Gut Feelings: The Link Between the Mind and Gut

by Julia Reese

It’s five in the morning and you’re freaking out. You have a paper due in three hours that you’ve barely started, even though you stayed up all night trying to work on it. Your fingers are shaking, your head is pounding and you feel close to crying. You’re stressed – and recent research has shown that your digestive tract may be to blame.

More specifically, the 100 trillion microbes that call the human digestive tract home could have a role in your mood. These microbes (outnumbering the human cells in the body by at least 10:1) are first introduced to the body during vaginal delivery. By the time a child reaches the age of one, they have what is called a “microbiome” in their gut that resembles an adult’s in complexity. The vast majority of these microbes are beneficial to humans, aiding in digestion and homeostasis through a mutualistic relationship. Scientists have known for a while that the brain can affect these microbes through the gut-brain axis, which is the pathway of communication between these two systems.

Recently, researchers are learning that this axis may be bidirectional – meaning that the microbes in your gut may affect your brain, ultimately altering your mood and perhaps influencing the development of mental conditions such as depression, anxiety and autism. The gut microbiome is made up of over 1,000 different species of microorganisms, but the exact strains and composition ratios of these organisms vary between individuals based on factors including genetics, age and diet.

One of the ways that scientists study the connection between the gut and anxiety is the use of germ-free animals. These animals (typically mice) are born and raised in sterile environments, and therefore have no microbiota in their digestive systems. When researchers expose these mice to stressors, they fing that they have an exaggerated response to the stress compared to the response of mice with typical gut microbiomes, as measured by the release of adrenocorticotropic hormone. These results suggest that the gut may interact with the brain through hormones in the neuroendocrine system, like the adrenocorticotropic hormone, which modulates the stress response.

In the case of anxiety, the gut and brain work together to produce a behavioral response, which appears as symptoms of anxiety. However, microbes in the gut can also affect conditions that are directly controlled by the central nervous system, such as autism. Children on the autism spectrum typically present with gastrointestinal issues, adding an interesting layer to the linkage between the gut and brain. However, it should be noted that these children are often prescribed antibiotics and adhere to atypical diet regimens, which may lead to some of the differences between their gut microbiota and those of unaffected children. Nevertheless, a study showed that autistic children had a different concentration of short-chain fatty acids with known neuroactivity in their fecal matter compared to unaffected children. When lab animals were given high doses of these same fatty acids, they exhibited some behaviors that resembled autism.

Further research has revealed even more indicators that the brain and the microbes in the gut interact in a highly complex manner. For instance, it was found that probiotics can act as anti-depressants by normalizing levels of tryptophan, and they can also increase the pain threshold in conditions such as irritable bowel syndrome. However, not all research follows the same upward path. Researchers have also found that germ-free mice exhibit less anxiety-like symptoms in a maze test.

As research continues, hopefully more mechanistic causes of the link between the gut and mind will be found to help balance these opposing study results.