Have you ever wondered why you have butterflies in your stomach when you are nervous? As it turns out, your nervous stomach is not a mere coincidence.
Your gut and brain are indeed connected. Research indicates that the brain and the gut are in constant communication via the vagus nerve, which is the longest nerve in the body extending from the brain stem to the gut, branching along the way to reach other internal organs in the chest and abdomen.
Your brain is home to your central nervous system (CNS), whereas your gut houses the enteric (intestinal) nervous system (ENS). The ENS consists of some 500 million neurons (nerve cells) embedded in the very thin lining of the 30-foot long gastrointestinal tract, which runs from the esophagus to the anus. It also houses nearly 100 trillion microorganisms.
The microorganisms residing in the gut are mainly bacteria from about 1,000 species, however, 90% of which belong to Bacteroidetes and Firmicutes. Each person has a distinct and highly variable composition of gut microbes, although a core group of microorganisms are common to all humans. The composition of the gut microorganisms is called the gut microbiota, whereas the totality of the genes of the microbiota is called the microbiome. The genes of the gut microbiome outnumber the genes of the human body by about 200-fold.
Microbes in the gut play a crucial role in human body function. Aside from digestive health and the immune system, researchers are beginning to understand how the gut affects brain health too. An increasing body of evidence shows that gut dysbiosis, which is an imbalance between different types of microorganisms in the gut, may play a significant role in a number of psychiatric and neurological disorders, including:
- Alzheimer’s disease
- Migraine headache
- Obsessive compulsive disorder (OCD)
- Parkinson’s disease
The following will look at the gut-brain connection and how the two communicate with each other. It discusses what science has discovered on the relationship between gut dysbiosis and various brain disorders. Finally, are some suggestions on how to maintain a healthy gut.
How Your Brain And Gut Communicate
The brain has a direct effect on the gut. For example, the very thought of eating can release the stomach’s juices before food gets there. The gastrointestinal tract is sensitive to emotions, such as anger, anxiety, sadness, or elation, which can produce symptoms in the gut. This connection goes both ways. A troubled gut can send signals to the brain, just as a troubled brain can send signals to the gut.
The gut, its microbes, and the brain are connected by a complex communication and regulation system called the gut-brain axis. The gut, by itself, is unique in many ways:
- The gut has the largest body surface, 100 times more than the skin.
- The gut has more neurons than the entire spinal cord.
- The gut contains around 80% of the body’s immune cells.
- The endocrine (hormone-producing) cells in the intestinal wall make the gut the largest endocrine organ in the body.
- The gut interacts and receives signals from its population of 100 trillion microbes.
The vagus nerve is the main communication pathway in the gut-brain axis, but the endocrine and immune systems are equally involved. Communication in the gut-brain axis relies on:
- Neuronal signals (via neurotransmitters)
- Endocrine signals (via hormones)
- Immune signals (via cytokines)
Certain gut bacteria species produce neurotransmitters, including GABA (gamma-aminobutyric acid), serotonin, dopamine, noradrenaline, as well as neurotransmitter modulators like BDNF (brain-derived neurotrophic factor). In fact, more than 90% of the body’s total serotonin and more than 50% of the body’s total dopamine are made in the gut, not the brain.
The gut microbiota are involved in the release of a host of hormones that regulate digestion, mood, emotions, sexuality, energy expenditure, and the stress response. These include insulin, glucagon, ghrelin, leptin, cortisol, as well as various thyroid and sex hormones.
Gut bacteria also make molecules like short-chain fatty acids (SCFAs) when they ferment dietary fibers in the colon. SCFAs, which consists of mainly acetate, butyrate, and propionate, can stimulate the secretion of hormones.
Additionally, SCFAs can cross the intestinal barrier, reach the bloodstream, get past the blood-brain barrier (BBB) to enter the brain, and act directly on it. Butyrate, in particular, is beneficial for restoring and maintaining the integrity of the intestinal barrier and the BBB.
When the intestinal barrier becomes permeable, it is a condition known as leaky gut syndrome. This allows undigested food molecules, viruses, and bacteria to get into the bloodstream. Leaky gut is associated with autoimmune diseases as well as food allergies and sensitivities.
Interactions between the gut microbiota and the immune system are extensive. Immune cells monitor the microbial composition of the gut. When there are significant changes, the immune cells alert the brain by activating the vagus nerve. Activation of the vagus nerve can change the properties of the intestinal barrier and its permeability to microbes, and modulate immune and inflammatory responses.
Gut bacteria can also trigger immune cells to release cytokines (cell signaling proteins) into the bloodstream. These cytokines can reach the brain and directly affect brain function.
Gut Dysbiosis And Brain Disorders
Accumulated evidence reveals that gut microbiota imbalance contributes to the development of various chronic diseases, like obesity and type 2 diabetes. In recent years, a significant number of studies have highlighted the correlation between gut dysbiosis and neurological disorders.
The influence of the gut on brain function begins early in life. At birth, infants acquire their gut microbiome from their mothers. During the first three years of life, gut microbiome diversity is low and variable, and undergoes drastic changes in composition.
From the age of three onwards, the microbial composition becomes stable and remains relatively unchanged in healthy adults, who possess mainly Bacteroidetes and Firmicutes. In adults 65 years and older, the gut microbiota undergoes considerable changes again.
Studies indicate that gut microbiota dysbiosis in mothers can result in the birth of autistic children. Autistic children often have gastrointestinal (GI) problems, such as indigestion, poor absorption, and overgrowth of bad intestinal bacteria. In many cases, probiotic supplementation that restores the gut microbiota mitigated autistic symptoms.
Anxiety and depression
Alterations in the gut microbiota can affect brain chemistry, including neurotransmitter signaling. Researchers found that the gut microbiota is able to influence executive function, cognition, mood, emotional regulation, stress response, pain, and social behavior.
The stress response is one of the functions in which the gut microbiota seems to play a crucial role. When faced with a stressful stimulus, hormones such as cortisol are released to regulate the stress response. Stress can alter the composition of the gut microbiota, and in turn, the gut microbiota can signal the release of stress hormones.
In studies using germ-free mice, which have no microorganisms in their guts, stressors like maternal separation early in life or prolonged restraint resulted in an exaggerated stress response, a hallmark characteristic of anxiety. The exaggerated response was reversed by administering probiotics.
In other studies, mice given oral antibiotics showed altered microbiota and increased anxious behavior, all of which were resolved within a few weeks after stopping the antibiotics.
Research has shown that patients with psychological disorders, such as anxiety and depression, have an altered constitution of the gut microbiome compared with healthy individuals. These patients also have a higher occurrence of inflammatory bowel disorders. When treated with probiotics, they reported a relief of symptoms with more happiness and less ruminative thinking.
Alzheimer’s disease (AD) and Parkinson’s disease (PD)
Throughout the aging process, humans undergo physiological changes that increase the susceptibility to neurodegenerative disorders, such as AD and PD. The incidence of GI diseases increases with age and the prevalence of diagnosed GI disorders is about 24% in people over 65.
Interestingly, a high percentage of patients with neurodegenerative diseases tend to have GI disorders. Patients with irritable bowel syndrome showed a high risk for the onset of AD. In PD patients, chronic constipation usually occurs ten or more years before the appearance of motor skill problems. Hence, it is suggested that gut dysbiosis may have an influence on the onset and development of these neurodegenerative diseases.
Current evidence shows that the gut-brain axis has a clear impact on migraine although the mechanism is yet fully understood. Migraine patients tend to have a number of other concurrent health conditions including:
- Gut infections (e.g. Helicobacter pylori, Candida, parasites, SIBO or small intestinal bacterial overgrowth, mold, and Lyme disease)
- Irritable bowel syndrome (IBS)
- Celiac disease (an autoimmune disease of the small intestine)
- Inflammatory bowel disease (IBD) such as Crohn disease and ulcerative colitis
In studies, treating the gut infections and supplementing with probiotics appeared to have significant reduction in the severity and frequency of migraine attacks. Dietary approaches such as increasing intake of SCFAs and omega-3 fatty acids, a gluten-free diet, vitamin D supplementation as well as weight loss (in case of obese patients) have shown favorable effects on migraine management too.
How To Maintain A Healthy Gut
The gut microbiome is greatly affected by various external conditions, including mode of delivery (vaginal vs. caesarean), dietary habit, lifestyle, drug use, and internal factors such as genetics and health status. Although you cannot alter your genes or how you were born, there are many things you can still do to make sure your gut stays healthy. The following are some suggestions for maintaining a diverse, stable, and robust microbiome with high gut lining integrity.
1. Eat a variety of probiotic foods
Eating fermented and cultured foods is the easiest, most effective, and least expensive way to make a significant impact on your gut microbiome. Examples include kefir, lassi (Indian yogurt drink), yogurt, natto (fermented soybeans), and pickled fermentations of cabbage, turnips, eggplant, cucumbers, onions, squash, and carrots. Eat a variety of these probiotic foods as each food provides different types of bacteria.
If you do not eat fermented foods on a regular basis, take a probiotic supplement. It is best to choose one with a variety of strains, not just one or two. The bacteria count is expressed in billion CFUs (colony forming units). Look for one that guarantees at least 10-20 billion CFUs at the expiration date.
2. Eat a variety of prebiotic foods
Prebiotics are dietary fibers that feed the friendly bacteria in your gut. In turn, the gut bacteria produce nutrients, such as SCFAs. SCFAs are the main nutrient source of the cells lining the GI tract; they help promote the integrity of the gut barrier.
Examples of prebiotic foods include dandelion greens, chicory root, garlic, onions, leeks, asparagus, Jerusalem artichokes, bananas, apples, whole oats, walnuts, flaxseeds, dark chocolate, red lentils, corn, jicama, and seaweed.
3. Avoid antibiotic overuse
Antibiotics is one of the greatest discoveries of modern medicine and have made once deadly infections easily treatable. However, broad-spectrum antibiotics are not targeted drugs. Not only will they kill the bad bacteria that cause infections, they will also destroy your beneficial gut bacteria.
Try not to take antibiotics unless it is absolutely necessary. Antibiotics are not effective on viral infections, only bacteria. After taking antibiotics, it can take months, if not years for the gut microbiome to recover; some people never fully recover from the use of antibiotics. Therefore, if you take antibiotics, make sure to reseed your gut with a variety of fermented foods and dietary fibers, and take a probiotic supplement.
4. Avoid meats and dairy products from factory-farmed animals
These animals are routinely fed low-dose antibiotics and genetically-modified soy or corn that have been treated with the herbicide glyphosate. Many studies have implicated that glyphosate leads to the destruction of gut flora. Chronic exposure to antibiotics increases your risk of antibiotic resistance as well.
5. Avoid processed foods and sugars
Ultra-refined foods, like white bread, chips, and sweets, make up nearly 60% of the average American diet. Excessive sugars not only lead to obesity and type 2 diabetes, they also feed the growth of pathogenic bacteria in the gut.
6. Avoid gluten and sensitive foods
Many people do not have the enzymes to digest gluten. If you are gluten intolerant and you continue to eat foods containing gluten, it will compromise the integrity of your gut lining barrier (leaky gut) and have a negative impact on your gut microbiota. For the same reasons, avoid your sensitive foods too. The five most common sensitive foods are wheat (gluten), dairy, eggs, corn, and soy.
7. Avoid excessive alcohol
Many studies have shown that excessive use of alcohol disrupts the microbiota composition, compromises intestinal permeability, and potentially contributes to the subsequent development of alcoholic liver disease. Recent studies in alcoholics showed lower numbers of Bacteroidetes and a greater abundance of Proteobacteria, which have the potential to be pathogenic. Alcoholics also have higher levels of endotoxin in the blood, indicating intestinal permeability.
8. Find ways to de-stress
Chronic stress is negatively associated with changes in your gut bacteria and your mood. It is not sustainable for your body to be always in a “fight or flight” mode. Effective strategies for reducing stress responses include:
- Maintaining a healthy social support network.
- Engaging in regular physical activities and relaxation practices.
- Getting an adequate amount of quality sleep each night.
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