For people with irritable bowel syndrome, finding relief often involves trial and error. A new study suggests that two common approaches, a low FODMAP diet and the antibiotic rifaximin, may be equally effective, while also pointing to a possible reason why results vary from person to person.
In a randomized clinical trial published in Clinical Gastroenterology and Hepatology, researchers found that both treatments led to similar improvements in symptoms among patients with IBS with diarrhea. The study also identified differences in participants’ gut microbiomes that were linked to whether they responded to one treatment, both, or neither.
“Importantly, we found that differences in the gut microbiome may help predict which patients respond to specific therapies,” said Dr. Allen Lee, assistant professor of internal medicine at the University of Michigan and lead author of the study. “We also identified a distinct microbial signature among patients who did not respond to either treatment.”
The study included 65 adults who were randomly assigned to either a 14-day course of rifaximin or guidance on following a low FODMAP diet, which limits certain carbohydrates that can be difficult to digest. After five weeks, both groups reported similar and significant reductions in abdominal pain and bloating.
Still, fewer than half of patients typically respond to either treatment, which has made managing IBS especially frustrating for many people. The researchers set out to better understand whether the gut microbiome might help explain these differences.
“Right now, treating IBS often involves a trial-and-error approach, where patients cycle through therapies before finding one that works,” Lee said. “This can be frustrating, time-consuming, and burdensome for patients.”
By analyzing stool samples collected throughout the study, the researchers identified patterns in gut bacteria linked to treatment response. Participants who responded to the low FODMAP diet tended to have lower levels of bacteria that break down carbohydrates at the start of the study and showed increases in microbial diversity over time.
Those who responded to rifaximin had different microbial features, including bacteria associated with producing short-chain fatty acids and modifying bile acids, which may make them more resilient to antibiotic treatment.
A third group stood out as well. Participants who did not respond to either treatment had a distinct microbial profile, suggesting that some patients may be less likely to benefit from common therapies.
The findings could eventually help guide more personalized treatment approaches. Instead of trying multiple therapies over time, clinicians might one day use microbiome data to better match patients with treatments that are more likely to work.
For now, the researchers caution that the results are early and need to be confirmed in larger studies. The trial was relatively small and conducted at a single center, and the microbiome findings are considered hypothesis-generating rather than definitive.
The study also does not suggest that one treatment is better than the other. Instead, it highlights that different approaches may work for different people, and that biology may play a role in those differences.
For patients, the results offer a clearer explanation for why IBS treatment can feel inconsistent. What works well for one person may not work for another, and part of that variability may come from the complex ecosystem of bacteria in the gut.
This study was supported by grants from the National Institutes of Health and additional support from the University of Michigan Medical School Microbiome Core and the Michigan Nutrition Obesity Research Core. Several authors reported consulting relationships or research funding from pharmaceutical and biotechnology companies, including Merck, Nestle, Salix/Valeant and others.