One way that the microbiome communicates with the brain is by stimulating the vagus nerve. The vagus nerve travels down the brain stem into the spinal column, innervates the abdomen and then spreads out to create a web-like cage for the digestive tract. Microbes in the colon ”tickle” the vagus nerve to communicate directly with the brain, stimulating production and secretion of neurotransmitters that contribute directly to mental wellness, including serotonin, dopamine and others. Additionally, the microbiota play a significant role in what kind of signaling molecules are produced in the gut that then communicate with the brain.
The microbiota also impacts a part of the brain called the amygdala. The amygdala "...is commonly thought to form the core of a neural system for processing fearful and threatening stimuli, including detection of threat and activation of appropriate fear-related behaviors in response to threatening or dangerous stimuli." The composition of the microbiota - both the overall population and the diversity of the ecosystem - impacts how the amygdala develops a response to stressful stimuli and the extent to which anxiety manifests in an individual. In this way, microbiota modification via diet, lifestyle and therapeutic probiotic supplements holds promise for mental health.
Another way in which the health of the microbiota contributes to the health of the mind/heart is via maintenance of the structural integrity of the digestive tract.
In addition to beneficial microbes like species of lactobacillus and bifidobacteria, we also host strains of streptococcus and staphylococcus, candida and other yeasts. These potentially pathogenic microbes make toxic compounds called lipopolysaccharides (aka endotoxins). Lipopolysaccharides (LPS) dismantle the harmony not just in the colon but in the small intestine too. When the microbiota is imbalanced and the potentially pathogenic microbes gain a stronghold, over time the accumulation of LPS in the digestive tract can cause intestinal permeability or "leaky gut syndrome." In a leaky gut, the barrier of the small intestine is inflamed and compromised to the extent that nutrients are not absorbed and larger molecules escape the digestive tract intact, causing inflammation throughout the body.
With leaky gut we are posed with two factors contributing to mental wellness:
- Micronutrients, amino acids, fatty acids and minerals needed for the production, secretion and reception of neurotransmitters may not be digested and absorbed sufficiently. Overtime, this can lead to nutritional insufficiencies and compromise mental health on a biochemical level.
- When unchecked over a prolonged period of time, the inflammation in the gut can make its way to the brain, disrupting the integrity of the blood-brain barrier. Inflammation there can cause a "leaky brain," which furthers the mental health imbalance. One way that anti-depressants like SSRIs work is via anti-inflammatory mechanisms.
Here are a couple action steps you can take now for your gut - and mental - health:
- Eat fermented foods every day. These are going to be found in the refrigerated section of your grocery store, not on the shelves!
- Work high fiber foods into your meal plan: legumes, artichoke hearts, avocado, flaxseeds, chia seeds and oatmeal are great ingredients to start with.
- Make bone broth with small joints, like chicken feet, necks and backs. When cooked slowly, glutamine (an amino acid that is used to repair inflamed cells in the small intestine) is broken down into an easily digested form. Sip bone broth every day or use it as a base for soups and stews, or use as a liquid for cooking whole grains and beans.
These recommendations are not going to be appropriate for everyone, so it's always a good idea to consult a health care provider to get an individualized wellness plan.
- Uma Naidoo.This is Your Brain on Food (2020).
- Baxter, M.G., Croxson, P.L. Facing the role of the amygdala in emotional information processing. Proceedings of the National Academy of Sciences of the United States of America. 2012 Dec;109(52):21180-21181. https://doi.org/10.1073/pnas.1219167110
- Tynan, R.J., Weidenhofer, J., et al. A comparative examination of the anti-inflammatory effects of SSRI and SNRI antidepressants on LPS stimulated microglia. Brain Behav Immun. 2012 Mar;26(3):469-79. doi: 10.1016/j.bbi.2011.12.011.