The intestinal microbiota: a major player in our health

Recently, the gut and the intestinal microbiota have experienced a resurgence of interest due to their influence on the functioning of the body and on the health status of individuals. In this article, we will discuss the basics of the functioning of the gut and microbiota as well as the consequences of changes in the latter on the health status of individuals. We will also address how diet and physical exercise can influence the functioning of the intestinal microbiota.

I. The gut: a crucial organ in the functioning of the body

The gut is an organ whose function is vital for the human body as it is responsible for the absorption of various nutrients used for the functioning of the body, thanks in part to its particular anatomy. Indeed, the gut is a tube whose walls are composed of:

• Blood vessels and lymphatic vessels to transport nutrients to other organs
• Muscle cells to move the intestine
• Immune system cells to protect the body from the entry of pathogens, such as bacteria or viruses.

It also contains neurons forming the enteric nervous system, which are numerous enough to be termed the “second brain,” whose main role is to control the movements of the intestine and the activity of the immune cells in the intestine².

Several intestinal pathologies, including irritable bowel syndrome and inflammatory bowel diseases, result from a dysfunction of the enteric nervous system and immune system cells. The main symptoms of these pathologies are abdominal pain, diarrhea or constipation, as well as bloating and nausea. Recent research indicates that the intestinal microbiota may also be involved in the development of these pathologies³.

II. The Intestinal Microbiota

The intestinal microbiota refers to the community of bacteria that live in symbiosis with the cells of our intestine. Unlike other bacteria, they are not pathogenic, i.e., they are not responsible for infections, and they are even essential for the normal functioning of our intestine and body. The microbiota population is complex and includes a large number of bacterial species, some of which are not yet known and some are more prevalent than others, such as Bacteroidetes and Firmicutes, for example.

The diversity of bacterial species within the microbiota is a crucial element for individual health, and it has been observed that low bacterial diversity is responsible for systemic inflammation, contributing to the pathophysiology of many chronic diseases, including brain diseases⁴ as well as cardiovascular and/or metabolic diseases^5,6. Moreover, maintaining a significant diversity of bacteria within the microbiota appears to allow healthy aging associated with a decrease in risks related to frailty⁷

Given that the intestine serves as a protective barrier to prevent the entry of pathogens into the human body, it is legitimate to wonder how it is possible that the microbiota can have such a significant influence on systemic pathologies. The bacteria of the microbiota are not metabolically inactive; they produce molecules that directly interact with the intestinal cells to influence their activity. Among these molecules, some are neurotransmitters that bind to neurons of the enteric nervous system, which in response modify their activity and thus that of the autonomic nervous system and the central nervous system. These interactions are not trivial, as it has been demonstrated that an alteration in the activity of the microbiota is responsible for mood changes, increased stress and anxiety, and a decrease in cognitive abilities³. Additionally, the bacteria of the microbiota are also capable of digesting dietary fibers, producing molecules that are beneficial to the body, including short-chain fatty acids (SCFAs) involved in intestinal function⁸, immune system activity⁹, regulation of blood pressure¹⁰, and the circadian rhythm¹¹.

Under certain conditions, the population of the microbiota can change and alter the intestinal environment, resulting in a state called dysbiosis, which is characterized notably by an increase in the permeability of the intestinal wall. Consequently, numerous pathogenic molecules will cross the wall and enter the body, activating immune cells located in the intestine. Furthermore, some of these molecules also travel into the bloodstream and activate immune cells contained within the blood. As during an infection, these activated immune cells secrete pro-inflammatory molecules, creating low-grade systemic inflammation. Inflammation is a normal process when it is elevated transiently. However, when it is constantly elevated, it becomes deleterious to the body and participates in numerous pathophysiological mechanisms that favor the emergence of many chronic diseases.

Dysbiosis can appear when a marginal population of bacteria takes advantage of a transient and/or lasting change in the intestinal environment to grow compared to the dominant bacterial populations¹². For example, a course of antibiotics or an unbalanced diet are two events that promote the development of bacteria responsible for dysbiosis, whereas under normal conditions, these bacteria would not have the capacity to grow. Indeed, in normal environmental conditions, a portion of the SCFAs produced by the bacteria of the microbiota is oxidized by the intestinal cells, drastically reducing the amount of oxygen present in the intestinal lumen. This makes it completely hypoxic, limiting the development of exclusively anaerobic bacterial populations that produce SCFAs through their metabolism⁹, establishing a state of symbiosis between the intestinal wall and the microbiota. As previously explained, during environmental changes, it is possible that bacteria not producing SCFAs may develop at the expense of pre-existing bacteria. Consequently, the intestinal cells will no longer oxidize SCFAs, and the intestinal tube will again contain oxygen, allowing other bacteria to develop. The absence of SCFAs activates immune cells, which then secrete pro-inflammatory molecules and thus establish low-grade inflammation in the intestine⁹. Gradually, intestinal permeability increases, allowing the entry of pro-inflammatory bacterial molecules into the bloodstream.

It is therefore crucial to maintain a healthy intestinal environment to have the most diverse and healthiest microbiota possible.

III. Modulating the intestinal microbiota through diet and exercise

Several one-time events can alter the environmental conditions of the intestinal lumen, but in the long term, the two most influential factors are diet, particularly prebiotics and probiotics, and physical exercise.

Prebiotics refer to various molecules in food that are used by bacteria for their metabolism and have health benefits¹³. These substances are found in dietary fibers contained in foods with whole flour, fruits, vegetables, and cereals and promote the growth of anaerobic bacteria populations that produce SCFAs¹⁴-¹⁷. A decrease in prebiotic intake thus reduces the bacteria’s ability to produce beneficial molecules for our body, increasing the incidence of inflammatory diseases, obesity, metabolic syndromes, as well as anxiety and chronic stress. Conversely, prebiotic supplementation has numerous benefits, including reduced chronic stress and anxiety, and increased brain plasticity³.

Probiotics, on the other hand, are not molecules contained in foods but are bacteria that provide health benefits when ingested in sufficient quantities¹⁸. They are primarily found in fermented foods, e.g., pickled cabbage like sauerkraut or Korean kimchi, yogurts, and also kefir and kombucha, with effects similar to prebiotics: improvement of metabolism, immunity, and endocrine function as well as a slowdown in cellular aging¹⁹,²⁰.

Therefore, the diet must be adapted to provide enough prebiotics and/or probiotics to the intestinal microbiota for its functioning. In addition to quality, the quantity of food ingested also seems important for maintaining a healthy microbiota as a slight caloric restriction over a short period, or even throughout life, promotes the growth of Lactobacillus-type bacteria which are associated with an increased lifespan²¹. Several studies have thus focused on the consequences of various diets on the microbiota, including the Western diet and the Mediterranean diet:

• The Western diet is characterized by a high intake of animal proteins, simple carbohydrates, and saturated fats, with low dietary fiber content, and has become very widespread in the global population in recent years. It is believed to promote the increase of bacteria from the Proteobacteria species, which are involved in the development of intestinal dysbiosis²².
• The Mediterranean diet is characterized by a diet consisting of cereals, nuts, and a large quantity of vegetables, coupled with fish and meat in smaller quantities. It therefore provides a diet that is predominantly composed of complex carbohydrates, unsaturated fatty acids, and a mix of animal and plant proteins. At the level of the intestinal microbiota, it is believed to promote the emergence of bacteria from the Bacteroides and Clostridium species, which are associated with an anti-inflammatory effect at the systemic level²³.

The nutrient composition of the diet is important for the normal functioning of the body, but it is also crucial for the quality of the intestinal microbiota. Therefore, it is interesting to know the effects of each macronutrient on the environment and the intestinal microbiota:

• Simple carbohydrates (glucose, fructose, and sucrose) promote a change in the microbiota population contributing to pathological alterations of the body²⁴, whereas when they are combined with dietary fibers in complex carbohydrates (starch, etc.), they promote the development of a healthy microbiota³.
• A diet rich in proteins would also contribute to a population of beneficial bacteria for the intestinal microbiota and the body, leading to increased secretion of SCFAs and branched amino acids²⁵. Moreover, it appears that the source of the proteins, whether animal or plant, influences the changes induced by the proteins on the microbiota²⁵. Indeed, a diet rich in plant proteins would promote the expansion of bacteria producing SCFAs, while a diet rich in animal proteins would allow the development of bacteria producing fewer SCFAs but a potentially carcinogenic molecule, TMAO²⁵.
• Similar to proteins, the different types of fatty acids (FAs) contained in lipids seem to have a differentiated effect on the population of the intestinal microbiota. Indeed, saturated FAs would contribute to a composition of the intestinal microbiota that promotes a pro-inflammatory environment and chronic diseases²². In contrast, mono- and polyunsaturated FAs are associated with an intestinal microbiota that has a significant anti-inflammatory effect on the intestinal mucosa and also at the systemic level²⁶,²⁷.

Consequently, it appears that a diet with a high proportion of complex carbohydrates, vegetables/fruits, plant proteins, and mono- and polyunsaturated fatty acids is the most beneficial for the intestinal microbiota.

The second factor influencing the intestinal microbiota is physical exercise, which is already known to have numerous direct benefits on the cardiovascular²⁸ and musculoskeletal systems²⁹. It seems that exercise also promotes the diversity of bacteria within the intestinal microbiota³⁰, regardless of the individuals’ dietary quality and clinical status, i.e., whether healthy or suffering from a chronic pathology. These beneficial effects are observable in the long term and are thus related to moderate or intense physical exercise carried out regularly. This is particularly interesting in populations of patients with chronic pathologies where an improvement in the quality of the intestinal microbiota was associated with an improvement in the clinical status of the patients, reinforcing the utility of physical exercise in the management of these pathologies.

Conclusion

The intestinal microbiota is a major player in individual health due to its roles in regulating intestinal function and the immune system, which are linked to the diversity of the population composing this intestinal microbiota. If this diversity is not maintained over time, the intestinal microbiota becomes a significant source of pro-inflammatory molecules causing local inflammation and contributing to numerous chronic pathologies. Therefore, it is necessary to maintain a high-quality microbiota through diet and physical exercise, two events that greatly influence its composition. According to scientific literature, a diet with a high proportion of complex carbohydrates, vegetables/fruits, plant proteins, and mono- and polyunsaturated fatty acids, as well as regular moderate or intense physical exercise, would be most beneficial for the intestinal microbiota.

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