Gut microbiome-targeted interventions are being explored as means of improving cognitive performance. That has led to the era of psychobiotics, defined as any exogenous influence—including but not limited to probiotics, prebiotics and bacterial metabolites—whose effect on the brain is microbiota-mediated.

A new meta-analysis of randomized controlled trials has found no significant effect on improving cognition for probiotics, prebiotics and fermented foods, whether alone or used in combination.

The authors analyzed a group of 22 individual studies that included a non-probiotic intervention as a comparator. Although 14 studies reported an improvement in at least one validated or self-reported cognitive screening and neuropsychological measure led by probiotics, prebiotics or fermented foods, the pooled meta-analysis found no significant effect on cognitive performance for any intervention.

The conditions in which the probiotic, prebiotic and fermented food interventions were studied include Alzheimer’s disease, cirrhosis, fibromyalgia, major depressive disorder, psychosis, mild cognitive impairment and stress. Studies were also carried out in healthy adults.

Some adverse events were also reported, mainly in the form of gastrointestinal symptoms such as diarrhea, bloating and flatulence.

Most studies had low risk of bias. When it comes to the null findings observed, it is acknowledged in the discussion that potential reasons include the limited number of small and short-term studies (lack of statistical power) as well clinical heterogeneity in terms of the participants studied, cognitive tests, and dose and formulation used in probiotic and prebiotic interventions. In addition, the subclinical population selected in most of the studies included in the meta-analysis may mask the detection of an intervention’s positive result in enhancing cognition.

This meta-analysis concludes that it is too early to recommend probiotics, prebiotics or fermented food interventions for improving cognitive outcomes. Large clinical trials with high quality designs in sufficient samples of healthy and clinical populations are needed in order to better elucidate to what extent probiotics, prebiotics or fermented foods are worth considering for mental and cognitive outcomes.



Sarkar A, Lehto SM, Harty S, et al. Psychobiotics and the manipulation of bacteria-gut-brain signals. Trends Neurosci. 2016; 39(11):763-781. doi: 10.1016/j.tins.2016.09.002.

Marx W, Scholey A, Firth J, et al. Prebiotics, probiotics, fermented foods and cognitive outcomes: a meta-analysis of randomized controlled trials. Neurosci Biobehav Rev. 2020;118:472-484. doi: 10.1016/j.neubiorev.2020.07.036.