Probiotics and health benefits: a journey through the scientific literature

In the article“Probiotics: what lies behind this name?” we left off with a very interesting question: what role can probiotics play in curing or preventing disease? Is their action exclusively at the intestinal level?

To answer these questions, we must learn to navigate our way through the research published in specialized academic journals, what are collectively called the “scientific literature.” And that may not be easy. Let’s see why.

Are probiotics good for health? Individual scientific articles are not enough

Much has been written about probiotics . A search with the keyword “Probiotics” on PubMed (a free search engine for scientific literature of biomedical interest)1 returns us 31976 results2. If we try searching with Google Scholar (a search engine for academic literature)3 the results rise to as many as 430 thousand! Perhaps there are a few too many.

We can try to refine the research by limiting it to articles that also discuss health benefits. We then use “Probiotics health benefits” as our search keys. Now the results on PubMed drop to “only” 1642 (114 thousand on Scholar). I don’t know about you, but I just can’t read 1600 scientific articles to see if the scientific community has the answer to my question. We could use a little help.

Fortunately, some scientists take it upon themselves to read scientific articles for us and help us navigate through this thicket of information. This is how reviews, or revisions (better known by the English term review ).

When dealing with topics related to medicine and therapies, in addition to individual scientific articles, one must take into account clinical trials , which are trial protocols on humans that aim to determine theefficacy and possible side effects of a drug or treatment. In these cases, the reviews that analyze and summarize individual research are systematic reviews and meta-analyses.

Individual scientific articles or clinical trials can sometimes give conflicting results. This may depend on how the experiments were conducted or how the results were interpreted. That is why in writing a review, one must take all parameters into account and try to highlight the information that emerges from the research, taken as a whole.

Probiotics and clinical trials: reviews from the Cochrane Collaboration

If during a walk in a forest we were to get lost, in order to understand more clearly where we are and decide which way to go, it would be helpful to climb a little higher to get a wider view. Even in the scientific field, there are those who do this work, detachedly and objectively analyzing the landscape of clinical trials conducted on a certain topic. The Cochrane Collaboration is a prestigious international initiative that produces and publishes systematic reviews of the highest quality and great scientific rigor regarding the efficacy and safety of health care interventions.

If we go looking in the Cochrane Library4 What has emerged from the numerous clinical trials on probiotics we notice two things; the first is that they have been Tested on so many diseases and disorders: eczema, colic, cystic fibrosis, Crohn’s disease, pneumonia, diarrhea, premature birth, otitis, respiratory infections, liver disease, gestational diabetes, and many others. This means that in tests conducted in the laboratory and on animals, there were positive results, which then motivated the extension of the research to humans as well. The second thing that stands out is that in many cases, the conclusion of the review team is that more studies are needed. Why?

The standards that Cochrane reviewers must hold when evaluating clinical trials are very stringent and are used to ensure that clinical trials are conducted with due scientific rigor so that the resulting information is supported by strong experimental evidence. That is why they control everything in detail, even the criteria by which patients are chosen to be included in the study. They are very lead-footed about it, in short! And they do well, because from their review often comes therapeutic advice: do we or do we not use this therapy to treat a certain disease? Better to be cautious when health is at stake.

A question of quantity, but also of quality: the need for uniform studies

In essence, then, today the scientific community has no doubt about theusefulness of probiotics in the treatment of certain disorders (particularly gastrointestinaldisorders ). For other diseases, however, the message from the clinical trials and reviews is that more studies are needed in this regard. And not just more studies, but better conducted studies.

So far, in fact, the various clinical trials have used different types of probiotics, different dosages, different treatment durations… in short: the trial has not been conducted uniformly, so it is difficult to amalgamate the results of such varied studies. Some studies do not even specify the bacterial strain used. Yet it has been shown that theeffect of probiotics is extremely strain-specific: with the same genus and species, one bacterial strain can have biological effects that another “sister” strain does not.

Our gut microbiota is extremely complex: it includes more than 1,000 different species of microorganisms, and it also changes greatly between healthy individuals, especially among children. It is important, therefore, that those conducting clinical studies do not further complicate the situation. One review indicates that in the 23 clinical studies reviewed, 9 different definitions of diarrhea are used. How, then, can one hope to make these data agree with each other?5 The use of standardized definitions is important.

Likewise, it is important to standardize methods for analyzing the results, particularly the symptoms and benefits that are reported. In doing so, each individual locally conducted study could turn into a tile within a mosaic of global scope, with the overall picture gradually becoming clearer.

Conclusions: positive results, but it still takes time

In short, in order to give a clear answer to the questions we asked at the beginning of the article, we have to wait. Because rigorous scientific studies on probiotics have only recently begun, everything is in flux. Thus, it takes time to be able to collect data that can be merged, evaluated, and compared, so as to arrive at a consensus of the scientific community on a certain topic.

In anticipation of these results, scientific research in laboratories around the world continues to study the effects of probiotics onintestinal well-being6, immune defense against respiratory viruses7,8,9, modulation of the inflammatoryresponse10 and other diseases. The results of research conducted in animal models are encouraging and shed light on the complex relationship between the microbiota and the rest of the body, an extremely complex and fascinating field to study.

If these results were also confirmed by meticulously conducted human trials, we might hope to find in probiotics valuable little allies in maintaining our well-being and preventing or treating certain diseases.



[2] Results of research conducted on 4/19/2021



[5] Parker EA, Roy T, D’Adamo CR, Wieland LS. Probiotics and gastrointestinal conditions: An overview of evidence from the Cochrane Collaboration. Nutrition. 2018;45:125-134.e11. doi:10.1016/j.nut.2017.06.024

[6] Salva S, Nuñez M, Villena J, Ramón A, Font G, Alvarez S. Development of a fermented goats’ milk containing Lactobacillus rhamnosus: in vivo study of health benefits. J Sci Food Agric. 2011 Oct;91(13):2355-62. doi: 10.1002/jsfa.4467.

[7] Tomosada Y, Chiba E, Zelaya H, Takahashi T, Tsukida K, Kitazawa H, Alvarez S, Villena J. Nasally administered Lactobacillus rhamnosus strains differentially modulate respiratory antiviral immune responses and induce protection against respiratory syncytial virus infection. BMC Immunol. 2013 Aug 15;14:40. doi: 10.1186/1471-2172-14-40.

[8] Chiba E, Tomosada Y, Vizoso-Pinto MG, Salva S, Takahashi T, Tsukida K, Kitazawa H, Alvarez S, Villena J. Immunobiotic Lactobacillus rhamnosus improves resistance of infant mice against respiratory syncytial virus infection. Int Immunopharmacol. 2013 Oct;17(2):373-82. doi: 10.1016/j.intimp.2013.06.024

[9] Clua P, Kanmani P, Zelaya H, et al. Peptidoglycan from Immunobiotic Lactobacillus rhamnosus Improves Resistance of Infant Mice to Respiratory Syncytial Viral Infection and Secondary Pneumococcal Pneumonia. Front Immunol. 2017;8:948. doi:10.3389/fimmu.2017.00948

[10] Zelaya H, Tsukida K, Chiba E, Marranzino G, Alvarez S, Kitazawa H, Agüero G, Villena J. Immunobiotic lactobacilli reduce viral-associated pulmonary damage through the modulation of inflammation-coagulation interactions. Int Immunopharmacol. 2014 Mar;19(1):161-73. doi: 10.1016/j.intimp.2013.12.020.

Roberto Cighetti

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