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Healthy Gut, Healthy Body

By Jill Koski

We are often told when faced with tough decisions to “go with our gut,” but should we really listen? Recent research into the gut-brain axis (GBA) has revealed an intricate system of communication that does not simply regulate the gastrointestinal tract but also affects emotion and cognitive function. The bacteria present in the intestines have been found to have a significant influence on these interactions. Gut microbiota and the brain communicate through both the central nervous system and the enteric nervous system by means of the vagus and pelvic nerves (Furness et al. 2014). This communication ensures that systems throughout our bodies are operating as they are supposed to. It is when this bidirectional flow of information is disrupted, however, that our bodies begin to respond in negative ways.

What are gut bacteria?

In order to fully understand the intricate connections between the brain and the gut, it is important to first discern what gut bacteria are and how they function in our bodies. In the average adult human, there are more than 1000 species of bacteria living in the intestines (Shreiner et al. 2015). The exact makeup of every person’s microbiome is unique due to differences in diet and lifestyle. Bacteria are present all throughout the body, but gut bacteria have the greatest impact on overall health. Lining the digestive system, typical functions for bacteria include nutrient absorption, mucosal barrier function, and signal transmission and reception (Hart et al. 2002). This means that gut bacteria are responsible for breaking down and extracting nutrients from the food we eat. Additionally, gut bacteria make up the mucosal barrier that protects the interior of digestive organs from the surrounding environment, and perhaps their most important function, gut bacteria send and receive signals through the enteric nervous system. Because the central nervous system and the enteric nervous system work hand in hand, gut bacteria are able to directly affect our emotions and thoughts. With this basic understanding, we can now explore how the gut and brain communicate and the effects felt throughout our bodies when they do.

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How can my gut affect my emotions?

Since 2007 when the Human Microbiome Project was first launched, countless studies have been conducted to explore and gain a better understanding of the microorganisms living in our guts. As more information was discovered about the microbiome, scientists started to question how these bacteria might affect our brains. Because microorganisms have been found to have the ability to secrete neurochemicals like serotonin, dopamine, gamma-aminobutyric acid, and other neurochemicals comparable to those produced by the central nervous system, they play a significant role in regulating mood and emotion (Lyte M. 2013). The absence or presence of bacteria that release certain neurochemicals can affect levels of depression and anxiety, as well as levels of happiness and ease.

After discovering that neurochemicals were secreted by gut bacteria, various studies were conducted to more fully understand the implications that gut bacteria have on emotional behavior. Dr. Mark Lyte was the first to test whether or not bacteria could induce stress. To do this, he fed mice a solution of bacteria known to be linked to food poisoning in humans, Campylobacter jejuni. The mice were then put in a plexiglass maze and observed. Those fed the bacteria were less likely to walk out onto unprotected ledges, showing that they felt more anxiety and stress than the control group of mice (Lyte M. 1998). These observations demonstrate how the absence or presence of one particular bacterium can significantly alter emotions and draws scientists to the conclusion that all species of bacteria play a large role in how we process and display our feelings.

Following Dr. Lyte’s study, more research was done on mice in order to understand gut bacteria better, but it was not until recently that scientists conducted an experiment involving humans. Published in 2017 by a team of researchers at UCLA, a test involving 40 healthy women from ages 18-55 identified behavioral trends based off of gut microbiota profiles. The researchers tested fecal samples from the women and divided them into two groups: those with a larger amount of bacteroides and those with more prevotella. These women were then shown images to provoke an emotional response while researchers took scans of their brains. The end results found that the group with more prevotella reported higher anxiety levels and distress after looking at the pictures (Tillisch et al. 2017). These findings further prove that there is a connection linking an individual’s microbiome to their emotional state.

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Can gut bacteria impact my thoughts?

Gut bacteria are not just changing how we are feeling; they are changing how we are thinking too. Research has shown that microbiota can have major impacts on cognitive function and might even be a significant factor in diseases such as Alzheimer’s and dementia. Before human infants are born their brains are underdeveloped, and they do not have any bacteria living in their gut. It is not until they pass through their mother’s birth canal that they are exposed to her microbiota. This initial colonization then plays a crucial role in the development of the brain over the next two years (Dinan et al. 2015). Gut microbiota also remain an important factor after development as it maintains optimal function of the brain, or if altered, decreases the function of the brain.

Multiple studies conducted to test the significance of gut bacteria in brain development have compared germ-free mice, which are free of all microorganisms inside and on their bodies, to mice born naturally. One, in particular, completed by Dr. Gareau and his team, examined the hippocampal function of germ-free mice through a novel object recognition test and a T-maze. Their observations showed that the germ-free mice had significant cognitive deficiencies in comparison to the control group (Gareau et al. 2011). These findings ultimately led them to the conclusion that the presence of gut microbiota is vital for proper functioning of the hippocampus which controls emotion, memory, and the nervous system. Without gut bacteria, many deficiencies in cognitive function can arise.

After studies involving animals concluded that there is a connection between gut bacteria and the functioning of the brain, scientists began to form studies to determine if this was the case in humans as well. They specifically set out to determine if gut microbiota had any effect on the development of cognitive function disorders. Diseases like Alzheimer’s and dementia become more common at older ages, and approximately 40% of all adults over 85 have some type of cognitive impairment (Hampel et al. 2011). A study conducted by Dr. Claesson and a team of researchers was able to associate a lower diversity of gut bacteria with elderly people compared to young adults and children. They also observed that the lowest diversity was experienced by those who were sickly and suffered from memory loss (Claesson et al. 2012). These observations give us reason to believe that the bacteria in our gut does, in fact, affect the way we think and how active our minds are.

What is still being discovered?

Studies conducted thus far have shown that our gut bacteria are crucial to every aspect of our wellbeing, not just the digestive system. With this knowledge of how gut bacteria affect our emotions and cognitive function, scientists have begun seeking ways to use the microbiota in preventative and therapeutic treatments of numerous disorders. Many studies are being conducted to discover ways that depression and anxiety could be treated by altering bacteria levels. Additionally, since it has been proven that the diversity of gut bacteria affect cognitive function, many researchers are attempting to develop cures for diseases like Alzheimer’s that cause memory loss. By constantly expanding our knowledge of the gut microbiome and how it acts in our bodies, we will be able to approach disease prevention and treatment in an entirely new way that will hopefully lead to more successful results and a healthier society.

References
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