Historically, it has been thought that the effect food has on our bodies was limited to strictly physical capacities, such as immune function, body weight or chronic disease risk. In recent years, however, research has been shifting to suggest that the impact food has on our daily lives may go far beyond the physical and can in fact play a crucial role in our mental health as well. There is currently a large amount of research being dedicated to exploring the relationship between the way food is broken down for energy in the gastrointestinal tract and how the brain is then able to utilize this energy for essential mental functions. It is becoming clear that this relationship, commonly known as “the gut-brain axis”, is much more complex than previously thought.
To be more specific, this axis encompasses the enteric nervous system, which is housed within the gastrointestinal tract, and the central nervous system, which includes the brain and spinal cord. The enteric nervous system is a huge component of the human microbiome, which refers to the collection of billions of living microorganisms that reside on and within the human body. These microorganisms (sometimes also called microflora) include bacteria, fungi, protozoa, and even certain viruses, all of which can actually be extremely beneficial to our health. They aid a wide variety of crucial human body functions, ranging from digestive regulation to neurotransmitter production. While these microorganisms exist all over the body, a large concentration of them are housed primarily within a pocket of the large intestine (called the cecum). The cecum is made up of several nerve endings that are involved with the enteric nervous system. The enteric nervous system is connected to the central nervous system via the vagus nerve, which specifically works to transport chemical messages from the gut to the brain and vice versa. Factors such as stress and poor dietary intake have been known to impair the functioning of this nerve and are associated with increased instances of gastrointestinal disorders, such as irritable bowel syndrome and inflammatory bowel disease.
We know now that the food we eat daily can affect the gut-brain axis. Research is showing that specific components in our diets can directly affect how well our gut and brain are able to function and communicate through their effects on the diversity of our microflora. Less diverse microflora is generally associated with decreased gut function and undesirable health outcomes. This communication (or lack thereof) between the gut and brain has rippling effects on a variety of bodily processes, such as our immunity, risk of chronic disease development and mental health. The current body of research is building an increasing amount of evidence around the direct impact of nutrients like carbohydrates, fats, probiotics, and polyphenols on the homeostasis of the relationship between our brain and gut.
For example, multiple recent studies have shown that the gut is largely responsible for contributing to the production of several important neurotransmitters, such serotonin and dopamine. These neurotransmitters can directly affect things such as our mood and stress response. A major fuel source for the creation of these neurotransmitters are carbohydrates such as glucose, fructose, sucrose, and lactose, which can be found in foods like grains, pastas, starches, baked goods, fruits, and even some vegetables. Diets that are lacking in carbohydrates have actually been associated with higher instances of mental health disorders, such as depression and anxiety. Regular consumption of carbohydrate-containing foods has been associated with an increase in the production of beneficial bacteria like Bifidobacteria and a subsequent decrease in less beneficial strains of bacteria, which has been observed to help reduce symptoms of irritable bowel syndrome.
Carbohydrates can also be found in substances like artificial sweeteners, which have grown in popularity over the last several decades. While artificial sweeteners are often seen as an attractive low-calorie alternative to natural sugars, research is emerging to suggest there may be some drawbacks to these substitutions. Some sweeteners such as sucralose and aspartame may cause an unfavorable alternation in our gut microflora which could lead to an increase in glucose intolerance and decreased insulin sensitivity. Decreased insulin sensitivity is associated with adverse chronic health conditions such as type 2 diabetes. For proper maintenance of gut microflora diversity and overall gut health, using natural sugars in place of artificial alternatives may prove to be the more favorable option.
There is also a type of carbohydrate, known as fiber, that has a unique effect on microflora function in the gut. Fiber is not readily digested by the human body and is found in a wide variety of foods, such as whole grains, cereals, fruits, vegetables, legumes, and certain kinds of nuts. Since fiber is not able to be broken down into individual sugar molecules for energy, like most carbohydrate foods can, it travels mostly intact all the way through the digestive system to the large intestine, where it will finally be fermented by microorganisms for energy. This fermentation process results in the formation of products known as short chain fatty acids. These short chain fatty acids act as a primary fuel source for cells in the intestines and are known to have a direct anti-inflammatory effect on the gut by impairing the production of inflammatory compounds. Short chain fatty acids also help to improve immune function by increasing the production of immune-related cells. Furthermore, these important compounds can also help to improve brain function by inducing the secretion of hormones such as GABA and serotonin, both of which help to regulate feelings like stress, anxiety, fear, happiness, wellbeing, and general mood, making them pivotal in the connection between the gut and brain.
Foods and supplements that contain probiotics are another heavily studied dietary subset and are also thought to be involved in the gut-brain relationship. Probiotics are a form of live bacteria that help to positively regulate the microbiome through the production of anti-inflammatory compounds. Probiotics can be found naturally in foods such as yogurt, kimchi, and pickles, and can also be manufactured in pill form. Adequate consumption of beneficial probiotics has been associated with lower blood cholesterol levels, improved insulin sensitivity and decreased risk of developing cardiovascular disease and type 2 diabetes. Additionally, recent research has found that probiotics may be beneficial in improving mood state and sleep and decreasing symptoms of depression and anxiety. This is most likely due to increased microflora diversity, which demonstrates the importance of a diet that incorporates appropriate amounts of probiotics.
Polyphenols also have a growing body of evidence to support their potentially advantageous effects on the microbiome and gut-brain axis. Polyphenols are naturally occurring, antioxidant-containing compounds found in things like fruit, tea, cocoa, and wine. The antioxidants found in these foods seem to be primarily responsible for positive impacts on gut function, as they have anti-inflammatory effects on the body. Consuming foods rich in polyphenols can have positive health outcomes, such as increased production of good gut bacteria, decreased risk of cancer, management of inflammatory bowel disease symptoms, increases in “good” HDL cholesterol, and reduced levels of blood triglycerides, which may contribute to the development of chronic cardiovascular conditions. The antioxidant effects of polyphenols have also been shown to protect the brain by reducing risk of oxidative damage, decreasing activation of inflammatory compounds, and balancing the production of serotonin within the gut. These outcomes are especially significant, because they suggest that polyphenols could be directly involved in the management of mood disorders.
Based on current research, it is becoming incredibly evident that the relationship between our brain and gut, and the ways our diet impacts that dynamic, is an extremely important and an often-overlooked aspect of our overall health. Through proper nutrition and nourishment of our bodies, we may be able to improve not only our physical wellbeing, but also our cognitive functioning and mental state. As research in this area continues to grow, it will be important to be aware of the foods that may be directly beneficial to our wellness and those that may end up being more of hindrance. Conclusively, a diet that is well-rounded in foods like fruits, vegetables, and whole grains seems to be one that is consistently favorable for improved mental and physical health status.
References:
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- Bonaz, B., Bazin, T., & Pellissier, S. (2018). The Vagus Nerve at the Interface of the Microbiota-Gut-Brain Axis. Frontiers in Neuroscience, 12, 49. https://doi.org/10.3389/fnins.2018.00049
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