WEDNESDAY, July 8, 2020 (HealthDay News) -- Probiotic supplements might help ease depression symptoms in some people, a new research review suggests.
Researchers found that across seven small clinical trials, various probiotics seemed to improve symptoms in patients with clinical depression -- at least in the short term.
The studies build on a growing research interest in the role of gut health -- specifically, the balance of bacteria dwelling there -- and brain health.
But experts stressed that the probiotic trials had a number of limitations, and it's too soon to draw any conclusions.
For one, a "placebo effect" cannot be ruled out, according to Sanjay Noonan, the lead author on the research review.
And, he said, besides being small, the trials did not look at the longer term: All lasted about two to three months.
According to Noonan, "no definitive statements can be made" on whether people with depression stand to benefit from probiotics.
"It would be conjecture to try and suggest anything about the long-term efficacy of probiotic therapy," he said.
Noonan and his colleagues at Brighton and Sussex Medical School in England reported the findings July 6 in the journal BMJ Nutrition, Prevention & Health.
Probiotics are live bacteria and yeast that naturally dwell in the body. Probiotic supplements are marketed as a way to restore a healthier balance of good bacteria.
The digestive system, in particular, hosts a vast array of bacteria and other microbes -- known as the "gut microbiome." And those organisms are believed to do more than just aid in digestion.
Research suggests the microbes are involved in everything from immune defenses to producing vitamins, anti-inflammatory compounds, and even chemicals that influence the brain.
Meanwhile, a number of studies have linked the makeup of the gut microbiome to the risks of various health conditions. These include brain-based conditions like Alzheimer's and autoimmune diseases like type 1 diabetes and rheumatoid arthritis.
In a 2019 study, researchers found that people with depression showed differences in specific gut bacteria, versus those without depression. Levels of two types of bacteria -- Coprococcus and Dialister -- were reported to be "consistently depleted" in people with depression.
But none of that proves a lack of those bacteria, or any others, actually helps cause depression. And for now, no one knows if any probiotics can help treat it.
"It's important to stress that this area of research is in an extremely early phase," Noonan said.
He said the point of this review was to look at the existing evidence on probiotics, and not to offer answers.
The trials in the review each contained fewer than 100 people. And they most often tested any of three probiotic strains: L. acidophilus, L. casaei and B. bifidum. One trial tested a probiotic combined with a "prebiotic" -- compounds that promote the growth of probiotics.
On average, the review found, study patients' depression symptoms improved over two to three months. Some trials, however, did not include a comparison group that took inactive supplements, to help account for the placebo effect. (That's the phenomenon in which people feel better simply because they are receiving treatment and believe it will work.)
Another issue is the trials give no clues on which bacterial strains might be helpful, according to John Cryan, a professor at University College Cork in Ireland.
"There is a tendency in the field to 'lump' all commercially available strains into the same category independent of the level of evidence there is," Cryan told the nonprofit Science Media Centre.
But, he added, "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."
Kevin Whelan, a professor at King's College London, made similar points. "Probiotics often contain different strains of bacteria, and we do not know if the supplements, sachets and fermented milks you find on supermarket shelves will work," he told SMC.
Whelan also stressed that most patients in the trials were taking antidepressants.
"So it is crucial that probiotics are seen as complementary to standard treatments recommended by your doctor," he said, "and not as an alternative."