Probiotics’ promise varies for different conditions
As the pathways by which gut dysbiosis may influence psychiatric disorders become clearer, the next logical step is to try to influence the composition of the microbiome to prevent and treat depression, anxiety, or schizophrenia. That’s where probiotics come in.
The evidence for the effects of probiotics – live microorganisms which, when ingested in adequate amounts, confer a health benefit – so far is the strongest for depression, says Viktoriya Nikolova, MRes, MSc, a PhD student and researcher at King’s College London. In their 2021 meta-analysis of seven trials, Mr. Nikolova and colleagues revealed that probiotics can significantly reduce depressive symptoms after just 8 weeks. There was a caveat, however – the probiotics only worked when used in addition to an approved antidepressant. Another meta-analysis, published in 2018, also showed that probiotics, when compared with placebo, improve mood in people with depressive symptoms (here, no antidepressant treatment was necessary).
Roumen Milev, MD, PhD, a neuroscientist at Queen’s University, Kingston, Ont., and coauthor of a review on probiotics and depression published in the Annals of General Psychiatry, warns, however, that the research is still in its infancy. “,” he says.
When it comes to using probiotics to relieve anxiety, “the evidence in the animal literature is really compelling,” says Dr. Ilardi. Human studies are less convincing, however, which Dr. Dr. Ilardi showed in his 2018 review and meta-analysis involving 743 animals and 1,527 humans. “Studies are small for the most part, and some of them aren’t terribly well conducted, and they often use very low doses of probiotics,” he says. One of the larger double-blind and placebo-controlled trials showed that supplementation with Lactobacillus plantarum helps reduce stress and anxiety, while the levels of proinflammatory cytokines go down. Another meta-analysis, published in June, revealed that, when it comes to reducing stress and anxiety in youth, the results are mixed.
Evidence of probiotics’ efficiency in schizophrenia is emerging, yet also limited. A 2019 review concluded that currently available results only “hint” at a possibility that probiotics could make a difference in schizophrenia. Similarly, a 2020 review summed up that the role of probiotics in bipolar disorder “remains unclear and underexplored.”
Better studies, remaining questions
Apart from small samples, one issue with research on probiotics is that they generally tend to use varied doses of different strains of bacteria, or even multistrain mixtures, making it tough to compare results. Although there are hundreds of species of bacteria in the human gut, only a few have been evaluated for their antidepressant or antianxiety effects.
“To make it even worse, it’s almost certainly the case that depending on a person’s actual genetics or maybe their epigenetics, a strain that is helpful for one person may not be helpful for another. There is almost certainly no one-size-fits-all probiotic formulation,” says Dr. Ilardi.
Another critical question that remains to be answered is that of potential side effects.
“Probiotics are often seen as food supplements, so they don’t follow under the same regulations as drugs would,” says Mr. Nikolova. “They don’t necessarily have to follow the pattern of drug trials in many countries, which means that the monitoring of side effects is not the requirement.”
That’s something that worries King’s College psychiatrist Young too. “If you are giving it to modulate how the brain works, you could potentially induce psychiatric symptoms or a psychiatric disorder. There could be allergic reactions. There could be lots of different things,” he says.
When you search the web for “probiotics,” chances are you will come across sites boasting amazing effects that such products can have on cardiovascular heath, the immune system, and yes, mental well-being. Many also sell various probiotic supplements “formulated” for your gut health or improved moods. However, many such commercially available strains have never been actually tested in clinical trials. What’s more, according to Kathrin Cohen Kadosh, PhD, a neuroscientist at University of Surrey (England), “it is not always clear whether the different strains actually reach the gut intact.”
For now, considering the limited research evidence, a safer bet is to try to improve gut health through consumption of fermented foods that naturally contain probiotics, such as miso, kefir, or sauerkraut. Alternatively, you could reach for prebiotics, such as foods containing fiber (prebiotics enhance the growth of beneficial gut microbes). This, Dr. Kadosh says, could be “a gentler way of improving gut health” than popping a pill. Whether an improved mental well-being might follow still remains to be seen.
A version of this article first appeared on Medscape.com.