How Gut Composition is Linked to Depression
Written by Ishita Singh
July 31st 2021
July 31st 2021
Did you know that the state of your gut can affect your chances of developing Major Depressive Disorder? Up until the 20th century, Major Depressive Disorder (or MDD) was attributed to the imbalance of chemical signals in the brain known as neurotransmitters (Flux & Lowry, 2020). Due to this, researchers thought that the most effective treatments for MDD should focus on maintaining balance between neurotransmitters. However, recent research has found links between the bacteria, fungi, and other microorganisms that reside in our large intestine, known as the gut microbiome, to the development of MDD (Flux & Lowry, 2020). With this information, we can obtain a holistic view of how depression occurs and create more effective treatment plans.
How exactly is our gut microbiome linked to the development of MDD? The gut microbiome affects our mood through pathways with the central nervous, immune, and endocrine systems (Irwin & Miller, 2007). Here is how some of these interactions take place:
The Gut and The Central Nervous System (CNS)
The CNS is in charge of our mind and bodily functions. Our gut microbiome affects the CNS in numerous ways. One way is by altering the digestion of the molecule tryptophan. When tryptophan is digested, it is taken up by the brain and converted into serotonin (a "feel-good" neurotransmitter). If an individual's gut microbiome decreases tryptophan digestion, less serotonin is formed, and the individual experiences low moods associated with MDD (O’Mahoney et al., 2015). Another way the gut affects the CNS is through functional changes. For example, altering the gut microbiome of mammals can lead to a change in ion channel behaviors of the amygdala and increase amygdala function (Neufeld et al., 2011). The amygdala is the part of our brain that heightens fear-related behaviors. Mammals with higher amygdala function will display more behaviors associated with depression.
The Gut and The Immune System
Our immune system defends us from disease and dysfunction. A primary defense mechanism of our immune system is inflammation, which manages the bacterial composition of our gut microbiome. However, it seems like hyper-inflammation is also linked to MDD (Flux & Lowry, 2020). Studies analyzing the number of endotoxins present in depressed individuals have revealed that these individuals have a higher number of endotoxins in their system and fecal microbiome (Stevens et al., 2018). Endotoxins are molecules that make up the membrane of certain bacteria.This points to the idea that individuals with higher amounts of bacteria in their gut experience increased inflammation, alteration of gut microbiome, and development of mood disorders such as MDD.
The Gut and The Endocrine System
Our endocrine system releases hormones to regulate body functions and processes. The HPA axis, a major part of the endocrine system, oversees the release of cortisol, a stress hormone. Research has shown that overproduction of cortisol by the HPA axis can lead to symptoms of MDD. Additionally, the composition of our gut can influence how much cortisol is released by our endocrine system. In some studies, altering the gut composition of humans in a beneficial way using probiotics resulted in individuals experiencing significantly lower amounts of cortisol release and better moods (Messaoudi et al., 2011).
Works Cited
Flux, M. C., & Lowry, C. A. (2020). Finding intestinal fortitude: Integrating the microbiome into a holistic view of depression mechanisms, treatment, and resilience. Neurobiology of Disease, 135. https://doi.org/10.1016/j.nbd.2019.104578
Irwin, M. R., & Miller, A. H. (2007). Depressive disorders and immunity: 2- years of progress and discovery. Brain Behav. Immune., 21(4), pp 374-383. https://doi.org/10.1016/j.bbi.2007.01.010
Messaoudi, M., Violle, N., Bisson, J. F., Desor, D., Javelot, H., & Rougeot, C. (2011). Beneficial psychological effects of a probiotic formulation (Lactobacillus helveticus R0052 and Bifidobacterium longumR0175) in healthy human volunteers. Gut Microbes, 2(4), 10.4161/gmic.2.4.16108
Neufeld, K. M., Kang, N., Bienenstock, J., & Foster, J. A. (2011). Reduced anxiety-like behavior and central neurochemical change in germ-free mice. Neurogastroenterol. Motil., 23(3), pp 255-265, 10.1111/j.1365-2982.2010.01620.x
O’Mahoney, S. M., Clarke, G., Borre, Y. E., Dinan, T. G., & Cryan, J. F. (2015). Serotonin, tryptophan metabolism and the brain-gut-microbiome axis. Behave. Brain Res., 277, pp 32-48, 10.1016/j.bbr.2014.07.027
Stevens, B. R., Goes, R., Seungbum, K., Richards, E. M., Holbert, R. C., Pepine, C. J., & Raizada, M. K. (2018). Increases human intestinal barrier permeability plasma biomarkers zonulin and FABP2 correlated with plasma LPS and altered gut microbiome in anxiety or depression, Gut, 67(8), pp 1555-1557, 10.1136/gutjnl-2017-314759
Image Source: Mood and Gut Bacteria with chalk on Blackboard by T.L.Furrer licensed under Shutterstock Standard License