The more scientific research I am exposed to the less inclined I am to expect simple solutions to complex problems. That said, a report from 2015 highlighted in The Guardian was one of the first accounts of probiotics that turned me on to the potential of gut flora to influence different aspects of health and wellbeing. It was reported that participants in an experiment took a capsule containing Bifidobacterium longum 1714 for a month. As a result they enjoyed lower levels of stress when compared to the control group that took a placebo. Stress was measured in terms of the levels of cortisol (a hormone associated with stress) found in the participants.
A second result of the investigation was that when taking the probiotic, memory function also seemed to be enhanced. The people taking part in the experiment took either the placebo for a month or Bifidobacterium longum 1714, then switched. It was a blind test so the participants didn’t know when they were taking the probiotic or the placebo. The effects of the reduced stress and improved memory were described as small but significant, make of that what you will. But the same effects were found in a similar experiment carried out on mice.
I’m not a great fan of experimentation on animals, not because experiments on animals are rarely replicated in a similar way on humans, or that it generally support poor science. Just because its cruel and not very ethical. However as the data exists I will draw your attention to it. Another study from 2015 indicated that B. longum 1714 had a positive impact on mice cognition and an apparent reduction on fear. So one probiotic strain appears to have an influence on both fear and a narrow range of cognitive function (or that both memory and fear/stress have a commonality that can be meditated by B. longum 1714). The fact that both mice and humans appear to have been effected in the same way is interesting. Although these are tentative findings and more studies demonstrating the same effects are necessary (replication).