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The Remarkable Link Between The Gut and Brain:Prebiotics and Probiotics at the Core!

The microbiota is a collection of trillions of microorganisms living in our intestines. These microorganisms benefit our health and communicate with our brain and central nervous system (CNS). Because our microbiota is responsive to external input (1) , it is important that we understand the relationship we have with our microbiota, how we can create a suitable environment for healthy microbiota, what behaviors we can avoid to prevent harming this special relationship, and how gut microbiota communicates with the brain. The terms prebiotics, probiotics, and fiber may be familiar, but what do they really mean and what do they have to do with gut-brain communication and microbiota?

Our Relationship with the Microbiota

Bacteria is not all that bad, although typically when we hear the word bacteria we think about a small germ that can make us sick. Contrary to this belief, there is a large amount (in the trillions) of bacteria that are beneficial to our everyday life. Healthy bacteria can be found on the skin, eyes, nose, mouth, upper throat, lower urethra, in the vaginal tract, in the lower intestine, and is especially abundant in the large intestine. (2) To put this into a better perspective, the human body has 3 x 10^13 human cells (that’s about 30,000,000,000,000), and living within our body there are 3.8 x 10^14 bacteria (that’s about 380,000,000,000,000).

There are 3.8 x 10 14 bacteria living within our body

Within the gastrointestinal tract (small intestine and large intestine, from now on referred to as the GI tract) there are more than 400 species of bacteria. The intestines alone contain about 100,000 billion bacteria. (2) These species of bacteria can provide benefits such as promoting the immune system, assisting in metabolism, assisting in the synthesis (creation) of vitamins and amino acids, specifically the B group vitamins and vitamin K, aiding in recovery time for athletes and defending the body against pathogens. Studies are surfacing that link a healthy microbiome to the prevention of chronic diseases such as: (3)

  • Crohn’s disease

  • Ulcerative colitis

  • Obesity

  • Type 2 diabetes

Other emerging studies show benefits to assessing microbiome colonies (different groups of bacteria species) and diagnosing certain cancers such as colorectal cancer. Another huge new area of research that highlights the benefits of a healthy microbiome are psychobiotics. A Psychobiotic is a combination of specific pre- and probiotic species, currently used for research purposes, to treat patients with mental health disorders such as schizophrenia, depression, and anxiety. (1,5)

Creating a Suitable Environment

First, let us start with what prebiotics and probiotics are and how they are different:

Prebiotics are defined as substrates used by the microorganisms living in our gut. (1) Fiber is a great source of prebiotics that greatly influences the gut-brain interaction.

Examples of dietary sources of fiber include:

Probiotics are defined as live bacteria that when consumed offer health benefits such as improving metabolism, immunity, and even slowing aging. (1)

Several studies have also shown the effects of probiotics and the microbiota on mood and anxiety in healthy individuals with depressive symptoms. (5)

Examples of probiotics include:

What do Prebiotics and Probiotics have to do with the Gut-Brain barrier?

Simply put, probiotics help to maintain a healthy balance among the bacteria in the gut while fiber is a popular source of prebiotics that feed these bacteria. (4)

How Pre and Pro Biotics can Help

Now that we understand the benefits that the microbiome can offer and the difference between a pre-and probiotic, let's dive deeper into how pre- and probiotics can benefit our health.

Prebiotics are especially beneficial to our microbiota. As we have learned prebiotics are a food source for the friendly bacteria in our gut. By feeding our microbiota prebiotic sources, we can diversify the different colonies living within our GI tract.

Having a diverse microbiota is beneficial as different strains of bacteria offer differing health benefits. An important feature of prebiotics is that they are often slow to digest. This feature is beneficial in lowering blood glucose levels, increasing satiety, and promoting weight loss. Probiotics can be used to treat and prevent many disease states. Clinical studies have shown a significant effect of using probiotics in treatments for obesity, insulin resistance, type 2 diabetes, and fatty liver disease. (6)

Probiotics have also shown effective in reducing allergies, such as eczema in newborn infants by supplementing the mothers, treating and preventing bacterial and yeast infections, increasing immune function by increasing the activity of natural killer (NK) cells, IgA, neutrophils, and IgE antibodies (all important biochemical players in immune function), and probiotics have shown to be effective in treating and preventing cardiovascular diseases by assisting in lowering cholesterol. (2) In addition to treating and preventing many disease states, probiotics are also helpful in the absorption of vitamins and minerals and assist the microbiota in generating amino acids, enzymes and metabolic sources of energy. (6)

Harming the Environment

The brain and the gut are in constant communication. This communication is known as the gut-brain axis. Studies have shown that our gut microbiome can influence our eating choices, our moods, our digestion, and many other key parts of our daily life. Now that we understand what our gut microbiome is and the importance of pre-and probiotics in diversifying and feeding this microbiome, let’s bring light to behaviors that negatively impact the diversity of the gut microbiome and thus its communication with the brain.


Stress signals from the body promote an inflammatory response within the GI tract in some individuals. This inflammation and stress signaling that occurs within the GI tract can encourage the overgrowth of pathogenic (unfriendly) gut bacteria.(7)

This overgrowth and shift away from

friendly bacteria and towards unfriendly bacteria can lead to depressive symptoms, leaky gut, irritable digestion, and lowered immune system function.

AA: Antibiotics

Antibiotics have been a revolutionary discovery in modern medicine. This wonder drug is used to kill off bacteria or make the environment difficult for bacteria to grow. The downfall to antibiotics, however, is that they are unable to decipher friend vs foe. This becomes problematic when our healthy gut microbiome is targeted and killed off by antibiotics.


According to research, diet is one of the most powerful predictors of gut health. (7) We have learned that what we consume directly affects which bacteria species will thrive in our gut and further aid in metabolism. Long-term eating habits that consist of plant protein, unsaturated fats, and fiber promote a healthy and diverse microbiome, however, a shift in one's dietary habits can create a notable effect on the gut microbiome within 24 hours. (7) A diet similar to the Western American diet, high in saturated fats, processed foods, and refined sugars, decreases the diversity of the gut microbiome, promotes leaky gut, lowers immune function, and further promotes chronic disease risk. In the United States, 100 billion dollars are spent annually on digestive disorders and the smoking gun is assimilation to the Western diet. Dr. Erika Ebbel Angle does a great job describing the role of diet and its impact on the diversity of the gut microbiome. In her TED talk, she shares that certain bacteria are responsible for the digestion of specific food components. (8) If we eat a diet high in saturated fats and refined sugars, the bacteria living within our GI tract will become highly populated with the bacteria that are able to further digest such a diet, while the bacteria that are able to digest foods such as leafy vegetables and lean meats will become scarce. (8) Figure 6 is the diagram Dr. Erika Ebbel Angle used during her TED talk to describe this relationship.

She then uses Figure 7 to show how this relationship can alter the chemicals produced within our gut that further communicate with our brain.

In the healthy gut example on the left, there is a balance between the biochemicals communicating within our brain. However, on the right, we can see the effects of a diet resulting in a limited gut microbiome and its biochemical communication with our brain.

Communication between the Gut Microbiota and the Brain

Considering the number of human cells and bacteria living within our gut are nearly parallel, we can understand that bacterial communication plays a significant role in our body's communication amongst itself. Studies using brain imagining have been able to show the remarkable impact that the gut microbiome has on brain function.(8) One study tested different strains of bacteria via fermented milk on healthy women. The brain scans from this study highlighted areas of grey and white matter within the brain that were influenced by the ingestion of specific strains of bacteria. Another study tested the bacterial strain B. longum on patients living with IBS (irritable bowel syndrome) and depression. The brain scans from this study showed positive impacts on areas of the brain responsible for negative emotional responses.(8) After this study, the patients reported reduced depression and increased quality of life. The study of the gut-brain axis is relatively new but the research available thus far shows the remarkable impact that the gut microbiome is having on the brain.



1. Cryan JF, O’Riordan KJ, Cowan CSM, et al. The microbiota-gut-brain axis.

Physiol Rev. 2019; 99: 1877-2013. DOI: 10.1152/physrev.00018.2018

2. Lecture 12: diseases of the lower GI tract. NSU MNT 6400 powerpoint

presentation. Published November 7, 2023. Accessed November 12, 2023.

3. Yin R, Kuo HC, Hudlikar R, et al. Gut microbiota, dietary phytochemicals and

benefits to human health. Curr Pharmacol Rep. 2019;5:332-344.


4. Holzapfel W, Schillinger U. Introduction to pre-and probiotics. Food Research

International. 2002; 35(2-3): 109-116.

5. Heidarzadeh-Rad N, Gokmen-Ozel H, Kazemi A, et al. Effects of a psychobiotic

supplement on serum brain-derived neurotrophic factor levels in depressive

patients: a post hoc analysis of a randomized clinical trial. J Neurogastroenterol

6. Markowiak P, Śliżewska K. Effects of probiotics, prebiotics, and synbiotics on

human health. Nutrients. 2017;9(9):1021. doi:10.3390/nu9091021.

7. Madison A, Kiecolt-Glaser JK. Stress, depression, diet, and the gut microbiota:

human-bacteria interactions at the core of psychoneuroimmunology and

nutrition. Curr Opin Behav Sci. 2019;28:105-110. doi:10.1016/j.cobeha.2019.01.011.



8. Ebbel Angle E. Your gut microbiome: the most important organ you’ve never

heard of. TEDxFargo. Published December 12, 2019. Accessed November 19, 2023.


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