The GI Tract: Our Own Private Ecosystem
The human body, as with nature in general, is endlessly fascinating. And every new discovery in medical research confirms this. Until maybe a few decades ago, the operation and maintenance of the human body was thought to be analogous to that of an automobile: many parts requiring maintenance and upkeep, operating for the most part independently to maintain life. It has, however, become clear that the human body is more like a supercomputer: all parts are communicating within themselves, with all the other parts, and providing the brain with continuous and precise details about our inner and outer environment. I will use the gastrointestinal tract to illustrate this, as there is a burgeoning amount of medical research being conducted on this system.
As we know the GI tract comprises the oral cavity, esophagus, stomach, small intestine (divided into duodenum, jejunum and ileum), the large intestine/colon, and the rectum. This is by far the most extensive sensory system of the body. The intestines, if opened and flattened, would cover the area of a basketball court. Why should this be? How complicated can ingestion of food, digestion and elimination be? It is now known that our GI tract, or gut, plays an integral part not just in digestion, but also in immune health, hormone production and release, the nervous system as well as regulation of our mood and maintenance of overall health. In fact, some scientists refer to the gut as the “small brain” or “second brain” due to its own extensive and complex networks of nerves and neurotransmitters. There are more nerves innervating the GI tract than the entire spinal cord. (!)
The GI tract has a surface area 100 times that of our skin. Each square inch of the small intestine is home to approximately 20 thousand fingerlike projections called villi; and the villi have microvilli. So that when food passes from the stomach into the small intestine, the liver and pancreas release digestive juices to the villi to break down the food. Each villus has a blood vessel that absorbs the food molecules, sending them back to the liver to remove or detoxify harmful substances. Then the nutrient-rich blood is sent from the liver to the heart and then to all body cells for energy. The entire small intestine is relatively “clean”, i.e., very few microbes (bacteria, virus, fungi) live there.
Perhaps the most interesting section of the GI tract is the large intestine aka the colon and for purposes of this discussion, The Gut. The gut is the home to our most elaborate ecosystem. In 2007 the NIH launched the Human Microbiome Project, the goal of which was to identify the specific microorganisms coexisting with us, in our colon. It was already known that other parts of our body coexist with microbes, most notably the skin, nostrils, mouth, eyelids, and the spaces between our teeth. However, the largest population by far is in the gut. More than 100 trillion microbes live in our gut, those of most interest to us being the bacteria. The one thousand bacterial species of the gut “microbiota” contain over 7 million genes. These genes allow the microbes to participate in constant communication with the brain. They code for the production of innumerable molecules facilitating this communication. No two humans have the same strains and species of microbes; thus, the microbiota comprises each individual’s unique internal ecosystem.
We depend upon this ecosystem to protect us from toxins and pathologic microbes, and to metabolize undigested food to obtain nutrients we would otherwise lose. We know that for an ecosystem to be healthy it must be diverse. And here is the role we play: to feed and protect our microbes, allowing them to keep the intestinal lining safe, and produce healthy metabolites to maintain the integrity of the gut-brain communication system. We do this by eating foods that are high in fiber, flavonoids and polyphenols as well as healthy fats- and avoiding ultraprocessed foods and also unnecessary antibiotics. A happy microbiome will serve us well, digestively, immunologically and emotionally.
Our microbes live inside the gut but also on the mucus coating of the colon. This coating prevents unwanted substances such as chemicals, inflammatory metabolites and pathogenic microbes from gaining access to the blood stream. It prevents unwanted inflammatory stimulation as the microbes live in close proximity to the gut’s immune cells. And perhaps the most recent discovery is that they protect the gut sensors which recognize signals from the brain indicating our emotional state, e.g., anxiety, happiness, anger. Then the microbes, by means of their own gene signaling, will produce and send information back to the brain. Our sense of self, emotional homeostasis if you will, is intimately regulated by these interactions that are occurring at all times. An emotion may start in the brain which then sends chemical information to the gut microbes which, when healthy and well-fed, will produce corresponding metabolites/chemicals, to complete the feedback loop. We have all experienced this communication in both directions. We may feel anxious, ill at ease emotionally, and our gut may respond with its own expression of this emotion such as diarrhea, cramping. Or perhaps an unpleasant gut symptom might clue us to an emotional discomfort- “a gut feeling”.
If the microbes are fed a constant diet of foods containing undigestible chemicals, excessive sugar or alcohol, they will look elsewhere for nutrition and start to break down the carefully maintained barrier between the colon and the blood stream. Gut immune cells might be stimulated leading to a low level of inflammation in the body. Or the poorly-fed microbes may adversely interact with the cells in the gut that produce and store crucial neurotransmitters. 90-95% of the body’s serotonin is produced and stored in cells in our gut. These cells respond to what we eat as well as to chemicals released by our brain (central nervous system) and our “little brain” (enteric nervous system). If the integrity of these cells is damaged this will affect our emotional state. The most common class of antidepressant medications, the selective serotonin reuptake inhibitors, obviously depends upon the production and presence of serotonin. Also, about half of another crucial neurotransmitter, dopamine, is produced in the gut. This is not to say that depression or other psychological conditions are caused by the gut, but there is strong evidence to suggest an important role in emotional regulation. We know there is a very strong neurally mediated communication between the gut and brain occurring all the time. What we eat and drink and how we treat our gut microbes plays a part in this process.
We can be good stewards of our physical and emotional health in many ways. One that we know is to make sure the gut microbes are well fed, enabling them to maintain healthy and productive communication with the brain. In so many ways “We are what we eat. In fact, in February of this year, a research report published in the American journal of Medicine, found that there are now “hundreds of novel ingredients never encountered by human physiology found in nearly 60% of the average adult’s diet and nearly 70% of children’s diets in the US”. The researchers believe that ultra-processed foods in the average American diet have become a “silent killer” just as we found that unrecognized high blood pressure was in the late 20th century. There are so many chemicals, preservatives, emulsifiers which, since undigestible, are available as unhealthy food for our microbiota, allowing them to promote rather than prevent disease.
Let’s aim to eat food that is as close as possible to its natural form, emphasizing greens, vegetables, legumes, nuts, whole grains, healthy fats (olive oil, omega 3’s from salmon), while minimizing animal fat, and foods processed with chemicals that would never be in anyone’s kitchen. A happy microbiota will enhance health and happiness.
References:
1. Mayer, Emeran, MD, The Mind-Gut COnnection, Harper Collins Publishers, 2017 Edition
2. Enders, Julia, MD, GUT, The Inside Story of Our Body's Most Underrated Organ, Greystone Books, 2015
3. Galoustian, Gisele, American Journal of Medicine, Feb 20, 2024, "Could ultra-processed foods be the new 'silent" killer?”
