(CNN) -- A gut-wrenching experience. Butterflies in your stomach.
Many of us instinctively feel the connection between our gut and our brain. That connection and how the range of bacteria residing in our digestive tract -- our microbiome -- might help treat mental illness has become a field of interest for scientists in recent years.
A new review of medical literature has suggested that probiotics -- foods or supplements containing microbes thought to exert a positive influence on our gut -- could help ease depression.
"This is good quality research but it is a review of relatively preliminary data," Allan Young, a professor of mood disorders at the Institute of Psychiatry, Psychology & Neuroscience, King's College London, told the Science Media Centre in London.
"So while this systematic review of the research literature supports the notion that pre and probiotics may be helpful for people with anxiety and depression, more research is needed. These data do make a case for larger trials to be carried out," said Young, who wasn't involved in the review.
The researchers from the University of Brighton and Croydon University Hospital in the United Kingdom looked at 71 studies published between 2003 and 2019 that looked at how probiotics, and prebiotics, compounds that help probiotics flourish, may help adults with depression and/or anxiety disorders.
Only seven were deemed robust enough to include in the systematic review, but all of those showed "significant improvements" when measuring the effect of taking pre/probiotics compared with no treatment or a placebo. While probiotic supplements either alone or in combination with prebiotics may be linked to measurable reductions in depression, the possible contribution to lessening anxiety was not yet clear, the study said.
The researchers said their review had several caveats: None of the included studies lasted very long; and the number of participants in each was small.
This made it difficult to draw any firm conclusions about the overall effects, how long they lasted and whether there might be any unwanted side effects associated with prolonged use, they said.
David Curtis, a retired consultant psychiatrist and an honorary professor at University College London, said that it was unlikely that probiotics would have an effect on mood.
"Although these published studies claim to show some benefits of probiotics on depression, we have no idea whether there were other studies which showed no effect which were not published," he told the SMC.
"People with depression should seek medical advice and not try to treat themselves with dietary supplements, which are not of proven benefit."
The review, which published on Monday in the journal BMJ Nutrition, Prevention & Health, said exactly how probiotics could ease depression was unknown, but the researchers suggested the effect could be twofold.
Firstly, probiotics may help reduce the production of inflammatory chemicals, such as cytokines, or they may help direct the action of tryptophan, a chemical thought to be important in the gut-brain axis in psychiatric disorders.
Secondly, people with depression also often have other underlying conditions, such as impaired insulin production and irritable bowel syndrome, and probiotics may influence how a person experiences depression by alleviating these conditions.
However, the case for the use of probiotics in the treatment of digestive disorders isn't clear.
For the majority of digestive diseases -- including digestive conditions like Crohn's disease, ulcerative colitis or irritable bowel syndrome, a review published last month by the American Gastroenterological Association said there's not enough evidence to recommend the use of probiotics.
"We are still learning about the numerous, complex communication pathways between the brain and the gut. It is certainly plausible that microbiome-targeting therapies could improve aspects of depression and anxiety by alleviating an individual's GI," said Dr. Geoffrey Preidis, spokesperson for the association and a pediatric gastroenterologist at Baylor College of Medicine and Texas Children's Hospital.
"The AGA's review of the current evidence base did not find sufficient evidence to recommend probiotics for IBS at this time. However, future clinical trials will address this knowledge gap, so we must continue to evaluate the rapidly changing evidence in this exciting field," he said via email.
Probiotics have become more popular as researchers have learned more about the role of our gut bacteria, or microbiome, on our gastrointestinal health, with probiotics promising an effective way of altering the microbiome for our benefit.
The studies in the review looked at 12 probiotic strains, primarily Lactobacillus acidophilus, Lactobacillus casei and Bifidobacterium bifidum. One study looked at combined pre-probiotic treatment, while one looked at prebiotic therapy by itself.
John Cryan, professor and chair of the department of anatomy and neuroscience, and principal investigator at the Alimentary Pharmabiotic Center, University College Cork, said that it was important to identify which strains had which effect.
"We know that strains really matter, and this review is not able to identify what it is about specific strains that render them with beneficial effects," he told the SMC.
"As this review highlights, there is a great need for longitudinal studies for different psychobiotic strains and diets both as standalone or as adjunctive therapies in anxiety and depression."
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