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Can a low FODMAP diet combined with a gluten-free diet (LF-GFD) be effective in helping reduce symptoms of irritable bowel syndrome (IBS)? The journal Digestive Diseases & Sciences recently published a systematic review and meta-analysis examining this very issue. Both of these diets have separately been used by people with IBS, with only a small number of studies evaluating their combined use. This systematic review included 4 randomized and controlled clinical trials as well as 4 cohort studies, and included 437 patients (221 on a LF-GFD diet and 216 on a gluten-free diet (GFD). This analysis concluded that the LF-GFD significantly improved bloating and pain Visual Analog Scale (VAS) scores, as well as the IBS Symptom Severity Scale (IBS-SSS), and IBS Quality of Life (IBS-QoL) scores. When compared to the GFD, the LF-GFD also improved self-rated anxiety and depression scores. Although this initially seems to be an encouraging finding, perhaps offering more evidence for a dietary strategy among people with IBS, the statistical analysis was poorly defined, the paper poorly referenced, and the conclusions largely unjustified, leaving the efficacy of the LF-GFD still somewhat vague. Yet there may be some value in parsing out what the studies themselves did find.
For example, one of the cohort studies mentioned in this review (but not cited) was not an actual cohort study but a review of studies that separately evaluated GFD and low FODMAP diets. While this review did not mention their combined use, it did provide some valuable strategic thinking about signs or symptoms that might prompt the use of one diet over another. For instance, extraintestinal symptoms or several relevant biomarkers may help to suggest an initial trial of a GFD before a low FODMAP diet, such as measuring serum anti-gliadin antibodies as a screening test. This is consistent with one of the other trials mentioned in the systematic review; in a prospective study of 50 participants with IBS, people with a positive antigliadin IgA or IgG were far more likely to have a reduction in IBS symptoms (75%) while on a GFD compared to those with negative screening (38%). A possible biomarker to provide guidance is especially helpful given the high prevalence of a “nocebo” effect among people with non-celiac gluten sensitivity, with one analysis reporting this to be as high as 40%.
One of the randomized and controlled trials mentioned in the systematic review did not evaluate the LF-GFD, but instead, compared a GFD, a low FODMAP diet (separately), and traditional dietary advice, with the latter representing 1st line care in the UK and including a combination of having regular meals, eating in moderation, good hydration, and a reduction in certain foods (e.g., alcohol, caffeine, fatty and spicy foods, beans, etc.). Comparing these 3 diets found a greater percentage of people with a 50% or higher reduction in IBS symptoms (IBS-SSS) with a GFD (58%) compared to both a low FODMAP diet (55%) and the traditional advice (42%). However, these differences were not significant. While the study may have been underpowered and too short in duration (4 weeks) to reach significance, there were other advantages of the traditional diet, such as lower cost, simplicity, and less social difficulty. Nonetheless, this trial did not in any way evaluate the combined LF-GFD.
Another of the randomized trials mentioned was a single-blinded 2-phase study in which participants with IBS initially followed an LF-GFD diet for 6 weeks, and then were randomized to one of 3 groups for the 2nd phase; one group continued this diet for an additional 6 weeks, one group had an unrestricted diet, and the 3rd slowly increased the amount of gluten (while remaining on a low FODMAP diet), starting at 8g gluten per day, and if symptoms did not worsen (more than 30%) increasing to 16g after 2 weeks, and finally to 32g after another 2 weeks. This study intended to quantify the number of people with IBS for which gluten played a role, as well as the dose of gluten needed to worsen symptoms. Although the authors surprisingly concluded that most people tolerated a high amount of gluten, this was not what the study found. They reported that 16 of 25 did not make it to 32g per day, i.e., they could not tolerate the increasing gluten intake.
The last of the randomized trials mentioned in this review was not at all appropriate; rather than patients with IBS, this was a trial that found additional benefit of a low FODMAP diet among people with celiac disease that were already following a gluten-free diet.
One of the better trials conducted in the IBS population was not even referenced in this systematic review. Published in the American Journal of Clinical Nutrition, this was a double-blind, placebo-controlled, 3-way crossover trial, which found that while a low FODMAP diet had a modest improvement in IBS symptoms, no differences were observed between gluten and placebo. Thus, although there may be some patients with symptoms of IBS that may be due to an undiagnosed gluten sensitivity or celiac, the evidence is still weak for implementing the combined LF-GFD without other relevant indications.
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