD at baseline, and the overlap group had more severe IBS symptoms, more continuous abdominal pain, more limitation of daily activities, more primary-care (p=0.001) and gastroenterologist (p<0.001) consultations, more new treatments started (p=0.007), and worse anxiety, depression and somatisation at 12 months (all p<0.001) (Barberio 2022, PMID 33839276). The two disorders also share a cause: the Walkerton outbreak produced excess dyspepsia at 8 years (multivariate OR 2.09, 1.58–2.78 broad definition; 2.30, 1.63–3.26 Rome II) alongside excess IBS (Ford 2010, PMID 20117111), and post-infectious FD has its own meta-analysis (Futagami 2015, PMID 25348873). The clearest practical consequence is for trials: more than half of IBS trial populations probably have concurrent FD, which the trials do not measure and which independently predicts worse outcome (Barberio 2022, PMID 33839276).

How often they overlap

Source Population FD in IBS IBS in FD
Andreev 2022, PMID 36286762 38 studies, Rome III–IV, 17,993 FD and 15,883 IBS patients 34.6% (28.2–41.4) 37.5% (31.5–43.8)
Andreev 2022 — Rome III only 32.0% (26.1–38.2), I²=98.2% 34.7% (29.3–40.3), I²=97.9%
Andreev 2022 — Rome IV only 42.6% (18.6–68.7), I²=99.0% 50.4% (38.0–62.9), I²=94.4%
Barberio 2022, PMID 33839276 807 UK adults meeting Rome IV IBS criteria 55.3% (446/807) —
Choi 2017, PMID 28160607 632 Korean outpatients (278 controls, 308 FD, 156 IBS, 110 both) — —
Nakov 2020, PMID 32919417 Bulgarian population-based study — —

Two things move these numbers. First, criteria set: Rome IV raises measured overlap by ~10 points relative to Rome III (Andreev 2022, PMID 36286762) — the same criteria-sensitivity that governs prevalence itself (diagnosis-and-rome-criteria). Second, setting: 55.3% in a self-identified IBS cohort applying Rome IV (Barberio 2022, PMID 33839276) versus ~35% pooled across mixed settings. Heterogeneity in the meta-analysis is extreme (I² 94–99%), so the pooled point estimates should be read as a range, not a value.

Overlap as a severity marker

The 12-month longitudinal comparison of IBS-alone versus IBS-with-FD (Barberio 2022, PMID 33839276; 807 baseline, 451 followed up):

Outcome at 12 months Direction in overlap group p
Consulted primary care physician for IBS Higher 0.001
Consulted gastroenterologist for IBS Higher <0.001
Number of new IBS treatments started Higher 0.007
IBS symptom severity More severe <0.001
Continuous abdominal pain More frequent <0.001
IBS limiting normal daily activities ≥50% of the time More frequent <0.001
Abnormal anxiety and depression scores More frequent <0.001
Somatisation Higher <0.001

The authors' conclusion is directed at trialists: "The natural history of people with IBS with overlap FD… is more severe than those with IBS alone. This has important implications for future treatment trials in IBS." No IBS treatment trial retrieved in this build stratified or reported on concurrent functional dyspepsia.

Clinic-based and population-based studies point the same way. In 632 Korean outpatients, overlap patients had more severe bloating, nausea, vomiting, hard stools, straining and incomplete evacuation, plus higher depression scores; bloating was an independent risk factor for overlap in both directions, and postprandial distress syndrome predicted coexisting IBS — especially IBS-C (Choi 2017, PMID 28160607). A clinic cohort in Brisbane examined prevalence and risk factors for the same overlap (von Wulffen 2019, PMID 30368683).

Triple overlap with heartburn

In two waves of a population-representative survey three years apart (1,312 individuals), complete overlap of IBS, FD and heartburn occurred in 2.1% (95% CI 1.9–3.7) in wave 1 and 2.0% in wave 2 — against a chance-expected 0.2% (p<0.001). Individuals with complete overlap were distinguished by higher rates of self-reported medically diagnosed asthma, higher psychological distress and greater sleep impairment, independent of age and sex. The authors conclude that this triple overlap "appears to be a distinct entity" with a profile of psychological morbidity, sleep disturbance and elevated atopy (Jones 2022, PMID 35293084). The temporal replication across two waves is the strongest evidence in this build that the overlap phenotype is stable rather than an artefact of a single survey.

A shared cause

Acute gastroenteritis produces both disorders. In the Walkerton cohort at 8 years (1,088 of 2,597 eligible provided data; 706 reported acute gastroenteritis), multivariate ORs for dyspepsia in exposed individuals were 2.09 (1.58–2.78) by a broad definition and 2.30 (1.63–3.26) by Rome II criteria. Prevalence was higher in women, smokers, and those with premorbid IBS, anxiety or depression, and in those reporting more than 7 days of diarrhoea or abdominal cramps during the acute illness (Ford 2010, PMID 20117111). The parallel IBS result from the same outbreak is on post-infectious-ibs (Marshall 2010, PMID 20427395). The 2024 pooled analysis of post-infectious DGBI reports post-infectious IBS at 14.5% and post-infectious FD at 12.7% after acute gastroenteritis, with ORs of 4.3 and 3.0 respectively (Porcari 2024, PMID 39013599); post-infectious FD has a dedicated meta-analysis (Futagami 2015, PMID 25348873).

The inference this supports: a single dated enteric insult produces excess disease in two anatomically distinct compartments at comparable rates. That is difficult to reconcile with organ-specific pathology and easy to reconcile with a shared gut–brain diathesis (Vanuytsel 2023, PMID 36657961; Nasser 2026, PMID 42462748).

A candidate shared mechanism: the duodenum

The most developed mechanistic proposal for why these two disorders travel together locates the origin of both above the colon. The argument (Talley 2025, PMID 40586878):

  1. FD and IBS symptoms overlap and often begin early in the post-prandial period, which is difficult to explain from a colonic origin.
  2. Low-grade duodenal inflammation — eosinophils and/or mast cells with increased permeability — is present in at least a major subset of both FD and IBS.
  3. This is supported by circulating small-intestinal-homing T cells and altered duodenal microbiota.
  4. The proposed pathway: food interacting with intestinal bacteria switches on small-intestinal immune activation, presenting antigens to the mucosa.
  5. Supporting observation: the low-FODMAP diet relieves symptoms in both IBS and FD, and also reduces common food-protein antigens (wheat, milk, soy) and urinary histamine — so its mechanism may not be purely fermentative.

The duodenal-integrity evidence on the FD side is direct: impaired duodenal mucosal integrity with low-grade inflammation in functional dyspepsia (Vanheel 2014, PMID 23474421), with duodenal acidification altering epithelial barrier function through a mast-cell-independent mechanism (Vanheel 2020, PMID 33060783) and corticotropin-releasing hormone increasing duodenal permeability and immune activation in healthy volunteers (Schol 2025, PMID 40167262). A six-food elimination diet has been reported to improve FD symptoms (Schol 2025, PMID 41354185). On the IBS side, the parallel findings are mucosal mast-cell activation and barrier dysfunction (brain-gut-axis-and-visceral-hypersensitivity; Vanuytsel 2023, PMID 36657961) and the mechanistic low-FODMAP trial showing FODMAP-driven, LPS- and TLR4-dependent, mast-cell-mediated barrier dysfunction in IBS-D (Gao 2026, PMID 40749856).

This is a hypothesis with converging support, not an established mechanism. Its author states it as such — "we hypothesize a major etiologic pathway" — and declares patents and commercial relationships relevant to FD diagnostics (Talley 2025, PMID 40586878). It is included because it is the only proposal that would explain the overlap, the food relationship and the shared post-infectious origin with one mechanism. A holistic pathophysiological overview of the functional GI disorders reaches compatible conclusions (Holtmann 2017, PMID 29421808); a 2026 review covers functional dyspepsia as a condition (Pasricha 2026, PMID 41499733).

The overlap does not stop at dyspepsia

IBS overlaps upward into reflux and outward into non-gastrointestinal central-sensitisation conditions.

Overlap Magnitude Source
Gastro-oesophageal reflux-type symptoms in IBS 42.0% (95% CI 30.0–55.0); OR versus non-IBS 4.17 (2.85–6.09); elevated in every geographical region and under every criteria set; direct overlap 14.2% (Rome II) to 26.7% (Manning) Lovell 2012, PMID 23032982 (13 studies, 49,939 participants)
Triple overlap IBS + FD + heartburn 2.1% observed vs 0.2% expected by chance (p<0.001), replicated at 2.0% three years later Jones 2022, PMID 35293084
Extra-intestinal central-sensitisation conditions Of 264 rural primary-care patients reporting IBS, only 3% (n=8) reported IBS alone; migraine 74%, depression 69%, anxiety 64%, fibromyalgia 52%. Symptom severity rose linearly with the number of comorbid conditions; EQ-5D fell from 0.8 (IBS alone) to 0.36 (IBS plus restless legs), p<0.01 Wang 2023, PMID 36807964

The rural primary-care figure is the most striking in this knowledge base: isolated IBS was reported by 3% of people who had it. If that generalises, the modal patient in an IBS trial has several other central-sensitisation conditions, none of which the trial measures. Cross-links fibromyalgia and migraine.

Why this is scoped as overlap, not as a separate condition

Three arguments, all supported above:

  1. Co-occurrence is the norm, not the exception — 34.6–55.3% depending on criteria and setting (Andreev 2022, PMID 36286762; Barberio 2022, PMID 33839276).
  2. The overlap predicts outcome better than either label alone (Barberio 2022, PMID 33839276).
  3. Latent-class work on the neighbouring functional bowel disorders reached the same structural conclusion — that the separate Rome categories may be better characterised as a spectrum than as discrete entities (Black 2022, PMID 35531932).

Against that, Rome V retained functional dyspepsia as a distinct gastroduodenal disorder, at 8.1% global prevalence and the most prevalent gastroduodenal DGBI (Sperber 2026, PMID 42613194). This knowledge base curates IBS; functional dyspepsia is described here only where it changes what is true about IBS. Comprehensive coverage of functional dyspepsia as a condition in its own right — its subtypes (epigastric pain syndrome, postprandial distress syndrome), its treatments, and its own controversies — is out of scope and would be a separate condition. A dedicated primer exists (Enck 2017, PMID 29099093).

Treatment implications

The only guidance addressing the overlap directly is the Asia-Pacific Association of Gastroenterology working-group guideline, which builds management algorithms for FD overlapping with GERD, epigastric pain syndrome with IBS, postprandial distress syndrome with IBS, and FD with constipation, on the explicit premise that "an overlap of symptom categories frequently occurs and has a negative impact on treatment outcomes" and that "there is a lack of guidance on their management" (Gwee 2023, PMID 36321167). Western guidelines (guidelines) do not address the overlap.

Practically, the treatments with the best evidence in IBS act on the shared axis rather than on the bowel specifically — gut–brain neuromodulators (gut-brain-neuromodulators) and brain–gut behavioural therapies (psychological-therapy) — which is consistent with, though not proof of, a shared mechanism.

Open questions

  • Does concurrent functional dyspepsia modify treatment response in IBS? Over half of IBS patients have it (Barberio 2022, PMID 33839276); a targeted PubMed search on 2026-09-02 retrieved no IBS treatment trial stratified by concurrent functional dyspepsia.
  • Why does Rome IV increase measured overlap by ~10 points relative to Rome III (Andreev 2022, PMID 36286762)?
  • Is the IBS–FD–heartburn triple overlap a distinct entity, as its stability across two survey waves and its asthma/sleep/distress profile suggest (Jones 2022, PMID 35293084)?
  • Why does one enteric infection produce excess IBS and excess dyspepsia eight years later (Ford 2010, PMID 20117111; Marshall 2010, PMID 20427395)? No mechanism has been established that predicts which compartment is affected.
  • Should the overlap be a diagnostic category with its own management pathway, as APAGE proposes (Gwee 2023, PMID 36321167) and Western guidelines do not?
  • Is duodenal eosinophil/mast-cell activation the shared mechanism of IBS and functional dyspepsia (Talley 2025, PMID 40586878)? The hypothesis is explicit, converging and untested as a causal claim.
  • If only 3% of primary-care IBS patients have IBS alone (Wang 2023, PMID 36807964), is "IBS" a useful unit of analysis for trials at all?
  • Why is gastro-oesophageal reflux four-fold commoner in IBS across every region and criteria set (Lovell 2012, PMID 23032982)? The authors call the reasons "speculative".
  • Does bloating mark a shared mechanism? It is an independent risk factor for overlap from both directions (Choi 2017, PMID 28160607).

References

  1. Andreev DN, et al. Prevalence of combination of functional dyspepsia and irritable bowel syndrome: a meta-analysis of studies using the Rome III-IV Criteria. Ter Arkh. 2022;94(9):1099-1108. PMID 36286762
  2. Barberio B, Yiannakou Y, Houghton LA, Black CJ, Savarino EV, Ford AC. Overlap of Rome IV Irritable Bowel Syndrome and Functional Dyspepsia and Effect on Natural History: A Longitudinal Follow-Up Study. Clin Gastroenterol Hepatol. 2022;20(2):e89-e101. PMID 33839276
  3. Choi YJ, et al. Overlap between irritable bowel syndrome and functional dyspepsia including subtype analyses. J Gastroenterol Hepatol. 2017;32(9):1553-1561. PMID 28160607
  4. Jones MP, Shah A, Walker MM, Koloski NA, Holtmann G, Talley NJ. Overlap of heartburn, functional dyspepsia, and irritable bowel syndrome in a population sample: Prevalence, temporal stability, and associated comorbidities. Neurogastroenterol Motil. 2022;34(9):e14349. PMID 35293084
  5. von Wulffen M, et al. Overlap of Irritable Bowel Syndrome and Functional Dyspepsia in the Clinical Setting: Prevalence and Risk Factors. Dig Dis Sci. 2019;64(2):480-486. PMID 30368683
  6. Nakov R, et al. Prevalence of Irritable Bowel Syndrome, Functional Dyspepsia and their Overlap in Bulgaria: a Population-Based Study. J Gastrointestin Liver Dis. 2020;29(3):329-338. PMID 32919417
  7. Ford AC, Thabane M, Collins SM, Moayyedi P, Garg AX, Clark WF, Marshall JK. Prevalence of uninvestigated dyspepsia 8 years after a large waterborne outbreak of bacterial dysentery: a cohort study. Gastroenterology. 2010;138(5):1727-36. PMID 20117111
  8. Marshall JK, et al. Eight year prognosis of postinfectious irritable bowel syndrome following waterborne bacterial dysentery. Gut. 2010;59(5):605-11. PMID 20427395
  9. Porcari S, et al. Prevalence of irritable bowel syndrome and functional dyspepsia after acute gastroenteritis: systematic review and meta-analysis. Gut. 2024;73(9):1431-1440. PMID 39013599
  10. Futagami S, Itoh T, Sakamoto C. Systematic review with meta-analysis: post-infectious functional dyspepsia. Aliment Pharmacol Ther. 2015;41(2):177-88. PMID 25348873
  11. Gwee KA, et al. Asia-Pacific guidelines for managing functional dyspepsia overlapping with other gastrointestinal symptoms. J Gastroenterol Hepatol. 2023;38(2):197-209. PMID 36321167
  12. Enck P, et al. Functional dyspepsia. Nat Rev Dis Primers. 2017;3:17081. PMID 29099093
  13. Black CJ, Houghton LA, Ford AC. Latent class analysis does not support the existence of Rome IV functional bowel disorders as discrete entities. Neurogastroenterol Motil. 2022;34(11):e14391. PMID 35531932
  14. Sperber AD, et al. Rome V global epidemiology and validation survey. Gut. 2026 Aug 18 (online ahead of print). PMID 42613194
  15. Vanuytsel T, Bercik P, Boeckxstaens G. Understanding neuroimmune interactions in disorders of gut-brain interaction. Gut. 2023;72(4):787-798. PMID 36657961
  16. Talley NJ, Duncanson K, Williams GM. Why are disorders of gut-brain interaction (DGBI) often food-related? Duodenal eosinophils and mast cells, small intestinal bacteria, food allergy and altered food intake in functional dyspepsia and the irritable bowel syndrome: a new paradigm. J Gastroenterol. 2025;60(10):1211-1220. PMID 40586878
  17. Vanheel H, et al. Impaired duodenal mucosal integrity and low-grade inflammation in functional dyspepsia. Gut. 2014;63(2):262-71. PMID 23474421
  18. Vanheel H, et al. Duodenal acidification induces gastric relaxation and alters epithelial barrier function by a mast cell independent mechanism. Sci Rep. 2020;10:17448. PMID 33060783
  19. Schol J, et al. The effect of corticotropin-release hormone on duodenal permeability and immune activation in healthy volunteers in a double-blind placebo-controlled study. Am J Physiol Gastrointest Liver Physiol. 2025;328(5):G457-G464. PMID 40167262
  20. Schol J, et al. The Six-food Elimination Diet Improves Symptoms of Functional Dyspepsia. Clin Gastroenterol Hepatol. 2025 Dec 5 (online ahead of print). PMID 41354185
  21. Holtmann G, Talley NJ. Pathophysiology of Functional Gastrointestinal Disorders: A Holistic Overview. Dig Dis. 2017;35 Suppl 1:5-13. PMID 29421808
  22. Pasricha PJ. Functional Dyspepsia. N Engl J Med. 2026;394(2):166-176. PMID 41499733
  23. Lovell RM, Ford AC. Prevalence of gastro-esophageal reflux-type symptoms in individuals with irritable bowel syndrome in the community: a meta-analysis. Am J Gastroenterol. 2012;107(12):1793-801. PMID 23032982
  24. Wang XJ, Ebbert JO, Loftus CG, Rosedahl JK, Philpot LM. Comorbid extra-intestinal central sensitization conditions worsen irritable bowel syndrome in primary care patients. Neurogastroenterol Motil. 2023;35(4):e14546. PMID 36807964
  25. Gao J, et al. Low FODMAP Diet Improves Colonic Barrier Function and Mast Cell Activation in Patients With Diarrhea-Predominant Irritable Bowel Syndrome: A Mechanistic Trial. Gastroenterology. 2026;170(1):132-147. PMID 40749856
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