Probiotic research seems much like a mosaic to me. Small tiles spotting across a big grid of conditions studied on one axis and probiotics tested on the other. The full picture is not yet visible, but becoming more and more focused as research progresses.
Different probiotic preparations have been tested in subjects suffering from irritable bowel syndrome (IBS). Reviews in this area are abundant. I found 11 reviews published over the past 24 months, including Santos and Whorwell 2014; Mayer et al. 2014; Didari et al. 2015; Tiequn et al. 2015; Ford et al. 2014; Hyland et al. 2014.
Common themes are repeated among these experts. IBS is an often debilitating condition, triggered by a diversity of factors and there are few treatment options. A disturbed microbiota may underlie development of this condition, providing a mechanistic link between probiotics and improved symptoms. Numerous human studies have shown that some probiotics can reduce symptoms and improve quality of life in IBS subjects. Probiotic influence on the gut microbiota could lead to reduced gut barrier permeability, calming of immune expression, interaction with the central nervous system resulting in reduced pain sensation and normalization of gut neuromuscular function (Hyland et al. 2014).
Changes that dietary microbes invoke in IBS subjects may also help attenuate symptoms by promoting homeostasis of gut microbiota. The ability of dietary microbes to promote stability of the gut microbial communities is an intriguing line of research. Although the ideal composition of a healthy gut microbial community is not known, a reasonable argument could be made that probiotic-enhanced stability of the gut microbiota would be a great benefit. Such stability could manifest either through promotion of resilience so that a potential perturbation has less of an effect, or through facilitating a return of a disturbed microbial community to normal. Such a function could be brought to bear after correction (for example by fecal microbial transplant, antibiotic treatment, prebiotic administration, or other microbe-targeted interventions) of some deficiency or aberration in a microbial community, with the aim of promoting stability of the newly established, healthier community. Viega et al. (2014) demonstrate that one probiotic yogurt can impact the gut microbial community. Additional studies are needed.
Reasonable mechanisms now exist for why probiotics may improve IBS symptoms. The challenge remains to find the best probiotic strains, doses and possible combination therapies to optimize beneficial effects. A challenge in conducting studies in IBS subjects is the heterogeneity of subjects and responses. One clear need is to better identify the characteristics of responders, so that therapies can be more targeted. Even with heterogeneous populations, currently used probiotics can be viewed as an intervention that is safe and inexpensive. Patients can determine for themselves if probiotics offer any relief. However, because IBS is a protracted condition, there is a need for long term studies. Most studies last only a few months at most.
But overall, the evidence for a role in probiotics in managing symptoms of IBS is encouraging.