What 33 Countries’ Diets Reveal About IBS-Like Gut Disorders

A new analysis of the Rome Foundation Global Epidemiology Study, the largest cross-national survey of digestive symptoms ever assembled, has found that everyday eating patterns track with the odds of having a disorder of gut-brain interaction (DGBI) – the modern term for conditions like irritable bowel syndrome (IBS), functional constipation, functional diarrhea, and functional bloating. Drawing on more than 75,000 adults across 33 countries, the researchers report that people on lactose-free diets were more likely to report these bowel disorders, while those eating bread- or rice-heavy diets were less likely to. The study, published in the Journal of Neurogastroenterology and Motility, is one of the broadest looks yet at how a person's usual diet, not a clinical intervention, relates to gut symptoms worldwide.

Lactose-Free Diets Linked to More Bowel DGBI Cases

The headline signal was consistency around dairy. In the internet survey, a lactose-free diet was tied to higher odds of IBS overall (odds ratio 1.26) and, more specifically, to diarrhea-predominant IBS (odds ratio 1.48). In the household survey, the association was even stronger: a lactose-free diet was linked to higher odds of any bowel DGBI (odds ratio 1.7), IBS overall (odds ratio 2.54), functional constipation, and both the constipation- and mixed-symptom subtypes of IBS. After adjusting for age, sex, religion, region, and living conditions, a lactose-free diet still tracked with diarrhea-predominant IBS in the internet sample and with constipation-predominant and mixed IBS in the household sample. The authors suggest a more mundane explanation than lactose itself being uniquely harmful: people with gut symptoms often cut dairy on their own, consciously or not, because lactose is a recognized trigger for bloating and diarrhea, yet the survey data suggest that strategy is not obviously working. Bread- and rice-predominant diets moved the other way. Among internet respondents, a bread-predominant diet (the most common pattern, at 38.2% of that sample) was associated with lower odds of IBS (odds ratio 0.86) and constipation-predominant IBS (odds ratio 0.78) after adjustment, and a rice-predominant diet showed a similar pattern (odds ratio 0.80 for IBS). Vegan and vegetarian diets, by contrast, showed no consistent direction at all.

Millions With IBS Already Self-Restrict Their Diets

Bowel DGBIs are not a niche complaint. IBS, functional constipation, and related conditions are among the most common reasons people see a gastroenterologist, and prior work from this same Rome Foundation research program has already shown that at least one DGBI is reported by roughly 40.3% of internet survey respondents and 20.7% of household survey respondents worldwide, a reminder of how common these symptoms are, even before this diet-focused analysis. People with these disorders frequently modify what they eat on their own initiative, often without guidance from a dietitian or physician, cutting dairy, avoiding certain grains, or experimenting with restrictive patterns like veganism in hopes of relief. That makes a global read on "usual diet" clinically relevant in a very practical way: if self-directed changes such as going lactose-free are not associated with fewer symptoms, and in this data set were associated with more, that has direct implications for how clinicians counsel patients and how public health messaging around diet and gut health gets framed, especially outside the Western countries where most prior diet-IBS research has been concentrated.

54,127 Internet and 20,973 Household Respondents Surveyed

The credibility of the study rests on its scale and design. The Rome Foundation Global Epidemiology Study surveyed adults in 33 countries spanning the Americas, Europe, Asia, the Middle East, and Africa, using a questionnaire translated into 21 languages with professional linguistic and cultural validation. Where most adults use the internet, researchers ran secure online surveys through the market research firm Qualtrics; in Bangladesh, India, Indonesia, Iran, Malaysia, and Nigeria, where that was not feasible, trained interviewers conducted face-to-face household surveys, with China and Turkey using both methods. This diet analysis included 54,127 internet survey subjects and 20,973 household survey subjects, all classified using the Rome IV Diagnostic Questionnaire, the standard clinical criteria for bowel DGBIs. Diets were defined operationally: vegan meant no milk, meat, fish, or eggs; vegetarian meant no meat or fish; lactose-free meant no milk products; and bread-, pasta-, or rice-predominant meant eating that starch four to seven days a week while eating the other two sources no more than three days a week. Adherence to vegan or vegetarian diets was low everywhere, ranging from 0.4% to 9.8% depending on survey and diet, while lactose-free and rice-predominant eating were markedly more common in household survey countries. The team ran both unadjusted comparisons and multivariate logistic regressions adjusting for age, sex, religion, geographic region, and living conditions to isolate the diet associations.

Survey Method Splits the Results in Two Directions

The clearest limitation in the paper is right there in its own data: internet and household respondents frequently told a different story about the same diets. A vegan diet showed no association with bowel DGBI in the household survey but was linked to lower odds of functional constipation in the internet survey, and after adjustment, was linked to higher odds of any bowel DGBI in the household survey (odds ratio 1.63), the opposite direction. A rice-predominant diet was tied to lower IBS in the internet sample but to higher diarrhea-predominant IBS in the household sample. The authors are candid that these are cross-sectional survey data, which can identify associations but cannot establish causation or even confirm which came first, the diet or the symptoms. Dietary intake was also self-reported through a questionnaire rather than measured with food diaries or biomarkers, and some diets, especially veganism, had so few adherents (as low as 0.4% in one sample) that the associations built on them carry real statistical uncertainty. Whether internet or household responses are more trustworthy cannot be determined with confidence; the researchers note that people may simply be more guarded discussing intimate bowel symptoms face-to-face than in an anonymous online survey, which could distort comparisons between the two methods more than it distorts the diet questions themselves.

Rome Foundation Calls for Longitudinal Diet Studies Next

The authors conclude that this large global survey has revealed dietary factors that may plausibly influence reported DGBI prevalence in the general population, while stressing that the picture remains incomplete. They explicitly call for longitudinal, prospective studies to sort out directionality: do dietary habits cause bowel DGBI, are they adopted in response to symptoms already present, or is it some mix of both depending on where someone lives and their culture and circumstances? The paper also flags dietary elements it did not capture, including fiber and prebiotic intake and consumption of artificial sweeteners, emulsifiers, and other additives, all candidates for future rounds of this research. No timeline for follow-up studies is given, but the underlying Rome Foundation database, already the source of prior papers on DGBI prevalence, quality of life, and psychological distress, remains active and available for further analysis, meaning more diet-specific findings from this same 33-country dataset are likely still to come.

Evidence rating

EMERGING — 82/100

Caveats:

• Cross-sectional design cannot establish causation
• Dietary patterns were self-reported via survey, not objectively measured
• Findings varied substantially between the internet-survey and household-survey populations, limiting generalizability

Large cross-sectional survey data across 33 countries show a correlation between certain diets and gut-brain bowel disorders, but the design cannot establish causation and relies on self-reported dietary intake, with inconsistent results between survey populations.

References

1. Daniel Bushyhead, Rutaba Khatun, Shrikant I Bangdiwala, Olafur Palsson, Ami D Sperber, Eamonn M M Quigley (2026). The Association Between Usual Diet and Bowel Disorders of Gut-Brain Interaction: Analyses From the Rome Foundation Global Epidemiology Study. Journal of Neurogastroenterology and Motility. https://doi.org/10.5056/jnm25137