What to remember:
The intestinal microbiota is composed of billions of microorganisms located mainly in the colon.
It actively participates in digestion : fermentation of fibers, production of microbial metabolites (short-chain fatty acids) and synthesis of nutrients.
It plays a key role in the education and regulation of the immune system.
It protects the body from pathogens thanks to a physical and chemical barrier effect.
An imbalance in the microbiota (dysbiosis) can promote digestive disorders, allergies or even chronic inflammation.
Introduction
Did you know that your gut is home to nearly 100 trillion microorganisms? This vast group, called the intestinal microbiota , plays a key role in the overall functioning of our body. Much more than just a player in digestion, the microbiota actively contributes to the proper balance of the immune system and the protection of the intestinal barrier .
But how does it really influence our daily health? And why is preserving this ecosystem essential?
In this article, we explain the central role of the gut microbiota in digestion and immunity – two closely linked functions, often overlooked, but essential for maintaining good health.
The intestinal microbiota, a key player in digestion
Definition and composition of the intestinal microbiota
The intestinal microbiota, often referred to as our "second brain," refers to the set of microorganisms that colonize our digestive tract. It is primarily composed of bacteria, but also yeasts, viruses, and fungi. Its primary location? The colon, where bacterial density is highest and diversity is richest.
But what exactly is its role in digestion? Much more than a simple passenger, the microbiota acts as a true functional organ, actively participating in the breakdown and assimilation of nutrients.
Fiber digestion and production of short-chain fatty acids (SCFAs)
Gut bacteria break down dietary fiber (which is indigestible by humans) through a fermentation process. This action produces SCFAs, such as butyrate, acetate, and propionate. These SCFAs have several beneficial effects:
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Energy source for colon cells;
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Local and systemic anti-inflammatory effect;
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Support for energy metabolism and glycemic regulation.
Synthesis of certain vitamins
The microbiota also participates in the synthesis of three essential amino acids: valine, leucine and isoleucine, in the synthesis of vitamins, notably certain B vitamins and vitamin K, thus helping to meet the nutritional needs of the body.
Optimizing nutrient absorption
By interacting with intestinal cells (enterocytes), microorganisms strengthen the integrity of the intestinal barrier and promote optimal nutrient absorption. They thus help limit the passage of unwanted molecules into the bloodstream and maintain a "selective" intestine: only the right molecules pass through.
The intestinal microbiota, a pillar of the immune system
Although we now know that the intestinal microbiota actively participates in digestion, its central role in the functioning of the immune system remains largely underestimated by the general public.
A role in educating the immune system
From birth, the gradual colonization of the digestive tract by microorganisms influences the development of the immune system. The microbiota interacts with gut-associated lymphoid tissue (GALT), which represents nearly 70% of the body's immune cells. This ongoing dialogue "educates" the immune system by helping it distinguish "friendly" elements (nutrients, beneficial microorganisms) from pathogens. As a result, the microbiota contributes to the balance between immune tolerance and defensive response.
A diverse and balanced microbiota is therefore essential to prevent the appearance of inappropriate immune responses (allergies, chronic inflammation, autoimmune diseases, etc.).
Barrier effect and protection against pathogens
The microbiota also plays a role as a physical and chemical barrier against pathogens:
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By occupying adhesion sites on the intestinal mucosa and consuming available nutrients, it prevents pathogenic bacteria from settling there (this is called competition).
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Some bacteria even produce natural antimicrobial substances (bacteriocins, organic acids), capable of inhibiting the growth of unwanted microbes.
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By stimulating the production of mucus and antimicrobial peptides by intestinal cells.
This microbial shield therefore actively participates in protecting the body against infections.
Regulation of inflammatory responses
Beyond its barrier function, the microbiota finely modulates immune and inflammatory responses:
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SCFAs derived from the fermentation of dietary fiber act as anti-inflammatory signals regulating local and systemic inflammation, modulating immune cell activity and strengthening the intestinal barrier. This promotes a balanced immune system.
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A healthy microbiota thus helps to maintain controlled inflammation, which is essential to prevent immune defenses from going haywire.
When the microbiota goes haywire: digestive and immune consequences
An unbalanced intestinal microbiota, known as dysbiosis, can have numerous repercussions on health, both digestive and immune.
Dysbiosis and digestive disorders
When the intestinal flora becomes unbalanced (lack of diversity, dominance of pro-inflammatory bacteria), the impact is often direct on digestive comfort:
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Bloating, gas and intestinal discomfort.
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Transit disorders: constipation , diarrhea, or even alternation of the two.
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Possible contribution to irritable bowel syndrome (IBS), where local inflammation, intestinal permeability and excessive fermentation play a key role.
These digestive disorders often reflect a weakened intestinal barrier and unbalanced fermentation of food substrates.
A close link with immune disorders
Dysbiosis also disrupts the regulation of the immune system:
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Increased risk of food or respiratory allergies, linked to poor immune tolerance.
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Promotion of autoimmune diseases (rheumatoid arthritis, Hashimoto's thyroiditis, etc.) via chronic activation of the immune system.
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Contribution to a low-grade inflammatory state, associated with many chronic diseases (obesity, type 2 diabetes, skin disorders, etc.).
In summary, an impoverished or unbalanced microbiota can weaken not only the intestine, but also overall immunity.
Conclusion
The intestinal microbiota is therefore not just a passive player in our health: a true organ in its own right, it directly influences digestion, the intestinal barrier, and the immune system. Its balance determines not only our digestive comfort, but also our ability to defend ourselves against pathogens, avoid excessive inflammatory reactions, and prevent many chronic diseases.
Taking care of your microbiota – through a diet rich in fiber, varied and minimally processed, and by limiting disruptive factors (stress, antibiotics , industrial food, etc.) – is an essential step in preserving your overall health.
In short: a healthy gut means a balanced microbiota, and a balanced microbiota is the key to a balanced body.
Sources :
[1] Inserm, 2021, Intestinal microbiota (intestinal flora) - A serious avenue for understanding the origin of many diseases, consulted from: https://www.inserm.fr/dossier/microbiote-intestinal-flore-intestinale/#comprendre-le-r%C3%B4le-du-microbiote-intestinal
[2] Manon Oliero, 2023, How do short-chain fatty acids take care of our intestine?, consulted from: https://www.gutmicrobiotaforhealth.com/fr/comment-les-acides-gras-a-chaine-courte-prennent-ils-soin-de-notre-intestin/
[3] Pham, N. H T., Joglekar, MV, Wong, W. K M., Nassif, NT, Simpson, AM and Hardikar, AA (2024). Short-chain fatty acids and insulin sensitivity: a systematic review and meta-analysis. Nutritional Reviews, 82(2), 193-209. https://doi.org/10.1093/nutrit/nuad042
[4] Gaboriau-Routhiau, V., & Cerf-Bensussan, N. (2016). Gut microbiota and development of the immune system. Medecine sciences: M/S, 32(11), 961–967. https://doi.org/10.1051/medsci/20163211011
