A Mediterranean diet beat standard first-line dietary advice for IBS in a UK trial

Dietary advice is the usual first move in irritable bowel syndrome, and the options are limited. Major society guidelines recommend traditional dietary advice as first-line therapy, holding the low-FODMAP diet — restrictive, resource-intensive and requiring dietitian supervision to do properly — in reserve as second line [s1]. A randomised trial published on 28 October tested whether the Mediterranean diet belongs in the first-line slot alongside traditional advice [s1].

It was designed as a non-inferiority trial. It returned superiority.

The trial

One hundred and thirty-nine people with IBS from across the United Kingdom were randomised to six weeks of the Mediterranean diet (68 participants) or traditional dietary advice (71 participants), delivered through an online virtual platform [s1]. The trial is registered as NCT05985018 and reports no funding source [s1].

Participants had a mean age of 40.4 years, with a range of 19 to 65, and 80% were women [s1]. Mean baseline IBS Symptom Severity Scale score was 309 (SD 90) [s1] — squarely in the moderate-to-severe range that the scale is designed to capture.

The primary endpoint was the proportion achieving a clinical response, defined as a reduction of 50 points or more on the IBS-SSS [s1].

The result

On modified intention-to-treat analysis, 62% (95% CI, 50% to 73%) of the Mediterranean diet group met the primary endpoint, against 42% (95% CI, 31% to 55%) on traditional dietary advice [s1]. The difference was 20 percentage points (95% CI, 4 to 36 percentage points; P = 0.017), which established both non-inferiority and superiority [s1].

The continuous outcome moved in the same direction. Mean IBS-SSS fell by 101.2 points on the Mediterranean diet against 64.5 on traditional advice, a difference of 36.7 points (95% CI, −70.5 to −2.8; P = 0.034) [s1].

Adherence was confirmed: Mediterranean Diet Adherence Screener scores rose significantly more in the Mediterranean group (P < 0.001) [s1]. That check matters in a dietary trial, because it distinguishes an ineffective diet from one nobody followed.

What did not differ

Secondary outcomes were largely null. There were no statistically significant differences between groups in changes to mood, somatic symptom reporting, quality of life, or diet satisfaction [s1].

The quality-of-life null is the one to sit with. IBS-SSS improved substantially more on the Mediterranean diet, but that improvement did not translate into a measurable quality-of-life difference over six weeks. Symptom scales and quality-of-life instruments measure different things, and six weeks is short for the latter to move. Still, a reader interested in how much better people felt overall, rather than how their symptom score changed, does not get an answer here.

The diet-satisfaction null cuts the other way, and favourably: participants were no less content on the Mediterranean diet than on standard advice [s1], which is relevant given that adherence is the binding constraint on every IBS diet.

Where this sits against the guideline

The British Society of Gastroenterology's IBS guideline, published in 2021, reviewed dietary, pharmacological and psychological therapies using GRADE, informed by an update of trial-based and network meta-analyses [s2]. It reclassified IBS as a disorder of gut-brain interaction rather than a functional gastrointestinal disorder [s2], and identified areas of unmet need for future research [s2].

Guidelines of that generation had essentially no randomised Mediterranean diet data in IBS to work with. The trial published this month is the kind of evidence that changes what a first-line recommendation can include — but a single 139-person trial does not by itself rewrite a GRADE-assessed guideline, and it should not be read as having done so.

The limitations the trial names

The authors list one limitation explicitly: no long-term data [s1]. Six weeks establishes that symptom scores improved during the intervention. It says nothing about whether the improvement persists at six months, whether participants stayed on the diet, or whether symptoms rebounded.

Several other constraints follow from the design. Dietary trials cannot be blinded — participants knew which diet they were on, and IBS symptom scales are self-reported, which is a route for expectancy effects in both arms. The trial was delivered online, which improves reach but means adherence and symptom reporting were remote rather than clinically observed. Eighty percent of participants were women [s1], consistent with IBS epidemiology but limiting what can be said about men.

And the comparator matters. The Mediterranean diet outperformed traditional dietary advice; it was not compared against the low-FODMAP diet, which remains the option with the largest evidence base for symptom reduction. Whether it matches low-FODMAP is untested here.

What it means in practice

The trial's own conclusion is that the Mediterranean diet showed non-inferiority and superiority to traditional dietary advice and represents a viable first-line dietary intervention for IBS [s1].

If that holds up in replication, the practical appeal is less about effect size than about burden. Traditional dietary advice and low-FODMAP both work by subtraction — identifying and removing triggers, with the low-FODMAP version requiring a structured elimination and reintroduction sequence. The Mediterranean diet is a pattern rather than a restriction, does not require dietitian-supervised reintroduction, and carries a large independent literature on cardiometabolic health.

What to watch

Replication in a larger sample, follow-up beyond six weeks, and a head-to-head against the low-FODMAP diet are the three studies that would settle where this belongs in the treatment sequence. Until they exist, this is one unfunded, unblinded, six-week trial in 139 people that produced a clear result.

This article describes research findings and is not medical advice.

Sources

• [s1] The Mediterranean Diet for Irritable Bowel Syndrome: A Randomized Clinical Trial — Annals of Internal Medicine, published online 28 October 2025. ClinicalTrials.gov NCT05985018. https://doi.org/10.7326/annals-25-01519
• [s2] British Society of Gastroenterology guidelines on the management of irritable bowel syndrome — Gut, 2021. https://doi.org/10.1136/gutjnl-2021-324598