How Gut Health Affects Mental Health

by Ericka Naegle MS Candidate, Dietetic InternNutrition

Everyone seems to be talking about the gut these days, and with good reason. Our gut health, and more specifically the health of our gut microbiota, or the approximately 100 trillion bacteria and other microbes that live in the gastrointestinal tract (1), have been shown to impact our overall health in a variety of ways. One exciting area of research focuses on the gut-brain axis, or the two-way communication pathway between the gastrointestinal tract and the brain. If you’ve ever felt butterflies in your stomach, you know the brain can impact the gut. Research now shows that the health of our gut can also influence things like our mood, anxiety, behavior and cognition.

What is a Healthy Gut?

A healthy gut is characterized by a diversity of microbes in our gastrointestinal tract, as well as a careful balance between the types of microbes present. When the gut is healthy, with a favorable balance of bacteria, the microbiota in the large intestine ferment fiber, resistant starches and other molecules, in turn producing gases, short-chain fatty acids (SCFAs) and some vitamins (2). The microbiota and the metabolites they form as part of the digestive process support immune function and maintain a healthy, non-permeable gut lining that keeps pathogens and other molecules from entering our circulation. More and more research is showing they are also involved in regulating our metabolism, weight, cognition and mood (3).

When the gut is in dysbiosis, or when the balance of “good” and “bad” bacteria is thrown off, our digestion and overall health can be disrupted (2). Leaky gut can occur when dysbiosis causes the gastrointestinal tract to become permeable, which can lead to neuroinflammation (4).

There is no one ideal gut microbiome. Everyone’s microbiota is different, but a few studies have shown certain types of bacteria to be associated with overall health and others to be associated with unfavorable outcomes, including obesity, chronic disease and mood disorders (5). Research on humans is limited, however. We don’t yet know whether an altered microbiota produces disease or results from disease.

The Gut-Brain Axis

We do know that the brain and the gut microbiota influence each other through the gut-brain axis (GBA). This axis is composed of the central, autonomic and enteric (intestinal) nervous systems, as well as the hypothalamic-pituitary-adrenal (HPA) axis (6). Through the nerves and hormones involved in the GBA, the brain controls intestinal activity, and the intestinal cells, under the influence of the gut microbiota, can influence emotional and cognitive functions in the brain (6).

The means by which our gut influences our mental health are still being explored, but the primary suspected mechanisms include:

  1. Neurotransmitters: Gut bacteria produce many of the neurotransmitters that control our brain function and mood, including serotonin, dopamine, norepinephrine, acetylcholine and GABA (7).
  2. Microbial metabolites: The results of microbial fermentation of certain foods include short-chain fatty acids, which have been shown to improve the integrity and function of the blood-brain barrier in mice (4). Metabolites of fiber fermentation have also been shown to reduce inflammation, a key risk factor in mental disorders (8).
  3. Immune system regulation: Different types of bacteria in the gut can cause the immune system to increase or decrease levels of inflammation, which we know directly impacts the brain (9).
  4. The vagus nerve: this nerve is a major two-way communication pathway between the gut and the brain. While the mechanisms are not fully understood, studies have demonstrated that specific strains of gut bacteria can alter our behavior via vagal nerve communication with the brain (10).

The State of the Research

Research shows that the types and amounts of bacteria and other microbes in our gut can influence our mental health through the above mechanisms.

Several studies have shown that changing the gut microbiota of mice changes their behavior and that behavioral traits can be transferred when gut microbes are transferred between mice (9). Further, when the microbiota of depressed patients has been transferred to animals, depressive behaviors have also been transferred, supporting arguments that the microbiota could be causative in depression (8).

Most of the research in humans is observational, which means it cannot assess causality, but studies have associated neuropsychiatric disorders including autism, schizophrenia, and attention deficit hyperactivity disorder (ADHD) to alterations in the gut microbiota (8). Patients with depression have been observed to have altered microbiota, including lower populations of Bifidobacterium and Lactobacillus, two prevalent and beneficial types of bacteria, compared to controls (8).

There is also evidence that the gut microbiota plays a role in neurodegenerative conditions like Parkinson’s disease and Alzheimer’s disease. Compared to controls, scientists have observed altered gut microbiota, including higher ratios of pro-inflammatory bacteria and lower concentrations of beneficial short-chain fatty acids, in Parkinson’s patients (8). Evidence related to Alzheimer’s is more tenuous. It has been proposed, however, that the alterations in the gut microbiota that are associated with type 2 diabetes and obesity contribute to why these two conditions are risk factors for Alzheimer’s (8). In mice, one study linked an altered gut microbiota to accumulation of amyloid plaques (8).

What Can We Do?

It is widely accepted that our gut health and mental health are linked. However, research has not yet established definitive guidelines for how we promote our mental health through our gut. Nonetheless, there are several steps we can take based on the available evidence:

  1. Probiotics: Eat fermented foods like sauerkraut, tempeh, kimchi, natto, kombucha and dairy products like yogurt (be sure that it includes live active cultures) and kefir (11). Probiotics have been shown to be beneficial for brain function (7) and mood in humans (12, 13, 14).
  2. Prebiotics: Eat fruits and vegetables containing prebiotics, the fiber fermented by gut bacteria. This fermentation process is what produces the short-chain fatty acids that can impact brain function and mood. Prebiotic foods include apple, asparagus, banana, barley, dandelion greens, endive, radicchio, jicama, Jerusalem artichokes (also called sunchokes), onions, garlic, peas, legumes and whole grains. (11). As with probiotics, studies have shown prebiotic-rich diets to be beneficial for brain function (7) and mood (15).
  3. Follow a Plant-Based Diet: Research has shown that people who eat plant-based diets have a greater diversity of gut microbiota (16).A study in mice indicated that diets high in saturated fat lead to depression-like, potentially because of altered gut microbiota (17).

References

  1. Dash, S., Clarke, G., Berk, M. & Jacka, F. N. (2015). The gut microbiome and diet in psychiatry: Focus on depression.Current Opinion in Psychiatry, 28(1), 1-6. 10.1097/YCO.0000000000000117
  2. Mahan, L.K., & Raymond, J.L. (2017).Krause’s food & the nutrition care process(14th edition). Elsevier.
  3. Young, C. (n.d.) What is the gut microbiome? Food & Mood Centre. https://foodandmoodcentre.com.au/2016/07/what-is-the-gut-microbiome/
  4. Lodgson, A.F., Erickson, M.A., Rhea, E.M., Salameh, T.S., & Banks, W.A. (2018). Gut reactions: How the blood–brain barrier connects the microbiome and the brain.Experimental Biology and Medicine, 243, 159–165. 10.1177/1535370217743766
  5. Wang, B., Yao, M., Lv, L., Ling, Z., & Li, L. (2017). The human microbiota in health and disease.Engineering, 3,71-82. dx.doi.org/10.1016/J.ENG.2017.01.008
  6. Carabotti, M., Scirocco, A., Maselli, M. A., & Severi, C. (2015). The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems.Annals of Gastroenterology,28(2), 203–209. www.ncbi.nlm.nih.gov/pmc/articles/PMC4367209/
  7. Galland, L. (2014). The gut microbiome and the brain.Journal of Medicinal Food, 17(12), 1261-1272. 10.1089/jmf.2014.7000
  8. Cenit, M.C., Sanz, Y., & Codoñer-Franch, P. (2017). Influence of gut microbiota on neuropsychiatric disorders.World Journal of Gastroenterology, 23(30), 5486-5498. 10.3748/wjg.v23.i30.5486
  9. Cryan, J.F. & Dinan, T.G. (2012). Mind-altering microorganisms: the impact of the gut microbiota on brain and behavior.Neuroscience, 13, 701-712.
  10. Fülling, C., Dinan, T.G., & Cryan, J.F. (2019). Gut microbe to brain signaling: What happens in vagus….Neuron, 101,998-1002. doi.org/10.1016/j.neuron.2019.02.008
  11. Institute for Functional Medicine. (2019).Probiotic and Prebiotic Foods[Toolkit Handout].
  12. Carpenter, S. (2012). That gut feeling.Monitor on Psychology, 43(8), 50. www.apa.org/monitor/2012/09/gut-feeling.aspx
  13. Steenbergen, L., Sellaro, R., van Hemert, S., Bosch, J. A., & Colzato, L.S. (2015). A randomized controlled trial to test the effect of multispecies probiotics on cognitive reactivity to sad mood.Brain, Behavior, and Immunity, 48, 258-264. doi.org/10.1016/j.bbi.2015.04.003
  14. Akkasheh, G., Kashani-Poor, Z., Tajabadi-Ebrahimi, M., Jafari, P., Akbari, H., Taghizadeh, M., Memarzadeh, M., Asemi, Z., & Esmaillzadeh, A. (2016). Clinical and metabolic response to probiotic administration in patients with major depressive disorder: A randomized, double-blind, placebo-controlled trial.Nutrition, 32(3), 315-320. dx.doi.org/10.1016/j.nut.2015.09.003
  15. Miki, T., Eguchi, M., Kurotani, K., Kochi, T., Kuwahara, K., Ito, R., Kimura, Y., Tsuruoka, H., Akter, S., Kashino, I., Kabe, I., Kawakami, N., & Mizoue, T. (2016). Dietary fiber intake and depressive symptoms in Japanese employees: The Furukawa Nutrition and Health Study.Nutrition, 32(5), 584-589. dx.doi.org/10.1016/j.nut.2015.11.014
  16. Conlon, M.A., & Bird, A.R. (2015). The impact of diet and lifestyle on gut microbiota and human health.Nutrients,7(1), 17–44. doi.org/10.3390/nu7010017
  17. Jørgensen, B.P., Hansen, J.T., Krych, L., Larsen, C., Klein, A.B., Nielsen, D.S., Josefsen, K., Hansen, A.K., & Sørensen, D.B. (2014). A possible link between food and mood: Dietary impact on gut microbiota and behavior in BALB/c mice.PLoS ONE,9(8), e103398. doi.org/10.1371/journal.pone.0103398