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 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 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’s some very intriguing observations both clinically but
also now more recently scientifically that make it highly plausible that there is an
integrate 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 really 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,
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. So in addition another intriguing finding
is that these serotonin cells, so 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
which 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’s 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’s 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

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