There are 10 times more bacteria in the body than your own cells, so it makes sense that these organisms affect our bodies in profound ways, ways that we are still slowly unraveling. Once again showcasing the complexity of this relationship, researchers have found that certain probiotic bacteria might help alleviate anxiety and improve memory.
After providing a small group of healthy men with doses of live Bifidobacterium longum 1714, researchers from University College Cork, Ireland, noted altered responses to a mildly stressful situation, and also a boost in memory test performance.
“Previous research showed that this strain was associated with an altered physiological response to stress in a rodent model, as well as changes in memory,” researcher Allen Andrew told IFLScience. “So we wanted to see if there is a similar effect in healthy human volunteers.”
While the findings are preliminary, this isn’t the first study to look into the interaction between the nervous system and the gut. We know, for example, that there are bacteria in the intestines that can produce nervous system signaling molecules, called neurotransmitters, that are also present in the brain, like the “happy hormone” serotonin.
“Our understanding is quite limited in this area,” said Andrew. “How the neurotransmitters in the gut interact with neurotransmitters in the brain requires more work. But these results do show that even in healthy volunteers, you can change nervous system activity by manipulating what is happening in the gut.”
For the study, yet to be published but presented at the Society for Neuroscience meeting in Chicago last week, 22 healthy male volunteers between the ages of 18 and 40 were given either a placebo for four weeks and then a probiotic pill for the same period, or the other way round. At baseline, and then again after each treatment, participants were subjected to numerous tests and given questionnaires to report anxiety, which was measured by the Spielberger State-Trait Anxiety Inventory.
One of the tests involved memorizing patterns in boxes, and the researchers found that the probiotic seemed to correlate with an improved performance. Another involved submerging the men’s hands in ice-cold water for up to three minutes and examining their stress responses. While anxiety was understandably found to increase across all visits, the increase was less when taking the probiotic as compared with the placebo. Moreover, after taking the probiotic, output of the “stress hormone” cortisol was lower over the course of the stressor.
“What is interesting about the memory test is that performance is associated with a brain region called the hippocampus, which is also influenced by stress,” said Andrew. “The HPA axis, which controls cortisol levels, interacts with the hippocampus, so the results seem to tie together.”
Participants were also asked to report daily stress levels throughout the study using an online questionnaire, which was again found to decrease towards the end of the probiotic stint.
There are obviously limitations to this study. For starters, the team has yet to prove that the bacterium even makes it to the gastrointestinal tract, but stool samples have been taken so hopefully later analysis will shed light on this. They also didn’t look at women, although Andrew says this is because the menstrual cycle can affect cortisol output, so the study would have been more difficult.
The study also didn’t look at potential mechanisms to explain the patterns seen, although the team thinks that vagus nerve signaling could be involved, one of the cranial nerves that supplies the digestive tract. Perhaps the bacteria release molecules that interact with the nerve fibers, but at the moment that is hand-waving.