Probiotics have been part of the human diet for centuries, and probiotic supplements have exploded in popularity over the last few decades. Today, there are endless commercial claims about the efficacy of probiotic-rich fermented foods and probiotic supplements for gastrointestinal disorders like irritable bowel syndrome (IBS), and many are supported by patient testimonials and anecdotes from physicians. As a result, probiotics have been the subject of considerable research interest since the start of the 21st century and interest continues to grow as alternative medicine becomes increasingly mainstream. For almost two decades, researchers have been conducting rigorous in vitro experiments, animal studies, and clinical trials, trying to determine whether probiotics could make a difference for patients with IBS.
By this point—after almost two decades of rigorous research—you would think that the scientific community might have come to a general consensus regarding the efficacy of probiotics. However, even the briefest dip into the scientific literature can leave physicians and patients with far more questions than answers. Some individual studies show promise, while others suggest that probiotics have relatively little impact on IBS patients. Even when you turn to large meta-analyses and systematic reviews, experts in the field universally conclude that it is not clear whether or not probiotics can effectively treat IBS. For practitioners and patients who are considering this body of literature, it is important to understand why no consensus has been reached, what we have found out, and what it all means for the question of whether probiotics can help patients with IBS.
Understanding the Existing Research on Whether Probiotics Help IBS
The lack of consensus on whether or not probiotics can help IBS patients is not due to an absence of research in the field. In one of the most recent systematic reviews of the potential role of probiotics in the treatment of IBS, published in the Journal of Infection in February 2018, researchers found over 800 potentially relevant articles published within the last fifteen years alone. However, despite careful analysis of hundreds of studies, the researchers were only able to characterize the available data as “promising” and conclude that “further studies are needed before probiotics can be considered a reliable treatment for IBS.”
Other meta-analyses and systematic reviews have drawn similar conclusions. This can be frustrating for patients since these types of studies are considered to offer some of the highest-level evidence when it comes to determining the efficacy of a treatment. In 2017, a group of researchers from University College Cork in Ireland and Houston Methodist Hospital in Texas collaborated to discuss the inconclusiveness of some of these studies in the journal Current Medical Research and Opinion. In their critique, the authors emphasized the reason a consensus has yet to be reached: the studies that have been conducted on probiotics simply are not methodologically comparable to each other. As a result, researchers conducting meta-analyses have not been able to collect enough data on single treatment options to draw concrete conclusions.
Consider a few of the common methodological distinctions between the studies that have been conducted on probiotics and IBS:
- Strain types and combinations. This is perhaps the widest source of variation between studies. Some researchers have examined the efficacy of a supplement containing only a single strain, while others have conducted studies using multiple strains in various combinations—most of which are distinct from each other.
- Dosage and duration of treatment. In every study, the researcher has to decide how much of a probiotic the patient will take, how often they will take it, and for how long—all of which can impact the finding of a single study. With so many study designs to consider, the data becomes much more muddled, and it becomes much harder to compare studies and verify findings.
- Delivery method. In some studies on probiotics and IBS, the focus is on the potential efficacy of naturally probiotic-rich foods, like yogurt. In others, probiotics are formulated into functional foods or supplements, and it is not clear whether the delivery method plays a role.
- Measure of efficacy. There is no single scale that researchers use when characterizing the effectiveness of probiotic supplements for patients with IBS. Some studies have focused on specific symptoms, like bloating and abdominal pain, while others have evaluated broader measures, like perceived quality of life. Even when individual symptoms are emphasized in a particular study, comparison can be difficult because the types of IBS are so varied. For instance, a patient who has IBS with Diarrhea (IBS-D) may see positive results from a supplement that makes little difference for someone who has IBS with Constipation (IBS-C).
Drawing Out Themes from the Existing Research Evidence
While the heterogeneity of studies on probiotics and IBS make it difficult for researchers to draw conclusions on the broad question of whether probiotics can help patients with IBS, there is one clear area of consensus: that this research is worthwhile. It is becoming increasingly clear that microbial diversity plays a role in the pathophysiology of IBS, which provides a strong theoretical foundation for the idea that probiotics might help resolve symptoms and improve quality of life. Although no clear, universal pattern of change in the microbiome has been identified, the latest research suggests that both the diversity and abundance of bacterial strains in the gut varies between IBS patients and the general population.
Moreover, there are certain themes that can be drawn out of some of the evidence, even by researchers who are most concerned about the heterogeneity of the existing research literature. Specifically, it appears that probiotic bacteria from the genus Bifidobacterium have the largest impacts on patients with IBS. Some researchers go so far as to specifically highlight B. infantis as the best-supported strain, while others continue to reserve judgment, holding out for more comprehensive studies in the future. However, it is important to recognize that this is not the only bacterial strain that has been implicated in IBS studies, so patients may still find strains in other genera to be effective.
Considering Probiotic Therapy Options
Ultimately, answering the question of whether probiotics can help IBS patients is a lot more complicated than it seems. For the research community, the jury is still out, but patients and practitioners can still act on some of the available evidence. Knowing that scientists agree that the microbiome may be involved in the pathophysiology of IBS, it may be valuable to try a probiotic supplement or another supplement that has been associated with microbiome function, such as butyric acid. The diversity of studies on the subject also indicates that results may vary depending on factors like the bacterial strain, dosage, and type of IBS, so patience may be helpful for physicians and patients looking for an optimal treatment to meet unique needs.
Allen AP, Clarke G, Cryan JF, Quigley EMM, Dinan TG. 2017. Bifidobacterium infantis 35624 and other probiotics in the management of irritable bowel syndrome: Strain specificity symptoms, and mechanisms. Current Medical Research and Opinion. 33(7):1349-51. https://www.ncbi.nlm.nih.gov/pubmed/28436237
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