For anyone with irritable bowel syndrome who has cycled through diets and prescriptions with little relief, a new trial offers a different starting point.
The specific mix of bacteria in your gut appears to predict whether a leading diet or a widely used antibiotic will actually calm your symptoms, and which patients are unlikely to respond to either.
The randomized controlled trial, led by researchers at the University of Michigan Medical School and published in April 2026 in Clinical Gastroenterology and Hepatology, followed 65 adults with IBS with diarrhea (IBS-D).
Participants were randomly assigned to a low FODMAP diet or the antibiotic rifaximin for 14 days, then tracked for five weeks.
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The two treatments produced comparable and significant reductions in abdominal pain and bloating, but fewer than half of patients responded to either one.
That mixed hit rate is why the microbiome findings matter. The team analyzed stool samples before treatment and looked for bacterial patterns that separated responders from non-responders.
What the microbiome signatures showed
Patients who improved on the low FODMAP diet, which cuts back on fermentable carbohydrates found in foods like onions, garlic, wheat, apples and certain dairy products, started out with fewer sugar-fermenting (saccharolytic) bacteria and showed rising microbial diversity as the diet took hold.
Those who responded to rifaximin, a poorly absorbed antibiotic that acts mostly inside the gut and is sold under brand names such as Xifaxan, tended to carry bacteria capable of producing short-chain fatty acids and modifying bile acids. That combination pointed to a microbial community more resilient to antibiotic exposure.
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A third group stood out for a different reason. Non-responders to both therapies were enriched in protein-fermenting (proteolytic) bacteria, a signature the researchers linked to treatment resistance.
Identifying those patients up front, before they invest weeks in a therapy that will not help, is the practical promise of the study.
Why the finding matters for patients
Irritable bowel syndrome is one of the most common gut disorders in the United States, and many patients spend years trying elimination diets, probiotics and prescriptions with uneven results.
Lee also cautioned that the trial is a first step, not a finished tool. "Importantly, we found that differences in the gut microbiome may help predict which patients respond to specific therapies," he said, framing the results as hypothesis-generating and in need of larger validation studies before microbiome testing becomes routine in clinics.
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For readers weighing what to do now, the practical takeaway is narrower than a personalized test. Both the low FODMAP diet and rifaximin remain evidence-backed options for IBS-D, and the trial does not change that.
If diarrhea, cramping or bloating are disrupting daily life, or if you recognize the common IBS symptoms but have not been evaluated, a gastroenterologist can help decide which of these approaches to try first and monitor whether it is working.
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This article is made and published by Mie Hermansen, who may have used AI in the preparation.