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A recent study published in the journal Science Advances indicates that microorganisms in our gut may influence significant aspects of how our brains moderate mood.
In particular, the piece, which is entitled “Landscapes of bacterial and metabolic signatures and their interaction in major depressive disorders,” suggests that the collection of attributes we typically refer to as depression may be, in some cases, the consequence of an imbalance in gut bacteria populations.
This is not a novel research premise: we’ve suspected for quite a while that our gut-biome may influence other aspects of our bodies, including our brains, and that suspicion bloomed into a near-certainty with the arrival of data suggesting that the over-enthusiastic application of antibiotics—which, among other things, often kill off huge swathes of gut bacteria; innocent bystanders of a separate effort to kill bacteria that are harming us elsewhere in our bodies—could cause side-effects ranging from an increase in one’s risk factors for Type 2 diabetes and liver disease, to heightened instances of asthma and diarrhea.
This new research, though, focuses on a very specific collection of consequences which seem to influence the chemicals we produce, including those that moderate mood and, typically, keep us psychologically stable.
It’s estimated that about 350 million people, worldwide, suffer from some form of depression. This is a significant source of disability, not to mention personal and interpersonal suffering.
Data collected by the World Health Organization shows an 18.4% increase in reported cases of depression between 2005 and 2015, and although there are no doubt other variables influencing this trend—the myriad consequences of an increasingly interconnected world, for instance, and the ever-present creep of economic globalization-related changes—this increase also parallels the growth in worldwide usage of pharmaceutical antibiotics.
We already know that some of these bacteria, despite residing in our guts, help our brains moderate the chemical cocktails we receive in response to different sorts of stimuli.
Some gut bacteria produce serotonin, which is a neurotransmitter that helps regulate mood and memory in humans, while others produce gamma-aminobutyric acid, which is an amino acid that blocks some types of brain signal and which is sometimes used as an anxiety- and mood-moderating medication.
The theory is that if we kill some of these bacterial populations, they no longer aid our brains in this way, and as a consequence, we’re less likely to stabilize, chemically, as we normally would.
Allostasis—the body’s holistic collection of methods for returning our bodies and minds to a balanced, maintainable, equilibrium state—may become more cumbersome and energetically expensive, and in some cases no longer even possible, when these bacteria aren’t adding their outputs to our bodies’ collective chemical concoctions.
Some people might then experience what seems to be a slow descent into sadness which can then spiral into clinical depression, while others may experience a sudden jolt into such a state; everyone’s different, and we still don’t understand enough about what role the bacteria in our guts play to say much for certain. But we do have sufficient data to say that some presentations of depression may be rooted in a gut-based imbalance, triggered by it, or amplified by it.
The introduction of relatively low-cost, high-speed gene sequencing technologies has allowed us to advance our knowledge about this sub-field quite rapidly over the course of the past decade or so, but there’s still a great deal we don’t know—even in terms of the fundamentals.
That said, more than 15,000 research papers were published on this subject in 2019, alone, and it’s estimated that the average human gut contains about 100 trillion microbes, with the whole of the human microbiome containing something like 8 million unique protein-coding genes, alongside the 22,000-or-so human protein-coding genes most of us have in our bodies. So there’s quite a lot of work to be done, a great deal of ground to cover, and plenty of relationships between these components to map and understand.
This won’t be a simple or quick process, but the benefits of doing so could be dramatic.
It’s thought that, alongside depression and predispositions for diseases like diabetes, gut biome composition may also play a role in conditions like bipolar disorder, schizophrenia, and aspects of what’s often called the autism spectrum.
Our contemporary grasp of the neurological importance of these gut-based organisms has some scientists referring to it as the “psychobiome”: the portion of our larger microbiome that influences our psychological state.
The degree to which this is the case is still in question, but it does seem nearly certain that despite its odd anatomical location, this cluster of bacteria are more important to our state of mind and overall sense of well-being than we suspected even just a few decades ago.
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