In the past decade, universities such as Washington University in St. Louis have been conducting research on probiotics’ effect on our gut and metabolism. Jeffrey I. Gordon at the Washington University School of Medicine found that Malawi children suffering from undernutrition did not respond to highly nutritious food. However, while this was the case, undernourished mice that received a transfer of gut bacteria from healthy mice did increase body weight. The influence of gut bacteria may reach farther than just the gut and metabolism; more recent research indicates that our nervous system may also be impacted by changes in the diversity of those bacteria, factors not traditionally associated with psychiatric medicine.
In April of this year, the scientific journal Cell published a paper describing how certain gut bacteria, known as indigenous spore-forming microbes, regulate serotonin synthesis and uptake. The authors determined that the bacteria produced lasting increases in the production of serotonin in mice with low initial levels of serotonin in their blood. Importantly, the results could be reversed with a cocktail of antibiotics. If the bacteria caused complications, those bacteria could be removed. Although introducing indigenous microbes into the gastrointestinal (GI) tract can act as a remedy for serotonin-deficient mice, the effects of such microbes on serotonin regulation in people has yet to be fully investigated.
One research group at the Leiden Institute of Brain and Cognition in the Netherlands studied the effects of common gut bacteria on mood in 40 individuals over the course of four weeks. The research showed that individuals who took a multi-species probiotic reported less recurrent negative thoughts than those in the control group after feeling depressed.
Meanwhile, in a similar study, Cambridge University tested for changes in overall emotional state rather than the persistence of negative thoughts. Researchers gave some participants milk containing a species of gut bacteria and others milk without the bacteria. Mood was recorded on the first, tenth, and last day, and the results showed that there was no statistically significant difference in mood between the participants who received the probiotic and those who did not
Although the Dutch study shows that negative thoughts are less prevalent, Cambridge concludes that the overall mood remains relatively unaffected. Importantly, none of the participants were clinically depressed or had anxiety disorders. Because people are generally content or indifferent, their mood may not have changed. A more significant change in mood may be seen in those who are depressed rather than those without psychiatric disorders, who display very little change from day to day.
Among those with psychiatric disorders, chronic fatigue syndrome (CFS) patients commonly suffer from anxiety and gastrointestinal (GI) abnormalities. A 2009 study conducted by researchers at the University of Toronto tested the emotional states of CFS patients before and after ingesting a strain of Lactobacillus casei. Patients taking the Lactobacillus powder demonstrated fewer anxiety symptoms and were able to more accurately recognize that they were taking a probiotic rather than a placebo compared to the group taking the placebo, showing that the probiotic had demonstrable effects on mood.
Despite an apparent improved emotional state, the underlying causes and the pathways through which the bacteria act on the nervous system are still not fully understood. So, before we start downing gallons of probiotics, a more complete investigation of altering the microbiome of the GI tract is necessary. This investigation may help cultivate discussion on how gut bacteria affect our digestive and emotional health.