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Irritable Bowel Syndrome is one of the most common disorders of brain-gut interaction (DGBI) with estimates of between 10-15% people affected nationally. The Rome IV Criteria are symptom-based diagnostic criteria providing a standardized IBS diagnosis. The cardinal IBS symptom is chronically recurrent abdominal pain, and predominant symptoms are subtype dependent – constipation, diarrhea, or mixed stool habits. Although once thought to be difficult to treat, most patients with DGBI can be successfully managed using an interdisciplinary approach with the understanding that gut-brain interactions are bidirectional, and that stress, emotions, and diet all play a role. Over the past decade, probiotics have gained both scientific and popular interest regarding their potential role in overall health and wellbeing. Yet, there is limited evidence-based data to support their use in specific GI disorders or more generally. Recently, yeast probiotics have gained attention and are being studied in comparison to their bacterial counterparts in their application to IBS.

In 2014, the World Health Organization (WHO) and Food and Agriculture Organization (FAO) released a formal consensus statement defining probiotics as “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host.” Most often made up of a single or multiple bacterial strain, probiotics can also be composed of yeast which serve their own function in maintaining a healthy intestinal environment. The term “microbiome” encompasses all micro-organisms that naturally exist in the gut including bacteria, viruses, and fungi referred to as the bacteriome, virome, and mycobiome, respectively. Even though poorly understood, there seems to be a strong synergistic interaction between these organisms which influence gut health. Given unique properties of yeast compared to bacteria, the mycobiome has subsequently piqued interest of both researchers and clinicians. Although the evidence is mixed and inconclusive, alterations in the gut microbiome has been associated with IBS – particularly the post-infectious sub-type in which the healthy gut microbiota is altered following an acute gastrointestinal infection. It has been proposed that supplementation with probiotics may help restore a healthy microbiome and improve gut function, even though evidence for clinical benefits is inconclusive. This lack of strong evidence for use of probiotics across several digestive diseases led the American Gastroenterology Association (AGA) to release a clinical guideline and consensus statement against their widespread use.

In contrast to the multiple strains of bacterial probiotics with some degree of health benefits, only one yeast genus – Saccharomyces spp – has shown any potential impact on gut health. One study suggested that fungal dysbiosis may lead to visceral hypersensitivity, a hallmark feature of IBS patients and, using an animal model, such hypersensitivity was reduced when fungicide administered. As described in a review article summarizing multiple preclinical studies, Saccharomyces may help maintain integrity of intestinal epithelial lining by decreasing trophic effects on the mucosa, biodegrading phytate (a storage form of phosphorus) to improve absorption of nutrients, and improving immune response while also having anti-inflammatory properties. It must be noted, however, that these findings are predominantly from preclinical research in mice and rats, or demonstrated at the molecular level, rather than from human studies in patients.

The clinical trials that do exist have primarily focused on two very closely related strains Saccharomyces cerevisiae (colloquially known as Brewer’s yeast and Baker’s yeast, described here) and Saccharomyces boulardii. Though nearly identical at a molecular level, S. boulardii has demonstrated more physiological resistance to heat and acid stressors.

One identified strain of Saccharomyces cerevisiae, named CNCM I-3856, has consistently shown benefit in human subjects with IBS. A randomized double-blind placebo-controlled trial of CNCM I-3856 found improvement of bloating, pain, and motility in patients with IBS-constipation subtype (IBS-C), which was then supported by a meta-analysis. A similarly well designed prospective, randomized placebo-controlled study in humans showed statistically significant improvements in abdominal pain and stool consistency after taking CNCM I-3856 at the dose of 2 billion CFU twice daily for 8 weeks. In June 2022, new research failed to re-demonstrate significant changes in bowel movement frequency or stool consistency between IBS-C patients who took CNCM I-3856 versus placebo, yet those taking the yeast probiotics did have a significant improvement in quality of life score for abdominal pain. Possible mechanisms of action for yeasts’ effects are their immune modulating properties and assistance in the generation of short-chain fatty acids (SCFAs) which have been shown in animal studies to play a key role in neuro-immunoendocrine regulation and reduction of inflammation. Certain advantages of yeast probiotics compared to bacterial probiotics include their higher resistance against bile salts, pH variations, and temperature changes along the GI tract and an ability to be co-administered with antibiotics.

It is important to acknowledge that not every person will respond in the same way to probiotics, even those with evidence of benefits as described above. As acknowledged by the AGA, there is not enough strong evidence of probiotics’ benefits to recommend their widespread use and there remains variability in which combinations, species, or strains are effective for different GI conditions, including subtypes of IBS. More clinical trials are necessary to develop clearer recommendations for their use in gastrointestinal disorders. Better understanding the gut microbiome and its role in DGBI may offer new targets for future therapies, which potentially include yeast probiotics.