How Your Gut Influences Your Mental Health: It’s Practically a Second Brain | Dr. Emeran Mayer
The Mind-Gut Connection is something that people have intuitively known for a long time, but science has only, I would say, in the last few years, gotten a grasp and acceptance of this concept. It essentially means that your brain has intimate connections with the gut and another entity in our gut, the second brain, which is about 100 million nerve cells that are sandwiched in between the layers of the gut.
And they can do a lot of things on their own in terms of regulating our digestive processes. But there’s a very intimate conversation between that little brain, the second brain in the gut, and our main brain. They use the same neurotransmitters. They’re connected by nerve pathways. And so we have really an integrated system from our brain to the little brain in the gut, and it goes in both directions.
The little brain, or the second brain, in the gut you’re not able to see it because, as I said, it’s spread out through the entire length of the gut from your esophagus to the end of your large intestine, several layers of nerve cells interconnected. And what they do is, even if you – and you can do this in animal experiments – if you completely disconnect this little brain in the gut from your main brain, this little brain can completely take care of all the digestive processes, the contractions, peristaltic reflex, regulation of blood flow in the intestine.
And it has many sensors, so it knows exactly what’s going on inside the gut, what goes on in the wall of the gut, any distention, any chemicals. All of this is being picked up by these sensory nerves, fed into the interior nervous system, the second brain. And then the second brain generates these stereotypic responses. So when you vomit, when you have diarrhea, when you have normal digestion, all of this is encoded in programs in your second brain.
What the second brain can’t do is it cannot generate any conscious perceptions or gut feelings. That really is the only ability that allows us to do this and perceive all the stuff that goes on inside of us. It is really the big brain and the specific areas and circuits within the brain that process information that comes up from the gut.
Still, most of that information is not really consciously perceived. So 95 percent of all this massive amount of information coming from the gut is processed, integrated with other inputs that the brain gets from the outside, from smell, visual stimuli. And only a very small portion is then actually made conscious. So when you feel good after a meal or when you ate the wrong thing and you’re nauseated, those are the few occasions where actually we realize and become aware of our gut feelings, even though a lot of other stuff is going on in this brain-gut access all the time.
When we talk about the connection between depression and the gut, there are some very intriguing observations, both clinically, but also now more recently scientifically, that make it highly plausible that there is an integrated connection between serotonin in the gut, serotonin in our food, depression, and gut function. On a clinical level, there’s a connection because many patients with depression also complain of constipation. So, a distinct dysfunction of the gut.
And often the medications that people with depression take, particularly the serotonin reuptake inhibitors such as Prozac and all the other drugs in this category, they often cause transient gastrointestinal dysfunction. So that’s on the clinical level. However, what makes it particularly interesting and still an open question is really that more than 95 percent of all our serotonin we have in our organism is really produced and stored in the gut in specialized cells, so-called enterochromaffin cells.
So, our major, by far, the largest store of that molecule that plays such a big role in modulating our mood and our wellbeing, also appetite, and pain sensitivity, is stored in the gut. And a lot of very interesting discoveries have been made more recently that makes this even more intriguing. So, this serotonin is synthesized in the gut from precursors that come from our food that we ingest, and the microbes that live in the gut are actually able, through chemicals that they produce, to stimulate the production of serotonin.
It’s been estimated, based on studies in animals, that 60 percent of the production is due to these signals that come from the microbes that live in our gut. In addition, another intriguing finding is that these serotonin cells, they’re sandwiched in between the cells that make the lining of the gut. One end sticks into the inside, samples everything that goes on inside the gut.
The other side, very interestingly, has an outgrowth that is connected through a synapse with sensory nerve endings. So, many vagal nerve endings. So, this is a cell that sticks into the gut, samples a lot of things that go on in our digestive tract. Then it produces serotonin largely through the influence of microbes that live in our gut, and then the signal, when this cell is activated, there are many things, chemical or mechanical stimuli associated with digestion.
It signals through this synapse directly into our brain through the vagus nerve into centers that have to do with – I mean ultimately with emotional regulation and emotion generation. So, even though we don’t have the proof for that, it would be very difficult to mention that there’s not a significant influence on our mood and that this system, if it’s out of balance, does not play a significant role in the pathophysiology of depression.
There are many food items that contain neuroreactive molecules signaling substances, including oysters, chocolate, and many other foods that to varying degrees contain either the precursors of serotonin or actually serotonin molecules. It’s possible that these contents of this molecule are the basis for some of these claimed effects of food items like oysters as an aphrodisiac or mood enhancer, like with chocolate.
People say they feel a lot better after they eat their daily piece of chocolate in the evening, which is pretty true. Not just from the taste, but from this chemical connection.