Human gut microbiota: what science says

The human gut microbiota is the collection of microorganisms — bacteria, viruses, fungi and archaea — that colonize your digestive tract and directly influence your overall health.

According to Inserm( 2023)your gut is home to approximately 100 trillion microorganisms [1], ten times more than the number of human cells in your body.

• It represents an ecosystem of more than 1,000 different bacterial species.
• It weighs between 1.5 and 2 kg in a healthy adult.
• It conditions 70% of the body's immune cells.
• It produces short-chain fatty acids that are essential for the intestinal lining.
• It communicates with the brain via the gut-brain axis.

1. Composition of the intestinal flora: what lives in your gut?

The intestinal flora is dominated by two major bacterial phyla: Firmicutes and Bacteroidetes, which together represent more than 90% of the bacteria present. Next come Actinobacteria, Proteobacteria , and Verrucomicrobia.

Each individual possesses a unique microbial signature, comparable to a fingerprint. This composition varies according to age, diet, geography, and lifestyle.

• Bacteria: major component, fermentation and immune role.
• Viruses (viromes): regulate bacterial populations.
• Fungi (mycobiome): of Candida is normal.
• Archaea: produce methane during fermentation.
• Commensal parasites: present without causing disease in the healthy host.

2. Essential roles of the human gut microbiota for your health

The gut microbiota performs essential metabolic, immune, and neurological functions. Without it, the digestion of dietary fiber would be impossible, and the intestinal barrier would rapidly weaken.

A study published in Nature Reviews Microbiology (2022) shows that gut bacteria synthesize essential vitamins B and K [2].

• Digestion: fermentation of fibers, production of short-chain fatty acids (SCFAs).
• Immunity: education of T and B lymphocytes, production of secretory IgA.
• Protection: barrier effect against pathogens through competition.
• Metabolism: regulation of blood glucose and fat storage.
• Neurology: 90% of the body's serotonin is produced via the gut-brain axis.
• Vitamin synthesis: vitamins B2, B9, B12 and K2.

Many readers tell us that understanding these roles has motivated them to make lasting changes to their diet. Discover the link between fatigue and digestion to learn more.

3. Intestinal dysbiosis: when the microbial balance is disrupted

Dysbiosis occurs when the diversity or abundance of intestinal microorganisms is disrupted. It is associated with many chronic diseases.

ANSES ( 2023 ) identifies several main triggering factors.

• Repeated or broad-spectrum antibiotic therapies.
• Ultra-processed food low in fiber.
• Chronic stress and lack of sleep.
• Prolonged sedentary lifestyle.
• Excessive alcohol consumption.
• Acute gastrointestinal infections.

The consequences of dysbiosis include IBS, inflammatory bowel disease (IBD), obesity, type 2 diabetes, and certain mood disorders.

4. Diet and gut ecosystem: the foods that feed your bacteria

Dietary fiber is the preferred fuel for beneficial bacteria. Prebiotics —inulin, fructo-oligosaccharides, pectins—stimulate the growth of Bifidobacterium and Lactobacillus.

Blond psyllium is one of the best-documented prebiotics for supporting gut flora.

5. Probiotics and prebiotics: how do they act on the intestinal flora?

Probiotics are live microorganisms which, when administered in sufficient quantities, have a beneficial effect on health. The EFSA strictly regulates health claims made about them.

A meta-analysis published in The Lancet Gastroenterology & Hepatology (2021) confirms the efficacy of Lactobacillus rhamnosus GG and Saccharomyces boulardii in the prevention of antibiotic-associated diarrhea [3].

• Dietary probiotics: yogurt, kefir, miso, tempeh, raw sauerkraut.
• Probiotic capsules: choose identified strains (genus, species, strain) and a concentration ≥ 10⁹ CFU.
• Prebiotics: non-digestible fibers that selectively nourish beneficial bacteria.
• Symbiotics: combination of probiotics + prebiotics in the same product.

To learn everything about probiotics and how they work, consult our dedicated guide.

6. Gut-brain axis: your gut, your second brain

The gut-brain axis is a bidirectional network connecting the intestinal enteric nervous system to the brain via the vagus nerve, the immune system, and microbial metabolites. The gut microbiota directly influences mood, anxiety, and cognitive function.

Research from INRAE​​( 2022)shows that certain intestinal bacteria produce GABA and serotonin, directly modulating emotional state.

• 90% of the body's serotonin is produced in the gut.
• The vagus nerve transmits microbial signals to the brain in real time.
• Dysbiosis is associated with an increased risk of depression and anxiety.
• Psychobiotics (probiotics with psychological effects) are the subject of active clinical studies.

7. Immunity and the human gut microbiota: an inseparable link

Approximately 70% of immune cells reside in the intestinal lining, in direct contact with commensal bacteria. The gut microbiota educates the immune system from birth and throughout life.

Bacteria produce short-chain fatty acids—butyrate, propionate, acetate—that strengthen the epithelial barrier and reduce systemic inflammation. A depleted gut microbiota is associated with increased intestinal permeability (leaky gut), a risk factor for autoimmune diseases.

Understanding what a probiotic is will help you choose the right supplement to support your immunity.

8. Factors that alter the composition of your gut ecosystem

The composition of the intestinal flora is not fixed. It is constantly evolving under the influence of internal and external factors.

• Mode of delivery: babies born vaginally inherit the mother's more diverse vaginal microbiota.
• Breastfeeding: promotes Bifidobacterium thanks to the oligosaccharides in human milk.
• Diet: the most powerful modifying factor in adults.
• Physical activity: Regular exercise increases microbial diversity.
• Antibiotics: can eliminate 30% of bacterial species in a few days.
• Medications: proton pump inhibitors, metformin, NSAIDs alter the flora.
• Chronic stress: alters intestinal permeability and bacterial composition.
• Aging: progressive reduction in diversity after age 65.

9. Diseases associated with an imbalance of the gut flora

Research has established strong correlations between an impoverished gut microbiota and numerous chronic diseases. These associations do not always imply direct causality, but they point to promising therapeutic avenues.

• Obesity and metabolic syndrome: high Firmicutes/Bacteroidetes ratio.
• Type 2 diabetes: reduction of butyrate-producing bacteria.
• IBD (Crohn's, ulcerative colitis): severe dysbiosis with chronic inflammation.
• Irritable bowel syndrome: disturbance of visceral motility and sensitivity.
• Depression and anxiety: via the gut-brain axis.
• Autoimmune diseases: rheumatoid arthritis, multiple sclerosis.
• Allergies and asthma: a hygiene hypothesis linked to early impoverishment.

10. Fecal microbiota transplantation: the therapy of the future

FMT involves transferring the intestinal flora from a healthy donor to a sick recipient. It has a success rate of over 90% in recurrent Clostridioides difficile infections .

A study published in the New England Journal of Medicine (2023) colitis C. difficile antibiotic-resistant. Clinical trials are exploring its use in obesity, diabetes, and IBD.

11. 7 practical habits to take care of your gut flora

Taking care of your gut ecosystem doesn't require expensive supplements. The simplest daily actions have the most lasting effects.

1. Eat 30 different plant foods per week — vegetables, fruits, legumes, whole grains, herbs.
2. Consume fermented foods daily — yogurt, kefir, sauerkraut, kimchi, miso.
3. Aim for 25-30g of fiber per day — target recommended byANSES.
4. Engage in 30 minutes of moderate physical activity — brisk walking, cycling, swimming.
5. Sleep 7 to 9 hours per night — lack of sleep disrupts bacterial composition.
6. Reduce ultra-processed foods — artificial emulsifiers and sweeteners damage the mucous membrane.
7. Manage stress actively — meditation, heart coherence, yoga reduce intestinal cortisol.

12. Human gut microbiota in children: a critical window

The first three years of life constitute a crucial programming window for the gut microbiota. Microbial colonization begins at birth and stabilizes around the age of 3.

A study by thePasteur Institute (2021) shows that early and varied dietary diversification is the main determinant of a rich and resilient gut flora [5].

• Breastfeeding: enriches the flora with Bifidobacterium longum.
• Introduction of solids at 6 months: gradual diversification is essential.
• Avoid non-essential antibiotics before the age of 2.
• Exposure to nature and animals: enhances microbial diversity.

13. Current research: what science is still exploring

Metagenomics—the sequencing of total microbial DNA—is revolutionizing our understanding of the gut ecosystem. Projects like the Human Microbiome Project and the European MetaHIT have mapped thousands of previously unknown species.

• Role of Akkermansia muciniphila in resistance to obesity.
• Psychobiotics and the treatment of treatment-resistant depression.
• Microbiome and response to cancer immunotherapies.
• Influence of gut flora on longevity and healthy aging.
• Microbial modulation in the prevention of neurodegenerative diseases.

Our comprehensive guide to probiotics and the gut microbiota details the latest clinical advances.

14. Dietary supplements and gut flora: what is proven

The probiotics market is worth over $60 billion globally. But not all supplements are created equal. Here's what science actually backs up.

• Lactobacillus rhamnosus GG: effective against traveler's diarrhea and in combination with antibiotics.
• Saccharomyces boulardii: probiotic yeast, effective in infectious diarrhea.
• Bifidobacterium longum: reduces the symptoms of irritable bowel syndrome.
• Inulin and FOS: prebiotics validated to stimulate bifidobacteria.
• Blond psyllium: prebiotic mucilaginous fiber, improves transit and blood sugar.

Warning: Supplements are not a substitute for a varied diet. Consult a healthcare professional before starting any prolonged course of treatment.

Frequently asked questions about the human gut microbiota

What exactly is the human gut microbiota?

It is the collection of microorganisms — bacteria, viruses, fungi, archaea — that populate your digestive tract. It contains more than 100 trillion individuals and more than 1,000 different bacterial species.

What is the difference between microbiota and microbiome?

The microbiota refers to the microorganisms themselves. The microbiome also includes their collective genetic material. The two terms are often used interchangeably in popular literature.

How can I tell if my gut flora is unbalanced?

Signs of dysbiosis include frequent bloating, irregular bowel movements, chronic fatigue, recurrent infections, mood swings, and unusual food reactions. A fecal metagenomics test can map your gut microbiota.

Are probiotic capsules really effective?

Some strains are clinically validated for specific indications. Choose products that specify the genus, species, and strain, with a concentration ≥ 10⁹ CFU per dose.

How long does it take to restore gut flora after a course of antibiotics?

Partial recovery is observed within 4 to 6 weeks. Some species may take 6 months to 2 years to recover. A diet rich in fiber and fermented foods accelerates recovery.

Does stress really destroy gut bacteria?

Yes. Cortisol released during chronic stress alters intestinal permeability and changes bacterial composition. The hypothalamic-pituitary-adrenal axis communicates directly with the gut via the vagus nerve.

Is it possible to improve one's human gut microbiota without supplements?

Absolutely. Diet is the most powerful lever. Thirty different vegetables per week, daily legumes, and fermented foods are enough to significantly enrich microbial diversity in a few weeks.

Does the human gut microbiota influence weight?

Yes. Studies show that obese individuals have a higher Firmicutes/Bacteroidetes ratio. Transplanting microbiota from obese mice to sterile mice induces weight gain, even without dietary changes.

Which foods are the worst for gut flora?

Ultra-processed foods containing emulsifiers (polysorbate 80, carrageenans), artificial sweeteners (aspartame, saccharin) and excess alcohol are the most detrimental to microbial diversity.

Does the human gut microbiota change with age?

Yes. Microbial diversity gradually decreases after age 65. Older people have less Bifidobacterium and more pro-inflammatory bacteria, which contributes to inflammaging.

Is fecal microbiota transplantation available in France?

FMT is authorized in France for recurrent Clostridioides difficile. It is performed in specialized hospital centers. Its use for other indications remains experimental.

How does the human gut microbiota influence immunity?

Commensal bacteria educate lymphocytes, stimulate the production ofIgA , and strengthen the epithelial barrier. A diverse gut microbiota reduces the risk of allergies, infections, and autoimmune diseases.

Key takeaways

The human gut microbiota is an organ in its own right, and its health influences the health of your entire body. Diversifying your plant-based diet, incorporating fermented foods, and limiting disruptors such as non-essential antibiotics are the most effective and best-documented actions.

Start today: add a fermented food to your next meal and aim for five different plant-based foods throughout the day. These small actions, repeated daily, will permanently transform your gut ecosystem.

Sources and studies

1. Gut microbiota (intestinal flora) — Inserm, 2023
2. The gut microbiome: what we know now and what we need to know — Nature Reviews Microbiology, 2022
3. Probiotics for the prevention of antibiotic-associated diarrhea — The Lancet Gastroenterology & Hepatology, 2021
4. Fecal microbiota transplantation for recurrent Clostridioides difficile infection — New England Journal of Medicine, 2023
5. Early-life gut microbiome and its impact on childhood health — Cell Host & Microbe, 2021