When I first heard about the
existence of a second brain in our bodies, I felt the urgent need to investigate
whether this was true or just a myth. What I found out is that there is plenty
of scientific evidence showing that in fact another organ- located within our bellies-
plays a critical role in mental and emotional functioning. Moreover, I learned
that both ‘brains’ are connected and control each other. I ask you then, which organ
would you think is our second brain? Liver? Kidneys? Intestines? Spleen?
Well, believe it or not the
answer is the intestines- also known as guts- and the fact is that they hold
something fascinating that will revolutionize the way we think about our mental
health. Although this “second brain”- as it is currently known- is not involved
in the thought process as we know it, it can directly influence processes like memory
and learning as well as mental states like anxiety, stress and depression. In
this post I invite you to understand how our guts can act as a brain.
HOW
DOES THE GUT RESEMBLE A BRAIN?
Whenever we think about the brain
we think of neurons and neurotransmitters, but our guts contain millions of
neurons as well, more than in either the spinal cord or the peripheral nervous
system. These neurons enable us to “feel” our bellies, but its large number
also gives us a hint that the gut does much more than digesting food and taking
care of the ‘dirty work’: they arm our guts with its own senses and reflexes, independently of the brain in our heads. As
for neurotransmitters, the enteric nervous system uses more than 30 types which
include serotonin, dopamine and glutamate, just like the brain. A very
interesting fact for example is that over 90% of our bodies’ reservoir of
serotonin- considered ‘nature’s anti-depressant’- is produced in our own
intestines and less than 10% is produced in our brains, so guts might play more
of a role in depression that we yet know.
NON-HUMAN
CELLS IN OUR GUTS CONTROLLING OUR BRAIN?!
A growing body of data shows that
there is something unique in the guts influencing mental and emotional
functioning, that cannot be found in the healthy brain and that is…...
microbes!!
You may think how on earth can
microbes participate in the brain-like functions of the guts and although there
is currently not a clear answer to this question, a fact is that gut
microbiota-formerly called gut flora- do
influence behavior and can alter brain physiology and neurochemistry. For
example, in
one of the most interesting papers I found in the field, scientists
demonstrated in mice that a subset of gut bacteria directly alter
neurotransmitter levels, specifically they promote serotonin production. This
is an interesting finding given that some antidepressant drugs work by doing
the same: boosting serotonin.
In another study it was
shown that changes in the composition of gut bacteria- by the use of antibiotics-
lead to behavioral changes. In this demonstration scientists gave a cocktail of
antibiotics to a cautious and shy mice strain named BALB/c, after which they
became bold and anxious. Interestingly, when the antibiotic regimen was stopped
their cautious behavior went back to normal.
These studies suggest that we could
change our behavior by changing the microbes that normally reside in our guts.
As a matter of fact, this was demonstrated in mice: in a follow up experiment
using germ-free mice, gut microbiota from the timid BALB/c mice was
transplanted to NIH Swiss mice- which are courageous and exploratory by nature-
and vice-versa. The result? Timid BALB/c mice became much more fearless
explorers while NIH Swiss mice suddenly grew more hesitant and shy. Impressive
right? microbiota driving the behavioral phenotype of the host, so we should really
think about the possibility of using beneficial- also known as probiotic-
bacteria to treat mood disorders, as I will discuss about further on this post.
HOW GOOD CAN GUT BACTERIA BE FOR US?
The idea that bacteria are good
for us has somehow been forgotten, but long ago some scientists such as the Russian
Ilya Metchnikoff noticed their importance. This guy was so radical that in
order to study a deadly cholera epidemic that had broken up in France, he
decided to suck down a drink of Vibrio
cholerae- the bacteria that produces the disease. His idea was to
understand why this disease struck some people and not others. If he had died,
this would be hell of a boring story, but remarkably he didn’t even get sick. I
don’t know how, but he recruited a colleague to do the same thing, although
this guy didn’t get sick either. But when he recruited another colleague to do
the same…. this poor guy got very sick and nearly died.
When Metchnikoff took his
experiments to the lab to find out the reason of such difference, he discovered
that certain species of bacteria in the human gut microbiome supported and
stimulated cholera, while others prevented it. He then reasoned, if swallowing
pathogenic bacteria could make you sick, then swallowing a beneficial one would
make you healthier. In general, he claimed that our gut bacteria are essential
for our health and that the right balance of microbes inside our bellies could
help battle diseases.
Metchnikoff studied many
different bacteria, especially one that was popular for yogurt-making in
Europe- Lactobacillus bulgaricum- the
ancestor of probiotics. He discovered how these bacteria prevented us from
getting sick: they could swallow undesirable microbes and toxins. This famous
process known as ‘phagocytosis’ made him win a Nobel Prize in 1908.
As an example of how science is
important for our daily lives, here’s a cool story about Metchnikoff
discoveries you can tell your friends: A Greek guy named Isaac Carasso heard
about Metchnikoff’s discoveries and soon realized its potential. After
emigrating to Barcelona, he started a yogurt factory and began selling his
first yogurts fermented with lactic bacteria imported from Bulgaria (L. bulgaricum) or strains carefully
selected by Metchnikoff himself ‘on prescription’. Since yogurt was not
well-known in Western Europe, he initially sold it as medicine on pharmacies.
Carasso’s yogurt company would later become ‘Danone’…. I guess you’ve heard
about it.
CAN PROBIOTICS MAKE US SMARTER AND
HAPPIER?
Probiotics are beneficial
bacteria that yield positive health outcomes. The most typically used are the
Gram-positive Lactobacillus and Bifidobacterium families, which do not
possess pro-inflammatory lipopolysaccharide chains and so their propagation in
the gut does not trigger full-fledged immunological reactions. Several studies
have shown that probiotics have emotional and cognitive effects in animals and
humans.
In one study scientists fed
one group of BALB/c mice with a broth containing Lactobacillus rhamnosus, while control mice got just broth with no
bacteria added. After 28 days, researchers tested the mice through several
behavioral tests and detected that mice fed with Lactobacillus showed significantly reduced signs of anxiety and
depression. The probiotic diet also caused them to produce lower levels of the
stress hormone corticosterone in a ‘forced-swim’ test as compared to control
mice. Some brain regions were also altered by the probiotic diet, showing an
increase in the number of receptors for gamma-aminobutyric acid, or GABA- an
inhibitory neurotransmitter that keeps anxiety in check. Lactobacillus have also been shown to have
cognitive effects as it improves memory by inducing changes in hippocampal
activity.
HOW CAN THE GUT COMMUNICATE WITH THE
BRAIN?
The gut communicates with the
brain via the vagus nerve- the largest nerve in the human body.
Nevertheless, the fascinating part is not the connection per se, but that about 90% of the fibers in the vagus nerve carry
information from the gut to the brain and not the other way
around. Thus, a lot of information might be sent from our guts to our brains,
but what could this information be about? how relevant is for the guts to
communicate with the brain?
In biology, a common way to
understand the meaning of a process is to interrupt it and see what happens. Then,
what if we cut the connecting wire that sends information from the guts to the
brain? This is exactly what
scientists from the ETH Zürich in Switzerland did to test rats. In this
animals, the brain could still send signals down to the guts, but the brain
could no longer receive signals coming from the guts. The idea was to study how
interrupting this communication path could affect brain and cognitive functions,
specifically the authors of this study were interested in the link between
innate anxiety and ‘learned’ or ‘conditioned’ fear. They put these rats under
certain conditions to which control rats would experience fear and anxiety and
they analyzed them by several behavioral studies. The result was surprising: rats
in which the afferent vagus nerve fibers were cut out, become significantly
less afraid. This outcome demonstrates that the guts somehow control fear
responses or as we just learned, guts impact in behavioral responses. We are
all familiar with that strange feeling in our belly when faced with a
threatening situation- the so-called “gut instincts”, well this study suggests
that they significantly impact the way we react to those type of scary situations.
The idea of a connection between
brain and guts is not new though. In fact, in the late nineteenth century the
physicians William James and Carl Lange proposed that the origin of emotions is
physiological, meaning that the physiological change happens first and emotion
is then experienced when the brain reacts to the information transmitted to it from
the viscera.
Many are still skeptical about
the link between brain and gut, between microbes and behavior and if it will prove
important in human health…. But maybe
it’s time to think twice.
