How are you feeling right
now? Happy or sad? Angry or content? The microbes living inside of you,
particularly those in your large intestine, are partially responsible for your
state of mind right now.
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| Bacteria from the genus Lactobacillus, a genus of bacteria often found in the intestine. A creative commons image. Source. |
Microbes—tiny creatures
that aren’t even part of the human body—influence the brain?
Yes, intestinal bacteria,
and the food we feed them, play critical roles in how we feel.
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| Diagram of the large intestine, where our gut bacteria live, A creative commons image. Source: Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436 |
Here’s how bacteria and diet
influence the brain:
Microbes talk to the
brain through the immune system. Dendritic cells—immune system cells in gut
lining—constantly monitor microbes for misbehavior. If the dendritic cells
sense that the intestinal bacteria are acting up, they trigger the release of
cytokines—compounds that cause gut inflammation.
Cytokines communicate
with the brain in two ways: as hormones and as signals sent via the vagus nerve—a
thick nerve connecting the gut to the brain. These hormonal and nervous system signals
make the brain reduce energy levels, and increase pain sensitivity. Cytokines
can even induce feelings of sadness often felt during a stomach ache or gut
infection.
It doesn’t require a
full-blown gut infection to induce gut inflammation—simply eating a diet high
in animal fat can cause the release of cytokines—and the subsequent mood drop.
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| Bacteria from the genus Klebsiella, a genus often found living in the gut. A creative commons image. Source. |
Microbes also signal the
brain through metabolites—byproducts of microbial digestion. The metabolites
microbes produce change based on what food we feed them. Metabolites travel
through the bloodstream and act as hormones in the brain to influence our
moods.
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| Bacteria from the genus Lactobacillus, a genus of bacteria often found in the intestine. A creative commons image. Source. |
Generally, food that is
good for humans also is good for our bacteria and causes them to release
beneficial metabolites. Some metabolites, like those released after eating
whole grains and vegetables, trigger release of serotonin, a neurotransmitter
which is linked to improved mood. Fatty, unhealthy food on the other hand is
likely to cause gut inflammation and the depressed mood that comes with that
condition.
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| Illustration of a human brain. A public domain image. Source. |
Gut microbes are so vital
to our emotional state and thought process that scientists are starting to
think of the gut, the brain, and the human microbiome as a connected system
rather than separate entities. This system is often called the
gut/brain/microbiome axis. Next time you are in a good mood remember to thank
your intestinal microbes!
Check out a previous post
I wrote on microbes, We Need Bacteria, to read about the connection between
microbes and food allergies.
