The gut is your second brain. How does the microbiome affect your well-being?

What is gut microbiome?

The gut microbiome is a community of microorganisms that naturally inhabit the human gut. The microbiome consists mainly of bacteria, but not only. It also includes viruses, fungi and other microbes that make up a complex ecosystem. The microbiome begins to form as early as in fetal life and changes dramatically during childbirth and infancy. However, it is important to remember that it evolves and transforms throughout life.

Various factors influence the composition and mutual proportions of the elements of intestinal microbiome:

• age,
• physical activity,
• diet,
• work,
• sterility of the living environment,
• hobby and leisure activities,
• diseases and medications.

The composition of each person’s gut microbiome is unique, although similarities in microbial composition are observed in people who are related and/or live in similar environments. It is even said that the microbiome is as unique as a fingerprint.

What is a healthy gut microbiome?

Despite appearances, it is very difficult for scientists to clearly define a model “healthy gut microbiome”. All because of the huge differences between the types and proportions of various microbes colonizing the gut in healthy individuals. Two people may have a completely different composition of gut microbiome, but each will still perform its function. So there is no universal ideal of a healthy microbiome that every person should strive for. However, it is known for sure that a healthy microbiome is a diverse microbiome.

Over time, scientists have identified a growing number of indicators to help them to recognize whether a person’s microbiota is properly balanced. What matters is the type of dominant microbes and their ratios, as well as the potential presence of microorganisms considered to be undesirable. However, dividing microbes into “bad” and “good” is a gross oversimplification. In healthy individuals, microorganisms live in harmony and balance, and support the functioning of the digestive, immune, and nervous systems.

There is no simple and unambiguous way to check whether the microbiome is “healthy”. Although the market for “microbiome tests” is growing rapidly, doctors stress that most of them are not yet reliable enough to give a realistic health assessment and to be recommended to the general population.

The gut – our second brain

Although the gut microbiome indirectly affects almost every system in the human body, many experts describe the connection between the gut and the brain as the most fascinating one. There is a reason why people talk about having butterflies in their stomach or a gut feeling.

Calling the gut a “second brain” is definitely not an exaggeration. The enteric nervous system includes the largest number of nerve cells in the body outside of the brain. Although it is not capable of “thinking” as such, it transmits more information to the brain than any other system in the body.

The bidirectional network of information exchange connecting the central and enteric nervous systems is called the brain-gut axis. The microbiome is just one part of the communication that the brain and gut use to exchange information.

This network includes the following routes:

• anatomical, or neurological – they affect the vagus nerve, the enteric nervous system and secreted neurotransmitters such as e.g. GABA, serotonin and histamine,
• endocrine – the microbiota influences the release of biologically active peptides that regulate, e.g. the stress response,
• humoral and metabolic – short-chain fatty acids (SCFAs) formed by the metabolism of the gut microbiota have an impact on gut-brain communication and can affect mood and well-being,
• immunological – the gut microbiota regulates the inflammatory and immune response, so it can affect all kinds of conditions accompanied by inflammation, from intestinal disorders such as IBS, to stress and serious mental disorders.

How does the microbiome affect your well-being?

The mere presence of mechanisms that allow for an efficient exchange of information between the gut and the brain is not sufficient proof that the gut microbiome has a huge impact on your well-being. Here is a more detailed explanation of this connection:

1. Production of neurotransmitters

Gut bacteria can produce neurotransmitters, or molecules that directly affect the mood. According to estimates, up to 95% of serotonin, known as the happiness hormone, is produced in the gut. Serotonin affects the mood – it can reduce feelings of anger, tension, stress or even pain. Bringing serotonin levels to a normal level definitely helps with overall mood improvement. Drugs that regulate serotonin levels are key to treating depressive disorders, PTSD or anxiety disorders.

Dopamine is another important transmitter. It is mainly produced in the brain, but it has been shown that the microbiome can also affect its levels. Dopamine is very important in feeling motivation and pleasure.

Another lesser-known neurotransmitter, the gamma-aminobutyric acid (GABA), has the ability to “inhibit” the nervous system. It has a calming, soothing and anxiolytic effect and helps to reduce stress. Gamma-aminobutyric acid is available in the form of supplements, but it is naturally produced by intestinal bacteria. A healthy, well-functioning gut microbiome therefore ensures the natural secretion of substances that calm the nerves.

2. Effects on the immune system and inflammation

The impact of the microbiome on the body’s immune system is now common knowledge. Gut bacteria are crucial in “training” immune cells to respond appropriately and proportionately to pathogens. Intestinal dysbiosis, a condition in which the composition of the microbiome is not optimal, leads to increased inflammation in the body. Chronic inflammation is associated with a number of ailments and diseases, including mental disorders – from mood disorders and general weakness to autoimmune diseases and depression.

3. Production of short-chain fatty acids

Microbes that populate the intestine feed on food debris, mainly fiber. As a result of fiber fermentation, they produce short-chain fatty acids (SCFAs) such as butyrate, acetate and propionate. SCFAs bring a number of benefits to the body:

• they have an anti-inflammatory effect, which is beneficial for brain health and for maintaining good mood and optimal energy levels,
• they nourish intestinal cells, thus improving the functioning of the entire digestive system,
• they directly activate the vagus nerve, which affects mood control and supports stress reduction (there is evidence that vagus nerve stimulation can also be used to treat depression, anxiety and trauma).

Read also: Somatic exercise: how does it help with stress management?

Blood-brain barrier vs. microbes

The blood-brain barrier is the barrier that separates the blood and all the substances circulating in it from the brain. It is a key structure that protects the brain from exposure to unwanted metabolites. It is selective, meaning that it allows for the transport of essential substances to the brain. Its "tightness" is crucial for maintaining the health of the nervous system.

Blood-brain barrier dysfunction has been linked to neurodegenerative diseases, multiple sclerosis and depression. Metabolites produced by the gut microbiome have been shown to affect the permeability of the blood-brain barrier. The microbiome can have both positive and negative effects on the barrier.

The above mentioned short-chain fatty acids support barrier integrity, but unwanted metabolites produced by gut microbes can also have a destructive effect and, for example, increase inflammation. It has been shown that certain probiotic strains can support the body precisely by having a positive effect on the blood-brain barrier. The right balance of microbes and prevention of intestinal dysbiosis is crucial for this very reason as well.

Microbiome vs. stress and cortisol

Cortisol, also known as the stress hormone, is secreted in the adrenal glands. Its production depends on the work of the hypothalamic-pituitary-adrenal axis (HPA axis). Gut bacteria can affect adrenal activity and directly regulate cortisol secretion. The microbiome also has an indirect effect on stress hormone levels – it affects the sensitivity of the pituitary gland and hypothalamus, which form an interconnected system of vessels with the adrenal glands. This occurs through effects on the intestinal barrier, various interactions with the immune system and the production of short-chain fatty acids.

An interesting fact: Studies conducted with germ-free mice, i.e. mice which have been stripped of their gut microbiome, indicate that a lack of gut microbiota is associated with increased stress reactivity and higher cortisol levels.

Some isolated studies have found that probiotic supplementation with Lactobacillus rhamnosus in humans may help lower blood cortisol levels.

How to support the microbiome to feel better?

You already know that the gut microbiome is really important for many systems in your body, including the nervous system and its main organ – the brain. Taking care of the proper composition of the gut microbiome is an indispensable part of supporting the health of the whole body and taking care of your well-being. Here are some changes worth making in your lifestyle to promote a healthy microbiome, starting with the most essential one:

1. The right amount of dietary fiber

Dietary fiber is the basic nourishment for intestinal bacteria. Getting enough fiber regularly is the most important step toward a healthy and diverse microbiome. The minimum recommended daily intake of dietary fiber is 25 g. However, Poles still consume too little of it (the median is about 15 g per day).

To increase your dietary fiber supply, use complex sources of carbohydrates (e.g., choose wholemeal cereals instead of refined ones). Eat vegetables and/or fruits with every meal. For optimal support of intestinal microbes, daily fiber intake can be increased up to 40-50 g (if this does not cause intestinal discomfort).

Read also: Diet for the Gut or for a Healthy Gut – Which Is Better for Athletes?

2. A varied diet rich in plant products

To promote a diverse (healthy!) gut microbiome, food variety is also important, with a special emphasis on plant-based food. It is recommended to consume at least 40 different plant products every week. The more varied the diet, the more microbes will find something to their liking in the food scraps. That’s another reason not to choose monotrophic diets, extremely low-carbohydrate diets or non-nutritional diet plans based on macronutrient counting alone.

Dietitian’s tip: An excellent way to increase the variety of plant foods is to make it a habit to eat a variety of salads every day. Mix some vegetables and fruit, enhance the flavor with herbs and sprinkle with seeds, grains or nuts. You can consume up to 10 different plant foods in one meal.

3. Natural probiotics and prebiotics

If you already have sufficient fiber intake, you can support your gut health and microbiome with natural probiotics and prebiotics. Natural probiotics are products that are sources of healthy bacteria. They include pickles, fermented dairy products (kefir, buttermilk, yogurt) and kombucha.

Natural prebiotics, or foods that probiotics “feed on” include, for example, inulin, resistant starch or various fiber fractions. Their excellent sources are chicory, artichokes, asparagus, onions, garlic, cooled starchy foods (cold potatoes, cold groats or cooled rice).

4. Taking antibiotics under medical supervision

Antibiotic therapy undoubtedly affects the gut microbiome, so any antibiotic intake should be justified. Rash intake of antibiotics is a simple path to intestinal dysbiosis and many complications. Make sure you only take antibiotics under medical supervision and continue the treatment for as long as your doctor indicates.

5. Avoiding excessive hygiene

According to many studies, close contact with nature, especially during childhood, helps to build a healthy and strong microbiome. Excessive hygiene and sterile living conditions have the opposite effect.

Relaxing in nature, gardening or eating outdoors (while observing food safety rules) is also a good way to strengthen the diversity of the microbiome.

6. Appropriate probiotic therapy

Appropriate probiotic therapy can also strengthen the gut microbiota, but it is listed last for a reason. The steps mentioned above are much more important. Probiotic therapy is not a substitute for a varied diet or adequate fiber supply, and diet alone can support the microbiome without probiotic therapy.

Probiotics which are beneficial for mental health, brain function or stress management are called psychobiotics. The most promising strands are:

• Lactobacillus acidophillus,
• Lactobacillus rhamnosus,
• Lactobacillus helveticus,
• Lactobacillus reuteri,
• Lactococcus lactis,
• Bifidobacterium longum,
• Bifidobacterium infantis,
• Streptococcus thermophilus,
• Bifidobacterium animalis.

However, the selection of a probiotic strain should always be made based on your individual needs. There are no “one size fits all” probiotics which will be perfect for everyone. Be aware that good probiotics are often relatively expensive, and those improperly selected will not help your health and may be a waste of money.

_References:

• _{Appleton J., The Gut-Brain Axis: Influence of Microbiota on Mood and Mental Health, Integr Med (Encinitas), 2018 Aug;17(4):28-32, PMID: 31043907, PMCID: PMC6469458.}
• _{Carabotti M., Scirocco A., Maselli M. A., Severi C., The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems, Ann Gastroenterol. 2015 Apr-Jun;28(2):203-209, PMID: 25830558, PMCID: PMC4367209.}
• _{Mansuy-Aubert V., Ravussin Y., Short chain fatty acids: the messengers from down below, Front Neurosci, 2023 Jul 6;17:1197759, doi: 10.3389/fnins.2023.1197759, PMID: 37483350, PMCID: PMC10359501.}
• _{Evrensel A., Ünsalver B. Ö., Ceylan M. E., Psychobiotics, Adv Exp Med Biol., 2019, 1192:565-581, doi: 10.1007/978-981-32-9721-0_28, PMID: 31705514.}
• _{Casertano M., Fogliano V., Ercolini D., Psychobiotics, gut microbiota and fermented foods can help preserving mental health, Food Res Int., 2022 Feb,152:110892, doi: 10.1016/j.foodres.2021.110892, Epub 2021 Dec 22, PMID: 35181072.}