Peer Reviewed
Sanjay Noonan, BMBS, on the Link Between Probiotic Therapy and Mental Health
A systematic review published online in BMJ Nutrition, Prevention & Health documents several studies that found reductions in depressive symptoms after probiotic therapy, either alone or in conjunction with prebiotics. In this Q&A, lead author Sanjay Noonan, BMBS, discusses the impetus for the study, limitations of the review, and how it could be used to guide future research on the topic. Dr. Noonan is a graduate of Brighton & Sussex Medical School in England and is currently working in stroke medicine as a junior doctor in London.
Q: What led you and your colleagues to conduct this research?
A: I have a longstanding interest in nutrition/diet and how it can contribute to the overall health of an individual. I feel the significance of diet is somewhat recognized, but certainly not as central to health care as might be valid. Because of this, I was keen to conduct research that focused on dietary intervention.
During my scoping reading, I became aware of the gut-brain axis (GBA). I delved quite deeply into the current research on the GBA and became more and more fascinated; the concept of the microbiome was something that stood out to me particularly as interesting. Another key area of interest was whether there were any potential intersections between GBA research/therapy and mental health. Research that concerned common mental health disorders—such as anxiety and depression—were again also of particular interest, as their prevalence and personal and societal impact are so significant in our world presently and therefore, they deserve careful attention, in my opinion.
I was aware from my reading that numerous studies had investigated the effect of pre/probiotics on the microbiome and sought to explore whether information existed on the impact of pre/probiotics upon an individual’s microbiome and subsequently on the GBA, and then to see whether this had any tangible effects upon an individual’s experiential relationship with the world and any mental health conditions.
I realized there was not a tremendous amount of clarity in the research on these concepts, yet I felt there may be some useful findings that, if examined together, could lead to more useful research in the future. As such, my team and I sought to create a review to bring together existing findings. Due to our aim, our research was very limited in its ability to bring new information to light; similarly, any information we did produce was extremely preliminary and sought not to make a particular point, but simply to guide future research in a way that might contribute to a better understanding of this topic.
Q: Please briefly describe your study and its findings.
A: A very succinct summary is that we identified 7 research articles that analyzed the impact of pre/probiotic supplementation on patients experiencing clinically diagnosed anxiety/depression. Results had to be presented in a way that could be quantitatively represented (eg, by the use of graded questionnaire). We recognized this method is limited in its capacity to yield useful information, but it enabled us to reduce any subjectivity in our interpretation of the results and within the original research.
We discovered that each of the articles found a quantitatively evident reduction in patients’ experience of depression following supplementary pre/probiotic or isolated probiotic therapy. We found no signficant results relating to the impact of supplementation on anxiety.
It is also necessary to explicitly highlight that these supplementary interventions were not specifically explored in isolation but also in conjunction with treatments approved by England’s National Institute for Health and Care Excellence (NICE) for the treatment of depression and/or anxiety; eg, some patients would also be taking a selective serotonin reuptake inhibitor (SSRI). Patients undergoing therapies not approved by NICE were not considered. Furthermore, given that it is a patient’s choice to not engage with certain therapies, it was acceptable to include studies where the only intervention was a pre/probiotic.
Q: Were any of the outcomes particularly surprising?
A: The outcomes were not particularly surprising because the review yielded no definitive information. The main function of this review was to collect the existing information and show what types of research exist and what they generally find. However, a great amount of heterogeneity exists within the analyzed studies and I can say with certainty that no definitive conclusions can be drawn from this article.
The most important sections of our review would be the discussion, limitations, and conclusion. Here, we detail some interesting potential applications for pre/probiotics, including how patients with certain comorbidities may benefit more greatly than others and items that should be included in future research, such as biochemical profiling to help identify potential impacts of supplementation and to further elucidate potential mechanisms for any effect.
Q: What are the possible near-term applications of these findings in clinical practice?
A: There are no clinical applications I could recommend in light of the information our article has yielded. However, I believe there is some value in it for teams planning to research this topic, as we have outlined some very important confounding variables and flaws with study designs within the limitations and discussion sections, which, if attended to, would give substantially more robust and meaningful information.
Q: Is there anything else pertaining to your research and findings that you would like to add?
A: I think it is extremely important to point out once again the preliminary nature of these findings and emphasize that this article would serve best as a reference point to frame future research and that substantive clinically relevant claims cannot be drawn from the findings. The reasons I feel this is pertinent are detailed in the limitations sections and also featured within the discussion.
(Editor’s Note: This interview has been edited for length and clarity.)
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