The Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols Diet Is Effective in Irritable Bowel Syndrome With Constipation: A Single Center Tertiary Care Clinical Practice Evaluation

Abstract

Background/Aims

The low fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAP) diet is a well-researched treatment for diarrhea-predominant irritable bowel syndrome (IBS-D), but its value for constipation-predominant IBS (IBS-C) is uncertain. Clinicians may hesitate to recommend it for IBS-C, fearing it could worsen constipation or offer little relief. This study aims to assess the diet’s effectiveness across all IBS subtypes, specifically investigating its impact on stool patterns in IBS-C and the correlation with symptom relief.

Methods

This prospective service evaluation involved 294 IBS patients (84% female, mean age 44.6) attending dietitian-led group sessions on FODMAP restriction (baseline) and reintroduction (follow-up). The primary outcome was “satisfactory relief of gut symptoms” via the global symptom question; secondary outcomes included changes in stool consistency, IBS subtype, and gastrointestinal symptoms.

Results

At baseline, 23.8% (70/294) had IBS-C and 35.7% IBS-D (105/294). All subtypes except IBS-M showed significant symptom relief at follow-up (P < 0.001), with no statistically significant difference between IBS-C (50.7%, 35/69) and IBS-D (44.8%, 47/105) (P = 0.158). After FODMAP restriction, 50.7% (35/69) IBS-C remained IBS-C, while 44.9% (31/69) shifted to unclassified IBS (IBS-U). Those who shifted (IBS-C to IBS-U) reported greater relief (74.2%, 23/31 vs 45.7% 16/35, P = 0.019) and reduced pain (P = 0.006), unlike those who remained IBS-C (P = 0.180).

Conclusions

IBS-C and IBS-D showed similar symptom relief after FODMAP restriction. Notably, 44.9% of IBS-C patients shifted to normal stool consistency (IBS-U), with greater symptom relief and reduced pain. FODMAP restriction does not necessarily exacerbate constipation in IBS-C and remains a viable treatment option in real world clinical practice.

INTRODUCTION

Irritable bowel syndrome (IBS) is a chronic functional gut disorder affecting ~4% of the global population,¹ characterized by recurrent abdominal pain and altered bowel habits, as defined by the Rome IV criteria.² The pathophysiology of IBS is not fully understood but it is recognized as a disorder of gut–brain interaction influenced by biopsychosocial factors.³

IBS is classified into 4 subtypes; diarrhea predominant (IBS-D), constipation predominant (IBS-C), mixed stool consistency; both diarrhea and constipation predominant (IBS-M), and unclassified; neither diarrhea nor constipation predominant (IBS-U).² IBS-C constitutes approximately one-third of all IBS subtype cases worldwide, 1 and in the United Kingdom.⁴ IBS-C is defined by more than 25% of bowel movements being Bristol stool form scale (BSFS) types 1-2 and less than 25% being BSFS types 6-7.2

Many IBS patients view diet as both the cause and solution to their symptoms, and they generally prefer dietary therapy over medications.⁵ In contrast, patients with IBS-C often report low satisfaction with over-the-counter laxatives and fiber supplements.⁶ Current IBS management guidelines recommend diet therapies, including the fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAP) diet, prior to any first line pharmacological interventions in all IBS irrespective of subtype.^7,8

The FODMAP diet was more effective than other diets for IBS in a network meta-analysis of 13 randomized controlled trials (RCTs), but most studies focused on IBS-D, limiting the evidence for its effectiveness in IBS-C.⁹ Furthermore, concerns exist that the FODMAP diet may exacerbate or fail to alleviate constipation symptoms in IBS-C.¹⁰ The lack of evidence and unknown impact on constipation may create hesitancy among clinicians when recommending the FODMAP diet to patients with IBS-C in clinical practice, and highlights a research practice gap.

In most healthcare settings, including the United Kingdom’s National Health Service, IBS-C is included in dietitian-led FODMAP treatment pathways in line with current guidelines.^7,8 Our center treats a large tertiary IBS population using a group FODMAP education program that accommodates all IBS subtypes.^11-13 Expanding research on IBS subtypes could support the development of subtype-specific guidelines, enabling more precise and high-quality nutrition care.

The aim of this study is (1) to compare the effectiveness of the restriction phase of the FODMAP diet across all IBS subtypes (IBS-D, IBS-C, IBS-M, and IBS-U), and (2) to describe the impact FODMAP restriction had on stool consistency in IBS-C and to explore the relationship between these changes and treatment outcomes.

We utilized data previously published in a FODMAP pathway comparison study in routine clinical care,¹¹ however this study presents new data on a different and specific topic with new insights.

MATERIALS AND METHODS

Study Design, Population, and Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols Diet Advice

This study represents a sub-analysis of our previous 4-week vs 8-week FODMAP group pathway service evaluation where full details of the original study and administration of the FODMAP diet in a group setting can be found.¹¹ However to briefly summarise the methodology; a prospective, observational design was used to evaluate the effectiveness of group-based FODMAP diet education, within a tertiary gastroenterology service, requiring patients to follow FODMAP restriction for 4-8 weeks. Patients with IBS diagnosed by gastroenterologists using National Institute of Health and Clinical Excellence criteria¹⁴ were referred to a dietitian for FODMAP guidance. Those suitable for group sessions participated, while those with specific needs (eg, non-English speaking) received individual appointments and were excluded from this analysis. The study took place before the COVID-19 pandemic, with all appointments conducted face-to-face. Group sessions were led by an expert dietitian in the FODMAP diet. The initial session (baseline) focused on FODMAP restriction, while the follow-up session covered FODMAP reintroduction and personalization. Patients received educational materials and had opportunities to ask questions.

Data Collection and Questionnaires

Demographic data and appointment dates were collected from electronic records while patient-reported outcomes were assessed using questionnaires (see below) completed at baseline and follow-up (after FODMAP restriction) as previously described.¹¹ Patients who missed the follow-up or did not complete questionnaires were excluded.

Questionnaires consisted of the following measures; global symptom relief assessed using the global symptom question (GSQ), a single question on overall symptom satisfaction.¹⁵ Patients who responded “yes” were deemed to have achieved satisfactory relief of their IBS symptoms. Gastrointestinal symptom severity measured using the gastrointestinal symptom rating scale (GSRS), which evaluates the severity of 11 individual GI symptoms on a 4-point Likert scale with responses choices of: “none,” “mild,” “moderate,” or “severe” symptoms.¹⁶ IBS subtype was determined based on stool consistency assessed using the BSFS17 and Rome IV criteria² at baseline and follow-up.

Dietary adherence was assessed using a 5-point Likert scale as responding to the statement “I followed the diet” (never, occasionally [a quarter of the time], half the time, often [3 quarters of the time], and always). In this study dietary adherence was classified as those responding > 50% of the time, that is, “half the time,” “often,” and “always.”

Ethical Statement

Ethical approval was not required for this observational evaluation of routine clinical practice, in accordance with Health Research Authority guidance and its decision tool. The project was therefore registered with the Gastroenterology Medicine Clinical Effectiveness and Audit team in line with local governance procedures for service evaluations.

Statistical Methods

The primary outcome was “satisfactory relief of gut symptoms” measured by the GSQ,¹⁵ used to compare the effectiveness of the restriction phase of the FODMAP diet across all IBS subtypes. Mc-Nemar’s test was used to analyze the improvement in satisfactory relief from baseline to follow up (after FODMAP restriction) in each individual IBS subtype. To determine any differences in satisfactory relief at follow up across IBS subtypes the proportion (%) of those achieving satisfactory relief in each individual subtype were compared to each other using a chi-squared test.

Our secondary outcomes included measures describing the impact of the FODMAP restriction in patients with IBS-C at baseline. Firstly, changes in stool consistency (BSFS) and related IBS-C subtype at follow-up were examined descriptively. Next, satisfactory relief (GSQ) at follow-up was compared using McNemar’s test between patients who shifted from IBS-C to IBS-U (ie, from > 25% BSFS types 1-2 to > 75% BSFS types 3-5, indicating normal stool consistency²) and those who remained IBS-C. Secondly to analyze reductions in individual GI symptoms within each IBS subtype, GSRS scores were dichotomized: “none” and “mild” as “no moderate/severe symptoms,” and “moderate” and “severe” as “presence of moderate/severe symptoms.” McNemar’s test was then applied to assess reductions in moderate/severe symptoms from baseline to follow-up. Thirdly to further explore the relationship between stool consistency changes and satisfactory relief, GSRS symptom changes from baseline to follow-up were compared between those who shifted from IBS-C to IBS-U and those who remained IBS-C, using McNemar’s test. All statistical analyses were conducted using IBM SPSS Statistics Version 26. A P-value < 0.05 was considered to be statistically significant.

RESULTS

Participants

We included 294 patients with IBS, ages ranged from 18 years to 86 years (44.6 ± 15.5) and 246 (84%) were female. IBS subtypes at baseline were IBS-D (105/294, 35%), followed by IBS-U (87/294, 30%), IBS-C (70/294, 24%), and IBS-M (32/294, 11%). Ten additional patients were included in this analysis compared to the original publication (n = 284)¹¹ due to these patients being excluded from the original data set due to lacking data on appointment times required for the purpose of the original study. Dietary adherence was assessed in 292 of 294 patients (2 were excluded due to missing data). The overall adherence rate was 78% (230/292). These levels were consistent across all IBS subtypes, including patients who switched subtypes following FODMAP restriction (Supplementary Information).

Effectiveness of Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols Restriction Diet Across All Irritable Bowel Syndrome Subtypes

Satisfactory relief in individual irritable bowel syndrome subtypes

Following FODMAP restriction, the proportion of patients reporting satisfactory symptom relief significantly increased across all IBS subtypes except IBS-M (Table 1). The highest rates of relief were observed in IBS-C, followed by IBS-U and IBS-D.

Table 1.

Satisfactory Relief Assessed by the Global Symptom Question at Baseline and Follow-up in Irritable Bowel Syndrome Subtypes Following Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols Restriction

IBS subtype	Baseline satisfactory reliefa number (%)	Follow-up satisfactory reliefa number (%)	P-value
IBS-C	6/70 (8.6%)	39/70 (55.7%)	< 0.001
IBS-D	12/103 (11.7%)	47/103 (45.6%)	< 0.001
IBS-M	4/32 (12.5%)	8/32 (25%)	0.204
IBS-U	21/87 (24.1%)	49/87 (56.3%)	< 0.001

^aSatisfactory relief assessed via global symptom question.

IBS, irritable bowel syndrome; IBS-C, constipation-predominant IBS; IBS-D, diarrhea-predominant IBS; IBS-M, IBS with mixed stool consistency (both diarrhea- and constipation-predominant); IBS-U, unclassified IBS (neither diarrhea- nor constipation-predominant).

P < 0.05 was considered statistically significant.

Satisfactory relief across all irritable bowel syndrome subtypes

IBS-C experienced the same level of satisfactory relief from FODMAP restriction compared to IBS-D and IBS-U. IBS-M experienced significantly less satisfactory relief than all other IBS subtypes (Table 2).

Table 2.

Comparison of Satisfactory Relief at Follow-up After Low-fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols Restriction Diet Across Irritable Bowel Syndrome Subtypes

IBS subtype at follow-up	n	Achieved satisfactory reliefa n (%)	Comparison vs IBS-C	Comparison vs IBS-D	Comparison vs IBS-U
IBS-C	70	39 (55.7%)	–	P = 0.1588	P = 0.9402
IBS-D	105	47 (44.8%)	P = 0.1588	–	P = 0.1136
IBS-U	87	49 (56.3%)	P = 0.9402	P = 0.1136	–
IBS-M	32	8 (25.0%)	P = 0.0041	P = 0.0463	P = 0.0025

^aSatisfactory relief assessed via global symptom question.

IBS, irritable bowel syndrome; IBS-C, constipation-predominant IBS; IBS-D, diarrhea-predominant IBS; IBS-M, IBS with mixed stool consistency (both diarrhea- and constipation-predominant); IBS-U, unclassified IBS (neither diarrhea- nor constipation-predominant).

P < 0.05 was considered statistically significant.

Impact of Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols Restriction on Stool Consistency in Constipation-predominant Irritable Bowel Syndrome and Relationship to Satisfactory Relief

After FODMAP restriction, 50.7% (35/69) of patients with IBS-C remained IBS-C, while 44.9% (31/69) shifted to IBS-U (Figure). Smaller proportions changed to IBS-M (2.9%, 2/69) or IBS-D (1.4%, 1/69). At follow up satisfactory relief was achieved in 45.7% (16/35) of “IBS-C to IBS-C” but in 74.2% (23/31) of “IBS-C to IBS-U” highlighting significantly more improvement in satisfactory relief for IBS-C patients when constipation is also improved (P = 0.019) (Figure). Individually both IBS-C to IBS-C and IBS-C to IBS-U achieved significant improvements in satisfactory relief at follow up (both P < 0.001).

Figure.

Impact of fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAP) restriction on stool consistency in constipation-predominant irritable bowel syndrome (IBS-C) and relationship to satisfactory relief. Among 69 patients with IBS-C who underwent a low-FODMAP diet, 44.9% (31/69) transitioned to an unclassified IBS (IBS-U) subtype. These patients achieved significantly more satisfactory symptom relief compared to those who remained IBS-C (P = 0.019). GSQ, global symptom question.

Gastrointestinal Symptoms

Gastrointestinal symptom response in constipationpredominant irritable bowel syndrome and constipationpredominant irritable bowel syndrome to unclassified irritable bowel syndrome

As we observed increased satisfactory relief in IBS-C patients who changed to IBS-U compared to those who remained constipated (IBS-C to IBS-C), we could assume this was due to the improvement in stool consistency. However, to understand if there were any additional factors that were different between IBS-C to IBS-C and IBS-C to IBS-U that could explain why there was more satisfactory relief, we compared GI symptoms between these 2 groups (Table 3). We did not include the IBS-C to IBS-M or IBS-C to IBS-D due to the small numbers (n = 3).

Table 3.

Reduction of Presence of Moderate/Severe Gastrointestinal Symptoms at Baseline Compared to After Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols Restriction in Constipation-predominant Irritable Bowel Syndrome Who Changed to Unclassified Irritable Bowel Syndrome and Who Remained Constipation-predominant Irritable Bowel Syndrome

Symptom	Group	Baseline n/N (%)	Follow-up n/N (%)	P-value
Pain	IBS-C → IBS-C	21/35 (60%)	15/35 (43%)	0.180
	IBS-C → IBS-U	16/31 (52%)	6/31 (19%)	0.006
Bloating/distention	IBS-C → IBS-C	31/35 (89%)	17/35 (49%)	< 0.001
	IBS-C → IBS-U	23/31 (74%)	4/31 (13%)	< 0.001
Flatulence	IBS-C → IBS-C	27/35 (77%)	10/35 (29%)	< 0.001
	IBS-C → IBS-U	18/31 (58%)	5/31 (16%)	0.002
Belching	IBS-C → IBS-C	8/35 (23%)	3/35 (9%)	0.063
	IBS-C → IBS-U	6/31 (19%)	2/31 (6%)	0.219
Borborygmi	IBS-C → IBS-C	15/35 (43%)	4/35 (11%)	< 0.001
	IBS-C → IBS-U	15/31 (48%)	6/31 (19%)	0.012
Urgency	IBS-C → IBS-C	13/35 (37%)	8/35 (23%)	0.180
	IBS-C → IBS-U	10/31 (32%)	6/31 (19%)	0.289
Incomplete evacuation	IBS-C → IBS-C	18/35 (51%)	19/35 (54%)	> 0.999
	IBS-C → IBS-U	12/31 (39%)	6/31 (19%)	0.070
Nausea	IBS-C → IBS-C	10/35 (29%)	8/35 (23%)	0.774
	IBS-C → IBS-U	4/31 (13%)	1/31 (3%)	0.375
Heartburn	IBS-C → IBS-C	9/35 (26%)	3/35 (9%)	0.109
	IBS-C → IBS-U	5/31 (16%)	2/31 (6%)	0.250
Acid reflux	IBS-C → IBS-C	11/35 (31%)	5/35 (14%)	0.109
	IBS-C → IBS-U	4/31 (13%)	3/31 (10%)	> 0.999
Lethargy	IBS-C → IBS-C	26/35 (74%)	17/35 (49%)	0.004
	IBS-C → IBS-U	19/31 (61%)	8/31 (26%)	0.003

IBS-C, constipation-predominant irritable bowel syndrome (IBS); IBS-U, unclassified IBS (neither diarrhea- nor constipation-predominant).

P < 0.05 was considered statistically significant.

When comparing a reduction in moderate/severe symptoms following FODMAP restriction using the GSRS we found a significant reduction in pain in IBS-C to IBS-U (P = 0.006) but not in IBS-C to IBS-C (P = 0.180). Bloating/distention, flatulence, borborygmi and lethargy reduced significantly in both groups (P < 0.05 for all), while belching, urgency, incomplete evacuation, nausea, heartburn and acid reflux did not reduce significantly in either group (Table 3). Although bloating/distention significantly reduced in both groups from baseline to follow up only 4/31 (13%) with IBS-C to IBS-U experienced bloating, while half still had bloating whom remained IBS-C (17/35, 49%). Similarly, it is worth noting that the symptom of incomplete evacuation did not change in IBS-C to IBS-C (18/35, 51% to 19/35, 54%) following FODMAP restriction, whereas in those who changed to IBS-U this symptom improved in half of patients (12/31, 39% to 6/31, 19%) (Table 3).

Gastrointestinal symptom response in all irritable bowel syndrome-subtypes

Regarding a reduction in moderate/severe GI symptoms measured using the GSRS, all subtypes experienced a reduction in bloating/distention and lethargy (P < 0.05 for all). All subtypes except IBS-M experienced a significant decrease in the presence of moderate/severe pain, flatulence, belching and borborygmi (P < 0.05 for all). All subtypes except IBS-C experienced a reduction in urgency (P < 0.05 for all). All subtypes except IBS-D experienced a reduction in heartburn (P < 0.05 for all). Only IBS-D experienced a reduction in incomplete evacuation. Finally, only IBS-U experienced a reduction in acid reflux and nausea (Table 4).

Table 4.

Reduction of Presence of Moderate/Severe Gastrointestinal Symptoms at Baseline Compared to After Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols Restriction in All Individual Irritable Bowel Syndrome Subtypes

Symptom (GSRS)	Timepoint	IBS-C n (%)	IBS-D n (%)	IBS-M n (%)	IBS-U n (%)
Pain	Baseline	41/70 (59%)	65/102 (64%)	23/34 (68%)	46/87 (53%)
	Follow-up	24/70 (34%)	35/102 (34%)	17/34 (50%)	21/87 (24%)
	P-value	0.002	< 0.001	0.109	< 0.001
Bloating/ Distention	Baseline	58/70 (83%)	80/103 (78%)	27/34 (79%)	55/87 (63%)
	Follow-up	23/70 (33%)	43/103 (42%)	19/34 (56%)	25/87 (29%)
	P-value	< 0.001	< 0.001	0.008	< 0.001
Flatulence	Baseline	49/70 (70%)	68/104 (65%)	24/34 (71%)	59/87 (68%)
	Follow-up	17/70 (24%)	38/104 (37%)	17/34 (50%)	18/87 (21%)
	P-value	< 0.001	< 0.001	0.092	< 0.001
Belching	Baseline	16/70 (23%)	36/103 (35%)	15/33 (45%)	26/88 (29%)
	Follow-up	6/70 (9%)	16/103 (16%)	14/33 (42%)	12/88 (14%)
	P-value	0.013	< 0.001	> 0.999	0.003
Borborygmi	Baseline	31/69 (44%)	55/101 (54%)	17/33 (52%)	35/86 (41%)
	Follow-up	11/69 (16%)	29/101 (29%)	15/33 (45%)	18/86 (21%)
	P-value	< 0.001	< 0.001	0.774	0.002
Urgency	Baseline	26/70 (37%)	79/106 (74%)	26/34 (76%)	32/87 (37%)
	Follow-up	17/70 (24%)	47/106 (44%)	17/34 (50%)	20/87 (23%)
	P-value	0.064	< 0.001	0.004	0.031
Incomplete Evacuation	Baseline	33/70 (47%)	49/103 (48%)	20/32 (63%)	29/86 (34%)
	Follow-up	28/70 (40%)	35/103 (34%)	15/32 (47%)	21/86 (24%)
	P-value	0.405	0.026	0.125	0.152
Nausea	Baseline	15/69 (21%)	20/104 (19%)	15/32 (47%)	23/87 (26%)
	Follow-up	11/69 (16%)	16/104 (15%)	9/32 (28%)	12/87 (14%)
	P-value	0.481	0.503	0.070	0.013
Heartburn	Baseline	14/70 (20%)	24/103 (23%)	15/33 (45%)	14/87 (16%)
	Follow-up	5/70 (7%)	17/103 (16%)	3/33 (9%)	6/87 (7%)
	P-value	0.022	0.230	< 0.001	0.008
Acid Reflux	Baseline	15/69 (22%)	22/103 (21%)	14/32 (44%)	19/87 (21%)
	Follow-up	8/69 (12%)	15/103 (15%)	8/32 (25%)	5/87 (6%)
	P-value	0.092	0.167	0.070	< 0.001
Lethargy	Baseline	47/70 (67%)	82/105 (78%)	32/34 (94%)	53/87 (61%)
	Follow-up	28/70 (40%)	53/105 (50%)	23/34 (68%)	32/87 (37%)
	P-value	< 0.001	< 0.001	0.012	< 0.001

GSRS, gastrointestinal symptom rating scale; IBS-C, constipation-predominant irritable bowel syndrome (IBS); IBS-D, diarrhea-predominant IBS; IBS-M, IBS with mixed stool consistency (both diarrhea- and constipation-predominant); IBS-U, unclassified IBS (neither diarrhea- nor constipation-predominant).

P < 0.05 was considered statistically significant.

DISCUSSION

This real-world evaluation of dietitian-led group FODMAP education demonstrated that patients with both IBS-C and IBS-D experienced meaningful improvements in global symptoms. Among those with IBS-C, approximately half maintained their subtype following dietary restriction, while nearly half transitioned to a more normal stool pattern (IBS-U). Improvements in stool consistency were accompanied by greater relief of abdominal pain and overall symptoms, indicating that FODMAP restriction may improve outcomes in constipation-predominant IBS without exacerbating constipation.

Results on the FODMAP diet in IBS-C are contradictory. Halmos and colleagues¹⁸ demonstrated the efficacy of a FODMAP diet in a cohort with more IBS-C than IBS-D, with improvements observed in both subjective and objective stool consistency. Despite these seminal results, theories evolved suggesting FODMAP restriction will exacerbate constipation due to reduction in small bowel water motility content and decreased colonic fermentation,¹⁹ despite no mechanistic studies being performed that can validate this hypothesis. Although reduced fiber intake during FODMAP restriction is often cited as a risk for worsening constipation, evidence remains inconsistent.²⁰ A controlled feeding trial found that fiber supplementation did not improve bowel symptoms or quality of life despite increased stool water content.²¹ In our group sessions, patients were advised to substitute high-FODMAP foods with balanced low-FODMAP alternatives rather than increase fiber specifically. Nonetheless, IBS-C patients improved stool consistency without targeted fiber advice, suggesting this concern may be less clinically significant. Furthermore, in the RCT conducted by Böhn et al,²² all IBS subtypes experienced a reduction in IBS symptom severity from FODMAP restriction. While the RCT led by Pedersen et al²³ found IBS-C did not improve following FODMAP diet, however, as the authors point out with only 5 IBS-C included the lack of effect is likely due to lack of power. More recently a smartphone app that supports users to lower FODMAP intake (ie, a modified FODMAP diet) improved overall IBS symptom severity in a higher percentage of IBS-C than other IBS subtypes at both short term and long term follow up,²⁴ providing evidence for manipulating FODMAP intake in the primary care IBS-C patient. One study provided all low FODMAP meals for patients and found the restriction of FODMAPs showed similar benefits for IBS-C and IBS-D.²⁵ Furthermore in clinical practice group FODMAP education sessions improvements across all IBS subtypes have been previously reported. 12,26 Taken together, and with additional clinical practice data added from our current study, the available evidence suggests the FODMAP diet should be used in IBS-C in clinical practice to improve global symptoms.

A novel finding of this study was that patients who transitioned from IBS-C to IBS-U following FODMAP restriction reported greater satisfactory relief and pain reduction compared with those who remained IBS-C (Figure). Previous evidence indicates that reductions in constipation can predict decreases in abdominal pain,²⁷ which may explain the higher rates of satisfactory relief among patients whose bowel habits normalized. Clinically, those who shifted from IBS-C to IBS-U also showed larger, though not statistically significant, reductions in bloating, distension, and incomplete evacuation (Table 3), all factors likely contributing to overall symptom improvement. Consistent with our findings, real-world studies have similarly reported improvements in stool consistency following FODMAP restriction. Nawawi et al²⁸ observed normalization from IBS-C (BSFS 1-2) to IBS-U (BSFS 3-4) in 47% of patients, while De Roest et al²⁹ reported improvement in 52% of those with hard stools prior to the diet. Likewise, in our cohort, 45% (31/69) of IBS-C patients transitioned to IBS-U following FODMAP restriction (Figure).

Future studies should clarify whether improvements in stool consistency, pain, or the combination of both were responsible for increased satisfactory relief. According to the U.S. Food and Drug Administration (FDA) guidance for IBS-C therapeutics, a clinical “response” is defined by concurrent improvement in stool consistency (or frequency) and at least a 30% reduction in abdominal pain.³⁰ Conley et al³¹ have proposed applying these more rigorous FDA-defined endpoints to low FODMAP diet trials, as their adoption could enhance the assessment of efficacy and better distinguish responders from non-responders.

Despite FODMAP restriction, some patients remained IBS-C and continued to report incomplete evacuation and pain (Table 3). This is clinically relevant as it highlights a group of patients who require further support. IBS guideline management suggest introducing laxatives after dietary advice and suggest further investigation for symptoms suggestive of a defecatory disorder,^7,8 while further dietary interventions specifically aimed at constipation could be implemented. 32 Data on laxative and/or medication use and/or overlapping anorectal disorders were not collected, preventing further analysis of their potential impact on constipation symptoms.

The limitations of an observational study conducted in a single center in routine clinical practice must be acknowledged. Stool consistency measures (BSFS) to determine IBS subtype were only taken once at baseline and once at follow up. Whereas 2 weeks is suggested to define IBS subtypes in research,³³ therefore the accuracy of the IBS subtypes has limitations. How much the change we observed in subtype was due to natural fluctuations,³⁴ and not FODMAP restriction, is impossible to say without a control group, as is any placebo effect. Additionally, there are a range of factors that could have influenced outcomes of satisfactory relief and changes in stool consistency in IBS-C not related to the FODMAP diet. For example, differences in baseline diets, differences in FODMAP intake during the diet, fibre intake during FODMAP restriction, use/changes of medications (either laxatives to improve constipation or other medications that may have contributed to constipation), and probiotics/prebiotics, all of which were all not assessed in this study and may explain individual patient discrepancies which should be evaluated in controlled trials.

In conclusion, overall 55.7% of IBS-C achieved satisfactory relief of symptoms following FODMAP restriction in a dietitian led group education setting, with no difference compared to IBS-D. Additionally 45.7% changed stool pattern from abnormal (IBS-C) to normal (IBS-U), highlighting FODMAP restriction does not necessarily increase constipation in IBS-C. Higher rates of satisfactory relief and pain reduction were achieved in IBS-C who improved stool consistency to IBS-U when compared to IBS-C who remained constipated. This novel finding should be further explored to help understand response rates. Despite encouraging findings from our clinical practice and existing literature, controlled studies evaluating the efficacy of the FODMAP diet in IBS-C compared with IBS-D remain limited. Moreover, the long-term effectiveness and sustainability of this dietary approach are poorly understood, underscoring an important gap between research and clinical practice. For clinical practice, clinicians should not shy away from offering a dietitian led FODMAP diet to the appropriate patient with IBS-C. Monitoring the diets impact at follow up is essential, especially considering some patients may continue with hard stool consistency that may impact overall satisfactory relief and/or reduction in pain, which may require further targeted dietary or pharmacological treatment.

SUPPLEMENTARY MATERIAL

Note: To access the supplementary information mentioned in this article, visit the online version of Journal of Neurogastroenterology and Motility at http://www.jnmjournal.org/, and at http://doi.org/10.5056/jnm25126.