Gluten-Free vs. Low FODMAP Diets for Irritable Bowel Syndrome: What a Major Research Review Reveals

Table of Contents

• Key Points
• Understanding Irritable Bowel Syndrome
• Why Diet Matters for IBS Patients
• How This Research Was Conducted
• Key Findings: The Gluten-Free Diet
• Key Findings: The Low FODMAP Diet
• Low FODMAP vs. Specific Comparison Diets
• Quality of the Evidence (GRADE Assessment)
• What This Means for Patients
• Practical Recommendations
• Study Limitations
• Future Research Needs
• Frequently Asked Questions
• Source Information

Key Points

• A meta-analysis of nine RCTs found insufficient evidence to recommend a gluten-free diet for reducing IBS symptoms.
• A low FODMAP diet reduced persistent IBS symptom risk by 31% compared to control diets.
• Low FODMAP diet benefit was large versus high FODMAP or usual diets (54-56% risk reduction), smaller versus alternative diets.
• Overall evidence quality for both diets was rated very low due to bias, heterogeneity, and imprecision.
• The low FODMAP diet is meant as a short-term 2-6 week diagnostic test, not a permanent eating plan.

Understanding Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder—meaning the digestive tract looks normal on tests but functions abnormally. IBS is characterized by altered bowel habits (such as constipation and/or diarrhea), abdominal pain, and bloating, all without evidence of organic disease.

The condition is remarkably common. Studies estimate the worldwide prevalence of IBS is 10–20%, making it one of the most frequently diagnosed digestive conditions. Beyond the physical symptoms, IBS takes a significant toll on quality of life.

Research has also shown that IBS frequently co-exists with anxiety and depression. Patients with IBS often experience a stepwise increase in these mental health conditions alongside the severity and frequency of their digestive symptoms.

Diagnosis can be made using symptom-based criteria (such as the Rome criteria), rather than requiring invasive testing. There are several treatment approaches, including medications that target the gut, therapies that modify the microbiome (the community of bacteria living in the digestive tract), and psychological interventions.

Why Diet Matters for IBS Patients

Although pharmacological therapies can be effective, many patients seek additional options. Surveys suggest that about 50% of IBS patients use approaches other than conventional medical therapy, either alongside it or instead of it.

Diet is a particularly common focus. Over 60% of IBS patients report that specific foods trigger their symptoms. This makes sense—the digestive tract is directly exposed to everything we eat, and certain foods can draw water into the bowel, ferment in the colon, or trigger subtle immune and nerve responses.

Early attempts at diet therapy based on food sensitivity testing (such as IgG antibody tests) showed disappointing results in randomized trials. However, two newer dietary approaches have generated substantial interest and research:

• A gluten-free diet (GFD): Eliminating gluten, a protein found in wheat, barley, and rye. Some researchers have hypothesized that a subset of IBS patients may be sensitive to gluten even without having celiac disease—a condition sometimes called "non-celiac gluten sensitivity."
• A low FODMAP diet: Reducing foods high in fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs). These short-chain carbohydrates are poorly absorbed in the small intestine and rapidly fermented by gut bacteria, producing gas and drawing fluid into the colon. This is now the most widely adopted dietary approach for IBS.

Prior systematic reviews of these diets reached conflicting conclusions, but all agreed that more data was needed. Since those reviews were published, several new randomized controlled trials have emerged. This study was designed to provide an updated, evidence-based assessment of both diets.

How This Research Was Conducted

The researchers performed a systematic review—a rigorous, protocol-driven method of finding and evaluating all available studies on a topic—combined with a meta-analysis, which statistically pools the results of multiple studies to reach a more reliable overall conclusion.

Literature Search

The team searched four major medical databases:

• MEDLINE (1946 to November 2017)
• EMBASE (1974 to November 2017)
• Cochrane Central Register of Controlled Trials (through 2017)
• Cochrane Database of Systematic Reviews (2005 to November 2017)

They also included abstracts and conference proceedings from Digestive Diseases Week (2014 to 2017), one of the largest gastroenterology meetings in the world. There were no language restrictions—studies in any language were considered.

Search terms included "irritable bowel syndrome," "gluten free diet," "FODMAP," "fructan," "diet restriction," and many others, combined with terms for clinical trials and randomization.

Study Selection Criteria

For a study to be included, it had to be a randomized controlled trial (RCT) that evaluated an exclusion diet (either a gluten-free diet or a low FODMAP diet) versus an alternative diet, a high FODMAP diet, or a usual diet. Studies also had to report improvements in either global IBS symptoms or abdominal pain.

Two independent reviewers screened all citations, and a third reviewer resolved any disagreements. When important data was missing from published reports, the authors contacted study investigators directly.

Outcome Measures

The primary outcome was global improvement in IBS symptoms. If a study did not report global improvement, abdominal pain was used instead. When a study used multiple definitions of improvement, the researchers selected the most stringent definition—for example, a 50% improvement in symptoms was chosen over a 25% improvement—because stricter criteria minimize the placebo response.

Secondary outcomes included general quality of life and any occurrence of adverse events.

Statistical Methods

Data was analyzed on an intention-to-treat basis, meaning patients who dropped out were counted as treatment failures. This is a conservative approach that provides a more realistic picture of how well a treatment works in real life.

The results were expressed as relative risk (RR) with 95% confidence intervals (CI). A relative risk of less than 1.0 favors the diet being tested. Data was pooled using a random effects model, and heterogeneity (variation between study results) was measured with the I² statistic. An I² greater than 25% was considered significant heterogeneity.

The risk of bias in each study was assessed using the Cochrane Risk of Bias Tool, and the overall quality of evidence was graded using GRADE methodology (Grading of Recommendations Assessment, Development and Evaluation)—the world's most widely accepted system for rating evidence quality.

Key Findings: The Gluten-Free Diet

The literature search identified 1,726 citations. After screening and full-text review, 72 studies underwent full manuscript review, 63 were excluded, and nine studies met eligibility criteria.

Only two randomized controlled trials of a gluten-free diet were found, involving a total of 111 participants. Both trials used an interesting design:

1. First, patients who responded to a gluten-free diet were selected.
2. They were then randomized to either continue the gluten-free diet, or to continue the diet but with gluten secretly added back ("spiked").
3. This was a double-blinded challenge design—patient and researchers did not know who was receiving gluten.

In both trials, a greater proportion of participants experienced a return (exacerbation) of their IBS symptoms when their diet was spiked with gluten, compared with those who stayed on the pure gluten-free diet. Each individual trial reported statistically significant results.

However, when the two trials were pooled, the combined result was not statistically significant (RR = 0.42; 95% CI 0.11 to 1.55). The 95% confidence interval—which represents the range of plausible true effects—stretches from 0.11 to 1.55, crossing 1.0. A result crossing 1.0 means we cannot be confident the diet works.

The reason for this disappointing combined result was marked heterogeneity between the trials (I² = 88%), meaning the two studies produced very different results from each other. One trial was rated low risk of bias, while the other was rated unclear risk of bias.

Neither gluten-free diet trial reported on adverse events or general quality of life. This is a notable gap, given that eliminating gluten can be restrictive and potentially affect nutritional intake.

Key Findings: The Low FODMAP Diet

Seven randomized controlled trials compared a low FODMAP diet with various control interventions, involving a total of 397 participants. The control groups received an alternative diet, a high FODMAP diet, a usual diet, or placebo supplements.

Overall, the low FODMAP diet was associated with a statistically significant reduction in global IBS symptoms compared with control interventions (RR = 0.69; 95% CI 0.54 to 0.88; I² = 25%).

In plain language, this means that patients on a low FODMAP diet had a 31% lower risk of having persistent IBS symptoms than patients on control diets. The 95% confidence interval (0.54 to 0.88) does not cross 1.0, giving us confidence this effect is real, although the size of the effect is uncertain.

The heterogeneity between these seven trials was modest (I² = 25%), at the boundary of what the authors considered significant. This suggests the trials were reasonably consistent in their results.

Quality of Life Results

Only one low FODMAP trial reported general quality of life. That study found no statistically significant difference between the low FODMAP group and the control group (mean difference = 1.30; 95% CI = -6.82 to 9.42). Because the confidence interval crosses zero, any true difference in quality of life could be positive, negative, or nonexistent.

Adverse Events

Two trials reported on adverse events but did not present data that could be extracted for analysis. Both reported that the diets were well tolerated with no serious adverse events.

Notably, none of the seven low FODMAP trials were rated low risk of bias, primarily due to lack of blinding. The authors point out that even recent studies that made excellent efforts to maintain blinding were still classified as not blinded, because information about the low FODMAP diet is freely available on the internet and cell phone applications—meaning patients could easily figure out which diet they were on.

Low FODMAP vs. Specific Comparison Diets

To understand why the results varied between trials, the researchers conducted subgroup analyses. The single biggest factor driving differences between studies was the choice of comparison diet, with 52% of the variation in effect size between subgroups not due to chance.

Low FODMAP vs. an Alternative Diet

Three studies compared a low FODMAP diet to an "alternative" diet—typically another structured diet designed as a rigorous control, such as a diet based on standard dietary guidelines. These studies included 271 patients.

There was a trend toward the low FODMAP diet reducing global IBS symptoms compared with alternative diets (RR = 0.82; 95% CI = 0.66 to 1.02), but this did not reach statistical significance. The upper bound of 1.02 is just barely above 1.0, suggesting the effect may be real but smaller than previously thought.

Low FODMAP vs. a High FODMAP Diet

One study by McIntosh et al. compared a low FODMAP diet to a high FODMAP diet in 40 patients. This study found a significant benefit for the low FODMAP diet (RR = 0.44; 95% CI = 0.23 to 0.83)—a 56% reduction in persistent symptoms, the largest effect seen in any comparison.

Low FODMAP vs. a Usual Diet

Two studies compared the low FODMAP diet to a patient's regular (usual) diet, including a total of 71 patients. Significantly fewer patients had persistent IBS symptoms on the low FODMAP diet (RR = 0.46; 95% CI = 0.25 to 0.84)—a 54% reduction.

Low FODMAP Elimination Followed by Challenge vs. Placebo

One study by Hustoft et al. took a different approach. They first placed patients on a low FODMAP diet, then took the 15 patients who responded and randomized them to receive either a placebo supplement or a supplement containing FODMAPs. Patients receiving placebo maintained their symptom improvement, while those given FODMAPs saw symptoms return. However, this small study did not reach statistical significance (RR = 0.44; 95% CI = 0.11 to 1.71).

Quality of the Evidence (GRADE Assessment)

The researchers used the GRADE system—the most widely accepted framework for rating evidence quality, used by more than 100 organizations worldwide—to assess the overall confidence in these results.

The overall quality of the data was rated "very low" for both diets. This rating was driven by several factors:

• Risk of bias: Most studies were rated unclear or high risk of bias, largely because blinding was not possible in dietary trials.
• Heterogeneity: Study designs varied substantially, particularly in the choice of comparison diet.
• Imprecision: The number of patients with symptom improvement was modest.

Specifically, only 189 participants in the low FODMAP diet trials experienced an improvement in their symptoms. GRADE methodology would ideally require 300 events for the data to be classified as robust. This shortfall is a major reason why the evidence rating is not higher.

The authors note that these evidence gaps could be improved with further trials using similar comparators. However, they point out a practical challenge: dietary trials receive no pharmaceutical industry support, and most federal funding agencies do not rank IBS as a priority area for research funding. They also acknowledge that researchers in this field deserve credit for recruiting as many participants as they have under these constraints.

What This Means for Patients

So what does this all mean for someone living with IBS? Let's break it down by diet.

For the Gluten-Free Diet

The evidence is clear: there is insufficient evidence to recommend a gluten-free diet for reducing IBS symptoms.

While both individual gluten challenge trials were positive, their results were so different from each other that pooling them produced no statistically significant benefit. The authors also cite another randomized trial suggesting that any benefit patients experience from cutting out gluten may actually be due to the reduction in FODMAPs that happens when wheat, barley, and rye are removed from the diet.

For the Low FODMAP Diet

The low FODMAP diet is a different story. Based on very low quality evidence, it is effective in reducing global IBS symptoms. Importantly, the magnitude of benefit depends heavily on what it is compared to:

• Compared to a high FODMAP diet: large benefit (56% risk reduction)
• Compared to a usual diet: large benefit (54% risk reduction)
• Compared to an alternative structured diet: smaller, borderline benefit (18% risk reduction, not statistically significant)

This pattern makes sense. Part of the benefit of the low FODMAP diet likely comes from the structure and attention of following any dietary plan—the "placebo effect" of dietary intervention. But the fact that the diet still outperforms alternative structured diets, even if borderline, suggests there is something specific about reducing FODMAPs that helps IBS patients.

Proponents of the low FODMAP diet may be disappointed the evidence is not rated higher. But the authors are clear: of all available dietary interventions, a low FODMAP diet currently has the greatest evidence for efficacy in IBS.

Practical Recommendations

The study authors offer several concrete recommendations for patients and healthcare providers:

1. Treat the elimination phase as a diagnostic test. The low FODMAP diet should be followed strictly for 2–6 weeks. This is not meant to be a permanent way of eating—it is a test to find out whether you are sensitive to FODMAPs.
2. If you don't improve, stop the diet. Patients who fail to improve after the elimination phase should not continue the diet. It offers no benefit for them and unnecessarily restricts their food choices.
3. If you do improve, reintroduce foods. Patients who respond should be guided through a structured reintroduction phase, adding FODMAP-containing foods back one at a time to identify which specific types of FODMAPs trigger their symptoms.
4. Personalize and liberalize the diet. Once sensitivities are identified, the diet should be relaxed and tailored to the individual. This improves long-term adherence and minimizes any negative effects on the gut microbiome.
5. Work with a trained dietitian. The complexity of diet therapy, the potential for nutritional deficiencies, and the time needed for proper counseling are all strong reasons to involve a registered dietitian with expertise in IBS. If that isn't possible, patients should be given high-quality teaching materials so they can implement the diet in a medically responsible way.

Study Limitations

This review has several limitations that patients should be aware of when interpreting the results. The sample sizes within each trial were small, and some studies used a cross-over design (where patients receive both diets in sequence).

The cross-over design is particularly problematic because researchers do not know the exact mechanism by which exclusion diets improve IBS symptoms. The benefit could come from osmotic effects on the intestine, a subtle immune response to certain foods, or changes in the gut microbiome. Without understanding the mechanism, it is impossible to determine an appropriate "wash-out" period between treatments.

There was also insufficient information to determine whether either diet works better in specific IBS subgroups—such as patients with diarrhea-predominant IBS (IBS-D), constipation-predominant IBS (IBS-C), or IBS with bloating as the dominant symptom.

Additionally, the blinding problem cannot be fully solved. Since information about the low FODMAP diet is freely available online and in apps, patients in trials can likely figure out which group they are in. Asking participants which group they think they were assigned to may not help, because patients who correctly guessed may not admit it.

Short-term dietary changes can also affect the gut microbiome and cause temporary symptoms simply from introducing new foods, making short challenge studies difficult to interpret. Ideally, challenge studies should last at least 8–12 weeks—but every study included in this review was shorter than that.

Future Research Needs

The authors outline a roadmap for stronger research in this area:

• More parallel-group RCTs: Future studies should use parallel designs (patients stay on one diet throughout) rather than cross-over designs.
• Mechanistic data: Trials should collect microbiome and metabolomics data alongside symptom data to understand how exclusion diets work. Some current trials have already done this.
• Better reporting of quality of life and adverse events: Most studies did not report these important outcomes, and future trials should address this gap.
• Long-term safety data: Exclusion diets may have nutritional implications and may adversely affect the gut microbiome, so long-term safety needs evaluation.
• Protect new diets from premature public awareness: The authors suggest that if new dietary approaches are developed, they should be kept as confidential as possible until sufficient RCT data has been collected—otherwise the blinding problem will plague the next generation of dietary trials too.

Frequently Asked Questions

What is the difference between a gluten-free diet and a low FODMAP diet for IBS?

A gluten-free diet eliminates gluten from wheat, barley, and rye. A low FODMAP diet reduces fermentable carbohydrates that draw fluid into the colon and produce gas. This review found insufficient evidence for gluten-free diets, while low FODMAP diets reduced symptom persistence risk by 31%, though evidence quality was very low.

How effective is a low FODMAP diet for irritable bowel syndrome?

Pooled results from seven trials with 397 patients showed a low FODMAP diet reduced the risk of persistent IBS symptoms by 31% compared with control diets. The benefit was larger against high FODMAP or usual diets (54–56% risk reduction) and smaller, borderline, against alternative structured diets. Evidence quality was very low.

Is a gluten-free diet recommended for IBS symptoms?

No. The review found insufficient evidence to recommend a gluten-free diet for reducing IBS symptoms. Two small double-blinded challenge trials showed symptom return with gluten, but combined results were not statistically significant and differed greatly from each other. Any apparent benefit may come from reduced FODMAP intake.

How long should I follow the low FODMAP diet?

The elimination phase should last only 2 to 6 weeks as a diagnostic test, not a permanent eating plan. If symptoms do not improve, stop the diet. If they do, reintroduce foods one at a time to identify triggers, then personalize and liberalize the diet with expert guidance.

Why is the evidence for these diets rated very low?

The GRADE assessment rated both diets as very low quality. Reasons include most trials having unclear or high risk of bias due to inability to blind patients, varying study designs, and too few symptom improvement events (189 instead of the ideal 300). No serious adverse events were reported in low FODMAP trials.

Does the low FODMAP diet improve quality of life in IBS patients?

Only one low FODMAP trial reported general quality of life, and it found no statistically significant difference compared with control. The confidence interval crossed zero, meaning any difference could be positive, negative, or nonexistent. Two trials reported no serious adverse events and said diets were well tolerated.

Should I see a dietitian before starting a low FODMAP diet?

Yes. The authors strongly recommend working with a registered dietitian trained in IBS because the diet is complex, has potential for nutritional deficiencies, and requires proper counseling. If a dietitian is unavailable, use high-quality teaching materials to implement the diet safely and responsibly.

Source Information

Original article title: Systematic Review and Meta-Analysis Evaluating the Efficacy of a Gluten-Free Diet and a Low FODMAPs Diet

Authors: Joanna Dionne, MD, MSc, FRCP, PhD(c); Alexander C. Ford, MB ChB, FRCP; Yuhong Yuan, MD; William D. Chey, MD, FACG; Brian E. Lacy, MD, PhD, FACG; Yuri A. Saito, MD, MPH; Eamonn M.M. Quigley, MD, FRCP, FACP, MACG, FRCPI; and Paul Moayyedi, MB ChB, PhD, FACG

Journal: American Journal of Gastroenterology, 2018; Volume 113, Issue 9, pages 1290–1300. ISSN 0002-9270.

DOI: https://doi.org/10.1038/s41395-018-0195-4

Funding: This work was supported by the American College of Gastroenterology Institute and the Canadian Institute for Health Research.

This patient-friendly article is based on peer-reviewed research. It is intended for educational purposes and does not replace personalized medical advice from your healthcare provider. Always consult with your doctor or a registered dietitian before making significant changes to your diet.