HOW THE GUT BIOME AFFECTS MOOD
The association between mental and digestive disturbances is an old one that can be traced all the way back to the era of Hippocrates. As described in the paper Microbiota and Neurological Disorders: A Gut Feeling, the link between mental and gut imbalance “stands as the single consistently linked comorbidity* described in the medical literature from ancient times to the present.”
As newborns, our microbes originate from our mother, colonizing the gut in the first days of life and developing into our own unique microbial signature. Once fully developed, the human intestine is a hotbed of nearly 100 trillion bacteria at a given time. This mini ecosystem is constantly adapting and, like all other body systems, striving to maintain balance. ⚖️
Today, researchers like Foster and Neufeld, who published the paper Gut–brain axis: how the microbiome influences anxiety and depression, report that gut microbiota “are an important player in how the body influences the brain, contribute to normal healthy homeostasis, and influence risk of disease, including anxiety and mood disorders.”
THE GUT-BRAIN SUPERHIGHWAY
So, how could food affect mood? To answer this, we have to look at how the gut connects to the brain.
The gut and brain are connected through the vagus nerve, known popularly now as the “gut-brain axis.”
The vagus nerve is not just one nerve, but a whole system of nerves that act as a neural superhighway extending from the brainstem all the way to the lowest base of the abdomen, touching most major organs on its way.
The vagus nerve system also works directly with the enteric nerve system (ENS), which governs the gastrointestinal system, contains about 100 million neurons, and is nicknamed the “second brain.”
Within the ENS and vagus nerve system are some very busy protein molecules called cytokines, constantly providing status updates to the brain about the state of the organs and systems they encounter.
The emergent theory is that a dysbiosis in the gut’s bacterial flora, even if imperceptible to the individual, is nevertheless communicated up the pathway, sending an alert signal to the brain and directing an inflammatory response — one of the body’s key lines of defense — to the site of imbalance. This “bottom-up” effect is directed by the central nervous system (CNS), which stands poised at all times to signal the immune system to defend us against potential threats.
Foster and Neufeld cite several studies that provide “clear evidence” of this signaling between the gastrointestinal (GI) tract and what essentially is the CNS’s emergency switchboard, via the vagus nerve system.
THE EMERGENCY ALERT SYSTEM
To visualize, imagine a car accident on a highway. Like the driver calling 911, when dysbiosis occurs in the gut, the gut calls out a stress signal to the CNS.
Like the police dispatcher radioing to the units on duty, the cytokines — those protein molecules that trigger cell communication and stimulate their movement toward problem sites — stimulate the CNS and the immune system to jump into action, sending cells to the site to cause inflammation as an emergency response.
In cases of food poisoning, these responses can literally save our lives. The issue is that even low-grade cases of bad gut bacteria or an imbalanced gut biome can have the domino effect of sending emergency signals to the brain and CNS. The basic idea is that this distress signal and the resulting inflammation, however minor, could have a noticeable effect on the brain of the individual, especially if the response is prolonged, causing the gut to be consistently inflamed.
In a sense, if the gut is in an ongoing state of imbalance due to poor diet or a bacterial imbalance, it’s always on alert. This means the nervous system is being repeatedly engaged — resulting, potentially, in anxiety.
In their paper Gut brain axis: Diet microbiota interactions and implications for modulation of anxiety and depression, Luna and Foster report that diet-related changes in gut microbiota have an influence on the gut-brain axis and “may in turn influence behaviours including anxiety and depression.” For example, a recent study in the journal Nature suggests that emulsifiers used in processed food can trigger inflammatory disease by disrupting the barrier between the immune system and the gut biome. Microbiologist Karen Madsen responded to the study's results in Science magazine saying, “It sends a really clear message that changes to our food supply are altering our microbiota and our health.”
To clarify, the millions of bacteria in the gut, if representing a balanced share of “good bacteria” (like those found in probiotics), are wonderful for stimulating digestion and fighting pathogens. But should they crossing the intestinal barrier into the blood stream, this is where problems arise. To that end, a healthy intestinal lining will have a mucosal layer keeping the intestine safely removed from gut bacteria. But if emulsifiers break down this protective layer, bacteria cells can reach the intestinal lining, causing (by a different pathway, but all the same) inflammation.
THE SOLUTION
As one promising solution, a paper simply titled Mood and gut feelings, Bienenstock and Li report on a study that reveals one specific dietary manipulation positively affecting memory and reducing anxiety-like behaviour: “significant increases in diversity of the microbiome.”
Essentially, diversifying the gut microbiome with a healthy diet with a good representation of probiotics can bring homeostasis to the GI system, thus reducing inflammation and ending the feedback loop of distress signals to the brain.
The incredible complexity of the gut microbiome is both what makes it so fascinating, and the reason that only recently have researchers finally had the proper tools to really study it in depth. As far as we’ve come in understanding the human body, this essential habitat of our makeup remains largely mystifying in its design.
As Foster and Neufeld put it, “the diversity of the microbiome determined to date extends far beyond what researchers expected. We are only beginning to understand how the diversity and distribution of these prominent phyla contribute to health and disease.”
One thing is clear — the gut's ecosystem is definitely not autonomous. It's a complex, ever-changing, living entity that is significantly wired into the superorganism that is the human body, and it can play a significant role in our mental wellness.
*GLOSSARY
co·mor·bid·i·ty (noun): the simultaneous presence of two chronic diseases or conditions.