The connection between the gut and the brain

One of the ways the human body is intricately connected in its anatomy and physiology is through what is known as the gut-brain axis. In the gut, or intestines, a variety of microorganisms, such as bacteria, fungi, archaea, and viruses, feed off of nutrients to support the body’s metabolic and immune functions. These microorganisms aren’t to be feared or rid of; in fact, it is important to maintain a healthy balance in the gut for regulation of bodily functions, particularly in the brain.
 
Pathways of Connection 
Research supports that there are likely four main pathways through which the brain and the microbiota of the intestines communicate. 
 
Nervous System: The microorganisms produce local signals in the sensory neurons connected to the vagus nerve, which plays a role in the body’s stress response and satiety signals. This means that a healthy intestinal microbiome can actually help you feel less stressed and avoid overeating. Additionally, the gastrointestinal (GI) tract produces approximately 90% of the body’s serotonin. Also known as “the happy hormone,” serotonin is a neurotransmitter important for sleep and mood and is the primary target for drug therapy for depression. 
 
Endocrine System: Endocrine cells in the intestines deliver signaling molecules based on the nutrients produced by the microorganisms. The nourishment of the microbiota impacts the release of hormones responsible for regulating blood pressure, mood, and the sleep-wake cycle.
 
Immune System: 70 to 80% of the body’s immune cells are located in the GI tract. The microorganisms can directly connect with the immune system as they guard the physical barrier of the intestines. Dysregulation in the microbiota often lessens the support of the lining and leads to inflammation related to intestinal hyperpermeability (a.k.a. leaky gut syndrome). In other words, an imbalance of the intestinal microbiome can actually contribute to damage of the intestines themselves and open you up to the development of food allergies, food sensitivities, and infection.
 
Metabolism: The most direct pathway is through the production of short chain fatty acids, which influence the sympathetic nervous system (the body’s fight-or-flight response) and cross the blood-brain barrier to regulate the development of microglia in the brain. Microglia are the brain’s immune system cells, and they continue to develop throughout a person’s lifetime. In particular, they are activated by disease states, such as infection. These cells contribute to neuroplasticity (the ability of neurons to alter their signaling associations depending on environmental stimuli) by developing and maintaining neural connections in the brain. Essentially, microglia allow our brains to learn and adapt through neuroplasticity. Signals from the microorganisms in the GI tract are essential to the optimal functioning of microglia throughout our lives.
 
Impacts of the Connection 
Recent studies have increased the understanding of the connection between the gut microbiota and neurodevelopmental disorders, such as Autism Spectrum Disorder, as well as neurodegenerative diseases like Parkinson’s. Another prominent interest in the field of study is the impact of the gut microbiota on mental health. Researchers conducting human clinical trials continue to study the impact of probiotics on the gut-brain connection by measuring markers of depression, anxiety, and other mental health disorders. A review of ten clinical trials describes reduction in symptoms of depression following probiotics. Probiotics are only one form of various dietary elements that promote the healthy regulation of the gut microbiome. Others include prebiotics, synbiotics, and short-chain fatty acids. 
 
Supporting the Gut Microbiome and the Brain
Eating foods with probiotics introduces live microorganisms to the intestines; meanwhile, foods with prebiotics (a type of dietary fiber) support the growth of the microorganisms. Synbiotics combine the harmonious effects of prebiotics and probiotics while also acting to increase concentrations of the short-chain fatty acids. Short-chain fatty acids are produced by the intestinal microbiome to provide energy to the intestinal cells, support metabolism, and help prevent or soothe inflammation.

---

Foods that could lead to dysregulation of the gut microbiome:

• Fast food
• Heavily processed foods
• Foods high in fat content
• Sparse inclusion of raw fruits and vegetables 

 
Examples of foods with beneficial probiotics:

• Kefir & water kefir
• Sauerkraut & kimchi
• Yogurt with active cultures 
• Fermented pickles & other fermented vegetables
• Kombucha & Jun

 
Examples of foods with beneficial prebiotics:

• Whole-grains such as oats & barley
• Vegetables like onions, tomatoes, asparagus, & artichokes
• Fruits such as berries & bananas
• Legumes

 
Ultimately, we believe that the strongest, most diverse intestinal microbiome will result from regular inclusion of whole and fermented foods. If you need extra probiotic support, check with your doctor to see if supplementation is appropriate for you. Examples of our favorite practitioner-grade probiotics designed to support the intestinal microbiome and its connection to mental health include:

• Ther-Biotic Complete (by Klaire Labs)
• SBO Probiotics Mental Clarity (by Ancient Nutrition)
• Complete Probiotics (by Dr. Mercola Premium Products)​  

---

This article is for educational purposes only and is not intended as medical advice. Please consult your licensed naturopathic physician or other licensed healthcare provider before making changes to your medications or supplements. Disclosures & Policies: This and other blog posts may contain affiliate links. Please view our disclosures page for more information. 

About the Author:
Daelyn Quinn is an undergraduate student at the University of South Florida. She studies Cell and Molecular Biology, as well as Spanish, and plans to attend a nationally accredited Doctor of Naturopathic Medicine program. Outside of academic and professional pursuits, she enjoys exploring cities with friends and family, running outdoors, and reading next to one of Florida’s many bodies of water.