By Donovan Makus
When we think of ourselves as human beings, we tend to view ourselves as one organism. We know that our bodies consist of many different organ systems, tissues, and, at the lowest levels, chemical molecules with new properties at every level. At our core, though, all those parts combine to form one human body. Much like we don’t exist in life alone, a healthy body doesn’t function alone–we form mutually beneficial partnerships with many microorganisms. While these microorganisms are dispersed throughout our body, the ones that form our gut microflora have wide-ranging effects on our bodies and lives and, as research evolves, we’re learning more about how they affect the very core of bodily function.
Before looking into what our gut biome does for us, it’s important to set the stage to understand who they are and what they do. Like many organ systems, we are not born with a completely functional gut biome; instead, it develops over time. Contact with our environment, our mothers, and other people helps build up our gut microbiomes from birth until we turn 3 years old. This is when our gut biome is mainly formed, consisting primarily of bacteria and also containing other organisms, like fungi. However, our gut biome isn’t done changing; it will adapt throughout our lives as microorganisms die and are replaced. This process is rapid as the number of cells in this gut biome is quite large, outnumbering our own human cell count by a factor of at least ten. Technically speaking, our gut microflora are not part of our bodies at all as our digestive tract is not “inside” our body. Our digestive tract is open to the external environment, and any food we digest is absorbed into our bodies through membranes. To visualize this it’s helpful to think of our digestive tract as the central hole in a donut. Inside this “external” environment live the constantly changing microbiota.
Having covered what our gut biome is, we’re left with a question: why do we have them? We generously provide them with a home and nutrients in the form of the food we eat, and providing them with these benefits comes at a cost to us. Humans can function without a gut biome, leaving us with the question of why we don’t try our best to eliminate them if they are acting as parasites. The exact ways that these microorganisms contribute to our functioning is complex, which tells us how our tiny partners can have an outsized influence on our lives. In exploring how we form a mutually beneficial relationship with these gut organisms, our first stop will be the gut itself.
Considering their location, the first and most obvious service these microbiotics provide is related to the digestion and absorption of nutrients from our food. Our body doesn’t store food intact; despite talk of “cheesecake sticking to our thighs,” all food we eat is broken down into the 3 main macromolecules; carbohydrates, fats, protein. We also absorb a range of micronutrients such as sodium and potassium. Our gut microbiota play a key role in extracting both micro- and macro-nutrients through the process of digestion. This area of nutritional research has been an area of great interest in recent years, with researchers studying the effects of different diets on mice to see if a poor gut microbiome could explain weight gain and obesity. For years, the familiar mantra of weight management has been “calories in, calories out,” but researchers have struggled to explain “calories out” fluctuations of up to several hundred calories between individuals. Energy, represented in calories, isn’t miraculously appearing or disappearing. This creates a vexing problem for science. We know that excess weight represents the storage of excessive energy, leading to the question of how body masses of animals–who were fed identical diets–could differ. Due to their role in digestion, our gut biome lets us extract additional calories from our food, and had been identified as a potential factor in this discrepancy. Studies of rodents, who had their gut bacteria purposely eliminated, found they had to consume 30% more calories than regular rodents merely to maintain their weight. Thanks to advances in microbiology, researchers are now investigating the role microbiota play in weight management. The research is still in its infancy and certainly not free from controversy related to contradictory research results. Some researchers, using mice, have discovered that a typical modern Western diet (high in saturated fats and sugar and low in fiber) leads to shifts in gut microflora that seemed to lead to weight gain and obesity. Other researchers found that transplanting an obese mouse’s gut microflora into a lean mouse caused the lean mouse to gain weight, without any increase in caloric intake. While even the most eager researchers are not suggesting changes in gut microbiota alone lead to the rise of obesity, they are correlated with a poor diet, which could be a factor in the obesity epidemic. This provides insight into the complex role of our gut flora in nutrition.
Despite their status as foreign elements suspended inside our digestive tract, gut microflora play a key role in helping us fight other, pathogenic (disease-causing) microbes. Their mere presence is beneficial as they prevent harmful bacteria from gaining a foothold in our digestive tract; however, taking up space isn’t their only role. Through their metabolic role, these microbes synthesize compounds that help train and power our immune system. Our gut biome also helps us to develop the thick lining of our digestive tract, helping protect us from potential pathogens. Finally, gut bacteria play a role in converting immune cells in order to activate them and keep our bodies functioning in peak condition. Through these dual roles centered around our digestive tract, our gut biome helps both power us and keep us healthy.
Though the main role taken by gut microbiota is related to metabolism and fighting infection, this isn’t the only role our tiny partners play. When we think of bacteria, our first choice for bodily systems dealing with them likely isn’t the nervous system with it’s blood-brain barrier–researchers have actually discovered links between gut bacteria and mental health. In 2004, scientists found that mice lacking gut biomes were more sensitive to stress, opening up new avenues of research. Gut bacteria are known to synthesize neurotransmitter molecules, which affect the nervous system and could help explain these findings. However, attempts to study mental health and the brain’s functioning relating to gut flora in humans have been contradictory or inconclusive and will require future research, presenting an exciting opportunity for some of the many Psychology students here at Concordia.
The evolving status of research on gut flora serves as an important lesson not to forgot that little things can have a big impact. Far from just affecting our digestion, these tiny microbes play a key role in many systems, ranging from immune protection to mental health, and we should let their outsized influence remind us that small things can indeed play large roles.