From Gut to Brain to Gut and Back Again
Gut health is about so much more than just the gut. I remember in college neuroscience classes, learning about neurotransmitters and how there is more serotonin in the gut than in the brain (about 95% of our serotonin lies in the gut). At the time, I was shocked to learn this. As far as I knew, neurotransmitters and the messages they sent were limited to the brain. When you think about it though, it makes a lot of sense that neurotransmitters have jobs in other parts of the body. We can feel the implications of the impact of our psychological state on our gut when we get butterflies or lose our appetite before a stressful presentation. This is also why individuals who take SSRIs to treat depression often have unwanted and sometimes serious digestive side effects.
But what about the other way around? What about the role of our gut health on our psychological health? The road goes both ways, and we see that in the common co-occurrence of psychiatric symptoms such as anxiety and depression with digestive disorders such as IBS.
Neurons line the walls of our entire gut, making up what is called the enteric nervous system, or the ?second brain?. One function of the brain in our gut is to control all aspects of digestion, from breaking down food, to absorbing nutrients, to excreting waste. However, there is more to it than that. About 90% of the fibers in the vagus nerve (the primary nerve that connects the gut to the brain) carry information FROM the gut TO the brain. So, the gut seems to be saying more to our brain than our brain is saying to our gut. And as it turns out, that exchange of information is vital to our health and survival. The signaling between the central nervous system and the enteric nervous system are important in the regulation of hormonal, immune, and other systems within the body.
Beyond that, research suggests that the gut microbiome may impact communication between the gut and the brain, and thereby impact behavior and psychological health. Probiotics, often used successfully to treat symptoms of IBS, also appear to be effective for reducing stress and anxiety and improving mood. It?s not clear exactly how this happens, though there is evidence to suggest that certain microbes influence the way the gut communicates with the brain. In mouse studies, probiotics were shown to have antidepressant and anxiolytic (stress reducing) effects. However, when the vagus nerve was severed in the mice, the same effects did not occur. This indicates that microbes somehow influence the messages that are sent from the gut to the brain directly through communication via the vagus nerve.
Findings related to the impact of microbes on psychological health are not isolated to mouse studies. There is clinical evidence for the role of probiotics in reducing stress and anxiety in human subjects as well. Specifically, the same probiotics found to improve digestive symptoms have also been shown to reduce anxiety, decrease cortisol (stress hormone) levels, and decrease depressive symptoms. So far, the effects seem to be dependent on the particular strain of bacteria, with Lactobacillus helveticus, Bifidobacterium longum, Lactobacillus reuteri, and Bifidobacterium infantus showing the most notable beneficial effects in some studies.
Data from research with antibiotics also offers some interesting findings. Unsurprisingly, use of broad-spectrum antibiotics leads to a reduction in the diversity of gut microbes. Many of us have experienced unpleasant digestive symptoms following completion of a course of antibiotics, which is almost certainly due to gut dysbiosis resulting from the killing-off of good gut bacteria. Experiments in mice show that, when gut bacteria are wiped out following antibiotic administration, the mice showing symptoms of increased stress responses and anxiety. In both humans and mice, antibiotic-induced gut dysbiosis can result in behavioral and brain chemistry changes. In mouse experiments, re-inoculation of the gut with healthy bacteria essentially erased the negative effects of antibiotics, with the mice again showing lower levels of stress and anxiety. There is reason to believe that the same effects could be seen in humans.
And here?s where it goes back again, or maybe where it started in the first place. Because signaling goes both ways, stress can impact composition of the gut microbiome itself. This may, in turn, contribute to stress response from microbiome disruption. It?s possible that there is a positive feedback loop in those with increased stress and anxiety. Likewise, a healthy gut microbiome can contribute to beneficial psychological states, which can further promote gut health.
So what does all of this mean from a practical perspective? My approach for now is to continue to eat plenty of fermented foods, take a quality probiotic supplement when I remember, avoid antibiotics unless absolutely necessary, and incorporate plenty of stress management into my life. There is evidence for the benefits of fermented food consumption from both ancestral tradition and more recent scientific studies. It?s hard to go wrong consuming fun, delicious, traditional foods, and doing so could go a long way towards improving overall health and wellness.
Bercik, P., & Collins, S. (2014). Microbial endocrinology: The microbiota-gut-brain axis.
Bested, A., Logan, a., & Selhub, E. (2013). Intestinal microbiota, probiotics and mental health: From Metchnikoff to modern advances: Part II – contemporary contextual research.?Gut Pathogens, 5(3), 1-14.
Cryan, J. & O-Mahony, S. (2011). The microbiome-gut-brain axis: From bowel to behavior.?Neurogastroenterology & Motility, 23(3), 187-192.
Foster, J. & McVey Neufeld, K. (2013). Gut-brain axis: How the microbiome influences anxiety and depression. Trends in Neurosciences, 36(5), 305-312.