The mind-gut connection: What is it and how did it evolve? That is the question posed for this short podcast (the first in a series of three on the topic) to gastroenterologist Emeran Mayer, MD, PhD, a pioneer of medical research into brain-gut interactions and author of The Mind-Gut Connection: How the Hidden Conversation Within Our Bodies Impacts Our Mood, Our Choices, and Our Overall Health. Here, Dr. Mayer talks about his interest in the mind’s relationship to the GI tract, some basics of what bacterial gut/brain communication is, and the evolutionary biology theories behind it. He even discusses the hypothesis that bacteria actually tells our bodies what to eat—like to crave sugary foods in obesity. Dr. Mayer's website is http://emeranmayer.com. Twitter: @emeranmayer.
David Carreon: Hey everybody this is David Carreon.
Jessi Gold: And this is Jessi Gold.
David Carreon: And this is Psyched, a psychiatry podcast. Today we have Emeran Mayer on with us. He was born in a small town in Bavaria, where his family ran a confectionary business since 1873. After deciding against taking over the family business he finished medical school at Ludwig Maximilian’s University in Munich, completed his residency training at Vancouver General Hospital in Vancouver, before moving to Los Angeles. Dr. Mayer is a professor in the department of medicine, physiology and psychiatry at the David Geffen School of Medicine at UCLA, and Executive Director of the G. Oppenheimer Center for Neurobiology of Stress and Resilience. And Co-director of the CURE: Digestive Diseases Research Center at UCLA. Dr. Mayer, thank you for joining us.
Emeran Mayer: It's a pleasure to be on the show.
David Carreon: So, you've done a fascinatingly broad set of work over your career, looking at your research interests from traditional healing, to hypnosis, to gut microbiology. How did you get such diverse research interests?
Emeran Mayer: Well, there was one common thread that goes through all of this from the very beginning and that is really I've always been interested in the interaction between the mind, the brain, and the body. And just have pursued this from really from college on, and so there's certainly a red line going, a straight line going through all of my visits to different topics in this area. I think right now ... so I was looking at the mind, brain, gut, micro environment interaction so that integrating all of these elements into one, and that's in some ways really been my interest from the very beginning.
David Carreon: Say more about that with the mind-gut connection. How do you conceive of the gut and how is there a connection?
Emeran Mayer: Well, I mean I've been interested in brain, gut interaction for the better part of my career. That topic was in some ways relegated to studying a disease called Irritable Bowl Syndrome, because that was sort of a classic entity where you had psychological commodity, GI symptoms, psychological interventions were beneficial. And then only about I would say seven years ago I developed this interest in including the gut microbes in this communication, so I think right now people are getting so excited about the microbes themselves. I mean there's always been this very intricate by directional communication between the brain and the gut. There's many interesting cells in the gut, but the microbes now play a role in using these various communication channels from the gut to the brain to be included in this dialogue.
Interestingly, and it was never been really great interest of people outside very narrow area of Neurogastroenterologists, until the microbiome arrived. Now, it's almost like everybody has become a gut microbiome expert of brain, gut microbiome expert. As you know there's many books written on this topic, the lay media loves it. And for me it's nice because at this stage in my career I get invited to all kinds of meetings outside of gastroenterology, including the APA meeting.
David Carreon: So, you said something that was rather astonishing I think to most people, that there's a dialogue between the microbes and the person, or the brain. Are there really like signals sent back and forth? How do you justify saying that this is communication?
Emeran Mayer: If you go back in evolution you'll find that microbes were obviously the dominant life form on planet Earth for billions of years, four and a half billion, a billion years, and then at some point there was a decision made by some algae in the ocean to settle inside the digestive system of a primitive marine animal, the Hydra. That animal itself was a floating digestive tube with a nerve net around it. Very similar actually to our GI tract, so our GI tract started ... like the earliest forms of life were GI tracts with an attached nervous system around it.
The microbes started living inside that system, and used their own communication signals that they had developed over four and a half billion years in the oceans to start communicating with the nerve cells of these floating digestive tubes. That actually explains a lot why the gut and the brain are so closely connected. And it also explains why the microbes have developed this ability, not just to communicate with the host but also to most likely exchange genes with this process lateral gene transfer, to the host. You can almost assume that the similarity of some neuro active substances that the microbes can produce, that we know today like GABA and tryptophan metabolites, where actually transferred then to the entire nervous system of these marine animals, and were really the origin of our own neurotransmitters ultimately. So, it's a very interesting story, which I think is quite convincing in terms of why that is happening.
David Carreon: Wait, so the thought is that there was the bacteria are sending their own genes into the host organism, and the host organism is incorporating those and then producing those things that the bacteria want?
Emeran Mayer: Yes. So, that's the assumption and that probably explains why the host has common language, it's common biological language, which goes from the microbes to various cells and receptors in the gut, including nerve cells. And also, since a lot of microbes live on plants, many plants have again, very homologous structures that sort of resemble some of our neurotransmitters. So, this clearly is a certain universality in terms of these signals, and the origin of all this is most likely the microbes that four and a half billion years to develop that language, had 400 times more genes than we humans have. And they have the ability, this ongoing evolution, I mean this is not a static system, so there's constant selection and that may be one of the advantages of having these microbes inside of us, because you can adapt to many things much better than our human system could adapt.
Jessi Gold: So, what you're saying is, there's things that the microbes signal for in us? Like, what kind of things would a microbe want our brain to be thinking, or doing, or can you explain that a little bit more?
Emeran Mayer: Yeah, so that's a very good question. I think right now it's more speculative, the answers, so somebody has suggested that the microbes may actually be able to hack into our reward system, and get us to do things, particularly to eat things that is beneficial for their own growth. And that is not been proven yet, I mean there's a recent study in fruit flies where it's been shown that microbes are able to change the behavior of these fruit flies. I mean obviously in humans and even in rodents we don't have this yet, but there's an interesting possibility for example in obesity where people develop this craving for high fat, high sugary foods. The normal composition of our microbes changes in a way that certain strains start dominating that then send signals to our brain, into the reward system, that makes us crave for these particular food items. Almost then it becomes like a parasitic relationship. It's no longer the symbiosis that exists normally, that everything is beneficial for both players. In this case the microbes then would take over in terms. It's really speculation. You know.
Jessi Gold: Yeah.
Emeran Mayer: But it's intriguing because we don't have the clear answer as what they would like to tell our nervous system, so I guess that's still really a mystery.