A meta-analysis of seven studies evaluating what effect probiotic bacteria may have on subjective levels of stress, anxiety, and depression found that both short- and long-term administration of a number of well-known probiotic bacteria is effective at generally reducing the subjective stress levels of healthy people. The meta-analysis included results gathered from populations amounting to a total of 1,146 clinical trial participants, marking ground for further research on this benefit, by which the mechanisms are still unknown.
 
High-level, long-term stress is considered to have a negative impact on the physical and mental health of people, in addition to adversely affecting daily life, with symptoms including elevated blood pressure and heart rate, and high correlations with behavioral issues, mental disorders, and unhealthy lifestyle choices.
 
Prior research has already identified that certain endogenous probiotic bacteria play a role in the development and function of the central nervous system, and that psychological stress appears to modify intestinal flora. Because of this, a subset of probiotics, dubbed “psychobiotics” are believed to have the potential to alleviate stress through certain bioactive factors, possibly by improving the synthesis and activity of serotonin, cortisol, and BDNF (Brain-Derived Neurotrophic Factor).
 
In the clinical trials assessed by the authors, significant improvements were observed in probiotics experimental groups in subjective stress level, as measured by a number of questionnaires and assessments, as was the stress-related subthreshold anxiety level. Most but not all of the evaluations involved participants’ self-reporting. Secondary outcomes that were assessed included cortisol level and adverse reactions within the clinical trial settings.
 
Dozens of single-strain and multiple-strain probiotics were assessed across the clinical trials, including those that came in capsule formats and more conventional, food-based delivery formats such as yogurt. Only one trial reported adverse reactions, however, there was no detailed description about the adverse reaction, the authors said.
 
“A large number of clinical trials based on microbiome-based innovative therapies (such as probiotics, prebiotics, and fecal transplants) have been carried out for stress-related diseases at present; thus, we conducted this research with a clear focus and address whether probiotic management is effective to stress,” the authors concluded. “This systematic review and meta-analysis showed that probiotic administration could reduce subjective stress level in healthy volunteers and may relieve stress-related subthreshold anxiety/depression level, but the effect on cortisol was not significant. In addition, adverse reactions were left undescribed; then, there is not enough support to draw conclusions about adverse effects. However, since the results of the meta-analysis are still unstable, there is a need for more reliable clinical evidence support and a deeper understanding of the strain specificity of probiotics before probiotics can be used to relieve stress.”