Much of what goes on inside your brain occurs as a consequence of what goes on inside your gut (1, 2, 3, 4, 5). If your gut is in bad shape, your brain will also be in bad shape. Conversely, if your gut is healthy and working well, then chances are your brain will also be healthy and well-functioning. This is very important to acknowledge, in part because it has widespread implications for medicine and psychology.
Why are the brain and the gut so tightly connected?
In order to understand why the human brain and gut are so tightly connected, we need to look into how these two organs work, as well as what their purpose is.
If I were to ask you whether you consider the things that are found inside your gut to be on the inside or the outside of your body, you would probably say that that you consider them to be on the inside. This is definitely the most obvious answer; however, whether it’s correct or not, that’s another matter. One could make the case that all of the microbes, vitamins, minerals, amino acids, and so forth that are present in your intestinal tract are actually not on the inside of your body, but rather on the outside, as a part of your body’s environment.
The walls of the long tube that runs through your body act as the most important interface between you and your external environment. Your body is constantly reacting to surrounding stimuli. Much of this stimuli originate from various substances and microscopic life forms that reside in your gastrointestinal tract.
Energy acquisition is very high on the list of priorities for all organisms. All living things, from plants to microbes to animals, require energy to survive. If they don’t get a hold of the energy they need to keep their cellular machineries running, they die. Hence, it’s not surprising that evolution has shaped organisms, including humans, in such a way that their brains are capable of responding to signals from their guts.
Our brains guide and to some extent control our behaviors, including behaviors related to food acquisition; hence, it makes complete sense that the gut, the place where nutrients is absorbed, is closely connected with the brain. If we take in very little food one day, the brain adjusts our behavior and metabolism accordingly as a result of signals it receives from the gut, as well as from other organs, including adipose tissue.
This is well known, and I think nobody’s going to dispute it. What’s not as well known though, but is gradually gaining mainstream acceptance, is that the signaling processes that take place between the brain and the gut extend far beyond those that are triggered by the digestion and absorption of nutrients.
Why have gut microbes evolved an ability to exert control over our behavior?
Your gut is, as you probably know, home to trillions of microscopic life forms. These organisms are not all identical with respects to the environmental conditions that they thrive under. Whereas some do well under anaerobic, acidic conditions, others are incapable of surviving in the absence of oxygen and/or quickly die if they are exposed to a low pH environment. This is very important to acknowledge, because it helps us make sense of why gut microbes have such a profound impact on our behavior and mental state.
A gut microbe that is capable of controlling or influencing the behavior of its host has an obvious fitness advantage over a microbe that doesn’t have this capability. The former bug may influence its host in such a way that the host ends up manipulating the environment of the microbe in a way that is beneficial to the microbe. The microbe could make this happen for example by producing compounds that travel to the brain of the host and trigger the host to seek out and consume foods that contain nutrients that the microbe can derive energy from. In other words, the microbe acts as a child who influences his parents, perhaps by crying or screaming, in such a way that he gets what he wants for dinner. A single microbial cell obviously isn’t capable of exerting much pressure on the human brain; however, large colonies of microbes certainly can.
Besides their role in shaping our food references and foraging behavior, gut microbes influence our behavior and brain function in many other ways. Perhaps most importantly, they interact with our immune systems. The immune system, in turn, interacts with the brain.
How evolutionary theory can help us restore the human microbiome
Microbes and humans have co-evolved. Throughout evolutionary time, selective pressures have obviously acted on both of these parties. In the natural environments our ancient ancestors occupied, it was highly beneficial, fitness wise, to be fairly physically active, healthy, and mentally sharp. Natural selection would have favored those ancestors of ours who did well under the conditions they found themselves in and whose biology matched well with the microbiotas that were produced by their diet and lifestyle practices. In other words, it’s not just microbes that have adapted to live alongside humans; humans have also adapted to live alongside microbes.
This is obvious, yet it’s often overlooked, which is unfortunate, seeing as it can help us make sense of what type of diet and lifestyle we should adhere to in order to construct a gut microbiota that matches well with our biological self. We have to acknowledge that the human genome was sculpted, via natural selection, over millions of years of evolution and that the vast majority of this evolutionary journey took place in a natural environment.
Throughout the time when we humans gradually replaced this natural milieu with the manufactured environment we currently find ourselves in, we’ve not only altered our living conditions, but we’ve also altered the microbial communities that our bodies interact with. There hasn’t been sufficient time or selection pressure for natural selection to adapt our bodies to our novel microbiotas. One doesn’t have to look far and wide to find evidence supporting this statement. Many diseases and health problems characterized by microbiota disturbances are today so common that they are often considered to be a normal part of human life.
Genome-microbiome mismatches underlie many mental health problems
More and more people are getting diagnosed with depression, the prevalence of autism keeps on increasing, a lot of people complain of brain fog and poor memory, and big chunks of the elderly parts of the population develop mental health-related degenerative illness. There’s little doubt in my mind that the aforementioned shifts in the human microbiota, driven by psychological stress, antibiotic use, and widespread consumption of highly processed foods, among other things, have greatly contributed to fueling this mental health crisis. A rapidly growing pile of evidence supports this statement (e..g., 6, 7, 8, 9).
Not only that, but there’s little doubt in my mind that the brain of the modern man is not working at peak capacity. The signals and nutrients that his brain receives from his gut, including its microbial constituents, are not the types of signals and nutrients it is evolutionary designed to require in order to thrive.
In order to make some headway towards exiting this mess, we have to acknowledge that we’re in the midst of an evolutionary mismatch situation characterized by a conflict between the human microbiome and the human genome. It’s time we stop blaming microbes for our problems and instead acknowledge that it’s largely our own fault that “unstoppable” superbugs have infiltrated hospitals all over the world and countless people have an unhealthy gut – and consequently also an unhealthy brain.
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