Recent advances in research have described the importance of gut microbiota in influencing interactions between the central and the enteric nervous systems. These brain-gut interactions appear to be bidirectional by means of neural, endocrine, immune, and humoral signals. Most of the data have been acquired using rodents (mice or rats) and pigs.
Evidence of microbiota-mental health interactions comes from the association of intestinal dysbiosis with central nervous system disorders (e.g. anxiety-depressive behaviours) and functional gastrointestinal disorders (e.g. irritable bowel syndrome) with mental health comorbidities.
Few human studies assessing the effect of probiotic supplementation on mental health through modulation of brain-gut pathways have been conducted to date. However, a recent systematic review of 38 randomized controlled trials in both animals and humans (25 were in animals, 15 in humans and 2 studies were conducted in humans and animals), led by Prof. Paul Enck from the Department of Psychosomatic Medicine and Psychotherapy at University Hospital Tübingen in Tübingen (Germany), has concluded that probiotics could be effective in improving psychiatric disease-associated functions and memory abilities. The paper by Wang, et al. found probiotics that showed efficacy in improving psychiatric disorder-related behaviours (anxiety, depression, mood, stress response) and memory abilities (including spatial and non-spatial memory) included Bifidobacterium (B. longum, B. breve, and B. infantis) and Lactobacillus (L. helveticus, L. rhamnosus, L. plantarum, and L. casei). Doses between 109 and 1010 colony-forming units (CFU) for durations of 2 weeks in animals and 4 weeks in humans showed sufficient effects. Although translations of animal studies to human studies suggest possibilities, further studies are worthwhile, especially in patients with mental diseases that usually show gastrointestinal comorbidities. In addition to behavioural measurements such as psychological questionnaires or scales, more neuroimaging studies in humans are needed in order to study what area is altered in the brain that causes behavioural changes after the consumption of probiotics.
On the other hand, a recent systematic review of 10 randomized controlled trials in humans (including 6 trials that were also included in the Wang, et al. systematic review), led by Dr. Paul Ritvo from York University in Toronto, Ontario (Canada), provides limited support for the use of probiotics in reducing anxiety-depressive symptoms in humans. Although it seems that probiotic supplementation could lead to psychological benefits, substantial methodological limitations were found as the main problem in generalizing its findings. The researchers emphasized that further follow-up intervention studies are needed in order to better understand the potential human mental health benefits of probiotic supplementation.
One of many possible mechanisms that could explain the role of gut microbes in affecting brain function is through modulating the level of microbial producers and consumers of gamma-aminobutyric acid (GABA). In this context a noteworthy and important step forward is the recent identification of a species of gut bacteria, called KLE1738, which can only grow in the presence of gamma-aminobutyric acid (GABA). KLE1738 is a newly discovered human gut bacterium that may have an unusual metabolism that is based on consuming GABA, the major inhibitory neurotransmitter of the central nervous system. Other mechanisms involving serotonin-producing bacteria could also have equal potential relevance for depression and other mood disorders.
On the whole, current research supports the idea that microbial communities in the gut may play an important role in mental health. Further human studies in this area are needed in order to elucidate which patients could benefit from probiotic supplementation for improving their mental function.
Pirbaglou M, de Souza RJ, Stearns JC, Motamed M, Ritvo P. Probiotic supplementation can positively affect anxiety and depressive symptoms: a systematic review of randomized controlled trials. Nutr Res. 2016, doi:10.1016/j.nutres.2016.06.009.
Wang H, Lee IS, Braun C, Enck P. Effect of probiotics on central nervous system functions in animals and humans – a systematic review. J Neurogastroenterol Motil. 2016, doi: 10.5056/jnm16018.