How Our Gut Affects Mood and Thinking

by Hunter Obeid
MBS 2018, Geisinger Commonwealth School of Medicine hobeid@som.geisinger.edu
Mentor: Dr. Michael Bordonaro, PhD

Shortly after we are born, bacteria begin colonizing our intestines. Most of these bacteria form a commensal relationship with our bodies, meaning that they co-exist inside us without causing any harm. Some bacteria even have positive effects on our bodies. Each person has a different composition of bacteria living inside of them, and factors such as diet, genetics, age, geography, and metabolism can influence the variations between individuals.

In recent years, researchers have begun investigating how the bacteria in our gut can communicate with the central nervous system, which is made up of the brain and spinal cord. The nervous system in the intestines, known as the enteric nervous system, synthesizes many of the same chemicals that the brain and spinal cord make. One of the biggest questions scientists are trying to answer is, “how exactly do bacteria communicate with our bodies?” It has been suggested that bacteria are able to produce chemicals that communicate with the brain through a pathway mediated by the vagus nerve that starts in the gut and travels to the brain. In addition, bacteria can release chemicals that travel through the blood and they also influence the body. This two-way communication system between the brain and the bacteria in our gut is referred to as the microbiome-gut-brain axis.

Experiments have been performed on animal subjects with and without bacteria in their digestive tracts. By comparing the two different groups, it is possible to examine the effects that bacterial colonization has during the early stages of development in animals. Changes in the composition of the microbiome resulted in obvious changes in behavior for test subjects. As shown by several of these experiments, there is a possibility that bacteria affect not only mood, but also memory and cognition.

The findings from all the investigations performed on the gut-brain axis have indicated some of the most important factors that regulate the makeup of the microbiome. Two of the most important factors that have been identified are diet and stress. For example, high levels of stress can cause the intestinal wall to become more permeable, or in other words, stress creates larger gaps in the intestinal wall lining. This allows bacteria to travel more easily so that they can communicate with the human body. In terms of diet, high-fat and high-sugar foods have been linked to changes in the important proteins needed for maintaining the integrity of the intestinal wall barrier, and have also been associated with issues like depression and anxiety.

Knowing all this information, scientists believe that it may be possible to utilize probiotics and certain foods to manipulate the bacterial species living within us to treat health problems like mental illness. Currently, if a patient is suffering from mental illness, doctors will recommend drugs to alter the patient’s brain chemistry. However, if it is possible to alter brain chemistry without using drugs, this could provide patients with a less invasive form of treatment, one that does not require pills.

It has been noticed for years that conditions such as irritable bowel syndrome and inflammatory bowel disease often coexist simultaneously with mental health disorders, but it was not until recently that a connection was made between the brain and the bacteria in our gut. Future research may show how different bacterial species can be used to directly influence brain areas that relate to specific behavioral changes.