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ANMS Blog

Microbiota and the brain-gut connection

Written by Stefanie Twist
Reviewed by Dr. Premsyl Bercik

April, 2019

In the past few years, attention devoted to microbiota and the brain-gut connection has increased. Gut microbiota encompasses the bacterial colonies, fungi, viruses, and other microbes that live within our gastrointestinal tract.(1, 2) Importance of bacteria in our GI tract should not be understated as they contribute to key biological processes in our body. However, when there are changes to this balance, it can be the reason symptoms develop.

Microbiota development starts at birth. Factors such as vaginal versus cesarean birth and breastfeeding versus formula shape the diversity and proliferation of strains of bacteria.(2, 3) Human breastmilk has nutrients which can help shape the microbiome and immune development of children. Infant’s initial exposure to microbiota is through the act of birth. Cesarean section births correlated with increased risk of children developing celiac disease, asthma, type 1 diabetes, and obesity.(4) This may occur because infants born vaginally are exposed to their mother’s vaginal microbiota while those born via cesarean are not.(2)

Infants who were breastfed had smaller colonies of Escherichia coli and Clostridiodes difficile and larger colonies of Bifidobacteria than those who were formula-fed.(5) Bifidobacterium is thought to exert anti-inflammatory effects within the gut mucosa and low levels of Bifidobacterium have been associated with obesity and allergies.(6) As we age, we graduate to solid food. Essentially, we become what we eat. Eating diverse array of healthy foods is important. Children who ate more fast food than their peers were more likely to develop a functional gastrointestinal disorder.(7) Additionally, a recent study highlighted eating a healthier diet significantly reduced depressive episodes.(8)

Byproducts from bacterial activity also affect our GI tract. Short-chain fatty acids (SCFA) are produced when bacteria ferment fiber in the gut. SCFA have anti-inflammatory properties and can cross the blood-brain barrier. High-fat diets are associated with inflammation and inflammatory cytokines, which have been associated with problems in serotonin synthesis.(9) They are also important for regulating visceral perception, motility, and mucous production in the gut.(6, 9) Changes in typical American diets have led to a decrease of fiber and an increase in sugars and carbohydrates. Multiple studies have found fiber to be beneficial for the health of our gut bacteria as well the lining of our intestines. Hence reductions in fiber consumption may contribute to our predisposition to western disease. However not all fiber is created equal and we still need to figure out which fibers are good and how they exert a beneficial effect.

Alteration in the gut microbiome is associated with changes in parts of the brain that control emotion and sensation.(6) This is important to consider in individuals with functional GI disorders (FGIDs) as this has the potential to explain why symptoms persist, even if diagnostic tests, such a colonoscopy, return negative findings. Stress activates the hypothalamic-pituitary-adrenal (HPA) axis, which causes a reaction to release adrenaline, noradrenaline, and cortisol. These chemicals have been shown to promote growth and function of Escherichia coli (adherence to the mucosa), Yersinia enterocolitica, and increase pro-inflammatory response.(9) Bacteria are also able to produce neurotransmitters, such as dopamine, serotonin, GABA (gamma-aminobutyric acid), and histamine. Neurotransmitters are chemicals used by the nervous system to communicate. Altered GABA levels have been associated with several central nervous system disorders, such as behavioral disorders, pain, intestinal motility, and gastric emptying.(10) Histamine is a neurotransmitter associated with immune response and can lead to inflammation. The Low FODMAP diet has been shown not only to alter microbiota colonies in the GI tract, but also reduces histamine levels, which leads to reduced immune activation and likely a reduction in symptoms.(11)

1. Ma Q, Xing C, Long W, Wang HY, Liu Q, Wang RF. Impact of microbiota on central nervous system and neurological diseases: the gut-brain axis. J Neuroinflammation. 2019;16(1):53.
2. Foster JA, Rinaman L, Cryan JF. Stress & the gut-brain axis: Regulation by the microbiome. Neurobiol Stress. 2017;7:124-36.
3. Kim H, Sitarik AR, Woodcroft K, Johnson CC, Zoratti E. Birth Mode, Breastfeeding, Pet Exposure, and Antibiotic Use: Associations With the Gut Microbiome and Sensitization in Children. Curr Allergy Asthma Rep. 2019;19(4):22.
4. Moya-Perez A, Luczynski P, Renes IB, Wang S, Borre Y, Anthony Ryan C, et al. Intervention strategies for cesarean section-induced alterations in the microbiota-gut-brain axis. Nutr Rev. 2017;75(4):225-40.
5. Penders J, Thijs C, Vink C, Stelma FF, Snijders B, Kummeling I, et al. Factors influencing the composition of the intestinal microbiota in early infancy. Pediatrics. 2006;118(2):511-21.
6. Hyland NP, Quigley EM, Brint E. Microbiota-host interactions in irritable bowel syndrome: epithelial barrier, immune regulation and brain-gut interactions. World J Gastroenterol. 2014;20(27):8859-66.
7. Shau JP, Chen PH, Chan CF, Hsu YC, Wu TC, James FE, et al. Fast foods–are they a risk factor for functional gastrointestinal disorders? Asia Pac J Clin Nutr. 2016;25(2):393-401.
8. Jacka FN, O’Neil A, Opie R, Itsiopoulos C, Cotton S, Mohebbi M, et al. A randomised controlled trial of dietary improvement for adults with major depression (the ‘SMILES’ trial). BMC Med. 2017;15(1):23.
9. Panduro A, Rivera-Iniguez I, Sepulveda-Villegas M, Roman S. Genes, emotions and gut microbiota: The next frontier for the gastroenterologist. World J Gastroenterol. 2017;23(17):3030-42.
10. Strandwitz P. Neurotransmitter modulation by the gut microbiota. Brain Res. 2018;1693(Pt B):128-33.
11. McIntosh K, Reed DE, Schneider T, Dang F, Keshteli AH, De Palma G, et al. FODMAPs alter symptoms and the metabolome of patients with IBS: a randomised controlled trial. Gut. 2017;66(7):1241-51.

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