I’m sure you’ve heard the phrases “go with your gut,” “a gut feeling” or a “gut instinct” before, but did you know there’s actually some truth to them?
Whether we know it or not, our gut has a huge impact on our brain.
The Gut-Brain Axis
The human intestines are home to billions of bacteria, together called the gut microbiome. This microbiome is unique to each person-as unique as a fingerprint! These bacteria have a bidirectional relationship with your brain. This relationship is called the microbiota-gut-brain axis (Liang, Wu, & Jin, 2018).
From birth, the bacteria in your gut help to form the structure of your brain. Research has shown that germ-free mice (mice that have been bred to not have a microbiome) had altered brain development when compared to the control group with normal bacteria in their guts. Specifically, these germ-free mice had abnormal amygdala, hippocampus, and myelination in the prefrontal cortex (Cenit, Sanz & Codoner-French, 2017). Scientists aren’t sure exactly why this is yet, but they know that the microbiome influences the development of your brain.
Though small (in size), these bacteria are mighty! They have several functions in the gut alone (such as digesting your food), but they also have several functions in the rest of your body, particularly as it relates to the brain. The gut microbiome has recently been implicated in several cognitive functions including: a person’s response to stress, mood and emotion regulation, memory formation, appetite regulation, among many others (Cenit, Sanz & Codoner-French, 2017).
How is this so?
As I said before, there is a bidirectional communication between your gut and your brain that is regulated by the Vagus Nerve, a very large nerve that runs from your brain all the way through your body and connects to your gut (Cenit, Sanz, & Codoner-French, 2017).
When the bacteria in your gut digest the good you eat, they break it down into smaller particles your brain can use. For example, the neurotransmitter serotonin (the “happy” chemical that is commonly referred to when talking about depression) is made from the amino acid tryptophan, which is found in foods like poultry (which you’re probably familiar with from hearing about it in relation to the Thanksgiving turkey). The brain can’t take the turkey that you eat on Thanksgiving and turn it into serotonin. Instead, once you’ve eaten a piece of turkey, the bacteria in your gut will help to break it down into amino acids, such as tryptophan, which will then be sent to your brain so that it can create serotonin.
Another function of these bacteria is to produce substances themselves . Your microbiome produces substances such as neurotransmitters, neurohormones, neuropeptides, and short chain fatty acids, that signal up to the brain to influence cognitive function, and thus your behavior (Tooley, 2020).
The production of these substances rely directly on what you eat. For example, certain bacteria strains produce certain neurotransmitters such as GABA, serotonin, noreadrenalin, dopamine, and acetylcholine. The presence of these specific bacteria strains in your intestines depends on the nutrients you eat through dietary choices. It has recently been shown that a typical Western diet (high amounts of sugar, saturated fat, and processed foods) has resulted in an important loss of several bacterial species, which has led to a reduction of microbial diversity (Oriach et al., 2016).
Leaky Gut
This loss of bacterial species can lead to an imbalance between the good and bad bacteria in the gut..what happens when your gut gets out of balance?
The gut-mircobiome has recently been implicated in several non-communicable diseases, many of which include mental illness such as depression, anxiety, ADHD, schizophrenia, Alzheimer’s, Parkinson’s, and even more recently, autism (Cenit, Sanz, & Codoner-French, 2017). What do all of them have in common? Chronic inflammation.
Typically, your gut has a very tight lining that separates it from the rest of your body. This lining is important because without it, particles that are in your gut will be released into your body.
Your gut is home to billions of bacteria, but unfortunately not all of these are friendly bacteria. Certain bacteria are pathogenic, and if they become too mighty in size, they can wreak havoc on your gut…and your brain. When the amount of “bad” bacteria outnumber the amount of good bacteria in your gut, it is said that your gut is in a state of “dysbiosis,” which basically means imbalance. This imbalance can lead to this otherwise tight gut lining to become permeable, and cause what is commonly known as “leaky gut” (Oriach et al., 2016).
When this happens, the cells lining your gut become inflamed, the tight junctions break, which creates space between these otherwise tightly packed cells. This allows particles to make their way out of the gut and into circulation. These particles can include some of the “bad” bacteria themselves, partially digested food, cytokines (hold tight to learn more!), and dangerous particles called Lipopolysaccharides and endotoxins (Oriach et al., 2016). When this happens, your immune system recognizes that something is off and becomes activated. It thinks that your body is being invaded by unrecognized molecules, and it starts to produce cytokines, which are the immune system’s messenger cells. These cytokines tell your body that something is wrong, and as your immune system tries to help make things better, it produces inflammation. Inflammation is a natural, healthy immune response that allows your body to naturally heal itself when faced with stressors such as a cut on your hand, or a virus entering your body. However, if this inflammation persists and becomes chronic, it can lead to widespread issues like the diseases I mentioned above (Cenit, Sanz, & Codoner-French, 2017).
Another issue associated with leaky gut is the inability to absorb nutrients (McCormick et al., 2019). When your gut is inflamed, particles make their way out into your system without being digested. This, obviously, leads to nutrient deficiencies and all of the health and cognitive issues that accompany those (check back in a few weeks for more info on this!).
The brain also has a tight lining that separates it from the rest of your body, called the Blood Brain Barrier. This barrier is extremely important in keeping toxins that might make their way into circulation somehow (like…through a leaky gut) away from the brain. When the lining of the gut becomes permeable (or “leaky”), and chronic inflammation increases, the BBB also starts to become leaky (Liang, Wu, & Jin, 2018). The same tight junctions that hold the cells of the gut wall together are found in the BBB, and those start to break due to increased inflammation. This allows for unwanted particles to make their way into the brain and cause neuroinflammation (inflammation in the brain). Neuroinflammation can lead to cell death in the brain (obviously something we want to avoid), which can lead to impairments in several cognitive processes such as learning and memory, but also early aging and increased risk of age-related diseases like Alzheimer’s and Dementia (Liang, Wu, & Jin, 2018). Inflammation is also implicated in several other mental disorders, which we will learn more about next week!
But first, let’s learn more about how what you eat can lead to leaky gut and widespread chronic inflammation.
Diet and the Gut Microbiome
Diet is the main factor that can impact the overall health of your gut. What a person consumes in way of their diet literally makes up the bacterial population in the gut. More than 50% variation in the gut microbiota is related to dietary changes (Oriach et al., 2016). Major dietary changes during adulthood can modify the gut bacteria in a matter of days. How is this so?
Certain bacteria consume certain nutrients found in foods. When we don’t give these bacteria the food they need to survive through our diet, we starve them off.
For example, some of the beneficial bacteria in our gut enjoy eating prebiotic foods that are high in fiber. These foods include bananas, oats, onions, asparagus, and whole grains. Certain pathogenic bacteria enjoy consuming sugar and saturated fats. Just like any other living thing, when you feed your bacteria the foods they need to survive, they become bigger, stronger, and can reproduce efficiently.
To keep our gut and our brains as healthy as possible, we want a large diversity of bacteria strains. As you learned before, certain bacteria strains produce certain neurotransmitters, vitamins, hormones, and other important substances that our brains use. Without a wide variety of bacteria, we risk not being able to create all of these things our brains need to perform optimally.
A typical Western Diet that is composed of high amounts of saturated fats, processed foods and refined sugars, and low amounts of fiber, has been shown to reduce the diversity of the bacteria in your gut. It has also been shown that this type of diet leads to a decrease in beneficial bacteria, and an increase in pathogenic bacteria (if you remember, that’s when “leaky gut” starts to happen) (Lopez-Taboada, Gonzalez-Pardo, & Conejo, 2020).
Whereas polyphenols (substances found in fruits, vegetables, cocoa, tea, whole grains, nuts, and olive oil), have been shown to improve dysbiosis by feeding the beneficial bacteria, and thus reducing inflammation and improving the integrity of the gut lining (Oriach et al., 2016).
Prebiotic fiber is extremely important as it not only feeds the good bacteria, but when these bacteria are fed they produce substances called Short Chain Fatty Acids (Oriach et al., 2016). These SCFAs make their way up to the brain via the Vagus nerve, and have several benefits related to cognition. Butyrate is a type of SCFA produced by bacteria consuming fibers, and it has been related to memory formation and long-term potentiation (ability to convert these memories into “long-term memory”) (Mohajeri et al., 2018).
Probiotics, which are live strains of these beneficial bacteria and are found in foods such as yogurt and kefir, are extremely important when it comes to gut health AND brain health. In fact, there’s a field of study called “psychobiotics” that is aiming to understand how to use probiotic supplements to treat mental disorders (Bermudez-Humaran, 2019). The way this works is that the probiotics that you consume are actually live strains of bacteria that live in your gut. In yogurt or other probiotic supplements, you’ll likely see “billions of live cultures” somewhere on the label. These live strains of bacteria, once eaten, will make their way down to the gut and start to repopulate your gut with the beneficial strains of bacteria that you need. There are numerous studies that show that probiotics help to improve gut dysbiosis, restore the integrity of the gut lining (healing “leaky gut”), and reduce chronic inflammation (Liang, Wu, & Jin, 2018, Mohajeri et al., 2018).
Heal Your Gut, Fix Your Brain
So how can you take your gut health into your own hands? Luckily, it’s not as hard (or as complicated) as it might sound! We basically want to take out everything that may be causing dysbiosis in the gut (aka the foods that are feeding the bad bacteria), and instead, give the good bacteria more of the foods they want. Here are some tips to help you accomplish that!
Add in a serving of probiotics a day: foods like yogurt, kefir (make sure both of these contain live bacteria strains), tempeh, sauerkraut, kimchi, or kombucha are all high in probiotics. If you don’t normally consume any of these foods, take it slow at first. Start with half a serving to get your gut used to the extra bacteria! Bloating is a normal side effect, and can last a week or so until your body adjusts
Consume prebiotics regularly: probiotics help to repopulate your gut with good bacteria, prebiotics will help to feed this good bacteria. Prebiotic foods include onions, garlic, leeks, asparagus, green bananas, oats, apples, and flaxseed.
Polyphenols: these nutrients have been shown to improve intestinal permeability (Peron et al., 2021). Polyphenols can be found in: berries, cocoa and dark chocolate, extra virgin olive oil, beans, nuts, and green tea.
Avoid processed foods high in trans or saturated fats. This includes most fast food (burgers, fries, hot dogs, fried chicken), pastries, margarine, corn soy or canola oils, muffins, cakes, or donuts, and even pizza.
Limit sugar consumption: I know, I’m pretty much ruining your life by telling you to try and limit suga. But your gut will thank you. Those bad bacteria enjoy eating sugar, which can drive dysbiosis and intestinal permeability…and the cognitive dysfunction that follows. Check ingredient labels to see if sugar is hiding in any of your most commonly consumed items (spoiler alert: it is).
Eat a diverse diet: The best way to ensure that your gut contains a wide variety of bacteria is to eat a wide variety of foods. Diversity in your diet is key to getting enough of the nutrients each of these beneficial strains need to survive.
So, was Hippocrates right when he said “all disease begins and ends in the gut”? When it comes to the brain, I definitely think he was on to something. If you take your gut health seriously, you’ll start to see improvements in many different parts of your life. Feed your gut what it needs, and it will in turn feed you what you want to perform optimally.
References
Benjamin J J McCormick, Laura E Murray-Kolb, Gwenyth O Lee, Kerry J Schulze, A Catharine Ross, Aubrey Bauck, Aldo A M Lima, Bruna L L Maciel, Margaret N Kosek, Jessica C Seidman, Ramya Ambikapathi, Anuradha Bose, Sushil John, Gagandeep Kang, Ali Turab, Estomih Mduma, Pascal Bessong, Sanjaya K Shrestra, Tahmeed Ahmed, Mustafa Mahfuz, Maribel Paredes Olortegui, Zulfiqar Bhutta, Laura E Caulfield, MAL-ED Network Investigators, Intestinal permeability and inflammation mediate the association between nutrient density of complementary foods and biochemical measures of micronutrient status in young children: results from the MAL-ED study, The American Journal of Clinical Nutrition, Volume 110, Issue 4, October 2019, Pages 1015–1025, https://doi.org/10.1093/ajcn/nqz151
Bermúdez-Humarán, L. G., Salinas, E., Ortiz, G. G., Ramirez-Jirano, L. J., Morales, J. A., & Bitzer-Quintero, O. K. (2019). From Probiotics to Psychobiotics: Live Beneficial Bacteria Which Act on the Brain-Gut Axis. Nutrients, 11(4), 890. https://doi.org/10.3390/nu11040890
Cenit, M. C., Sanz, Y., & Codoñer-Franch, P. (2017). Influence of gut microbiota on neuropsychiatric disorders. World Journal of Gastroenterology, 23(30), 5486. https://doi.org/10.3748/wjg.v23.i30.5486
Liang, S., Wu, X., & Jin, F. (2018). Gut-Brain Psychology: Rethinking Psychology From the Microbiota–Gut–Brain Axis. Frontiers in Integrative Neuroscience, 12. https://doi.org/10.3389/fnint.2018.00033
López-Taboada, I., González-Pardo, H., & Conejo, N. M. (2020). Western Diet: Implications for Brain Function and Behavior. Frontiers in Psychology, 11. https://doi.org/10.3389/fpsyg.2020.564413
Mohajeri, M. H., La Fata, G., Steinert, R. E., & Weber, P. (2018). Relationship between the gut microbiome and brain function. Nutrition Reviews, 76(7), 481–496. https://doi.org/10.1093/nutrit/nuy009
Oriach, C. S., Robertson, R. C., Stanton, C., Cryan, J. F., & Dinan, T. G. (2016). Food for thought: The role of nutrition in the microbiota-gut–brain axis. Clinical Nutrition Experimental, 6, 25–38. https://doi.org/10.1016/j.yclnex.2016.01.003
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Tooley, K. L. (2020). Effects of the Human Gut Microbiota on Cognitive Performance, Brain Structure and Function: A Narrative Review. Nutrients, 12(10), 3009. https://doi.org/10.3390/nu12103009