Gut microbiota: a complete guide to everything you need to know

Your gut is home to approximately 38 trillion microorganisms: this is your gut microbiota, and it influences far more than just your digestion. Immunity, mood, weight, skin—science confirms it: taking care of your gut flora means taking care of your overall health. This comprehensive guide explains what the microbiota is, where it is located, how it functions, why it becomes unbalanced, and above all, how to effectively strengthen it through diet and appropriate supplements. Whether you suffer from digestive issues, a weakened immune system, or simply want to optimize your well-being, you will find concrete answers here, based on the latest scientific data.

What is the gut microbiota?

The gut microbiota, long known as gut flora, refers to all the microorganisms living in your digestive tract: bacteria, viruses, fungi, yeasts, and archaea. Their number is estimated at 38 trillion, which is as many—or even more—than the number of human cells in your body. Their total weight is approximately 2 kilograms.

These microorganisms are not mere passengers. They form a complex ecosystem, unique to each individual like a fingerprint, shaped from birth by the mode of delivery, breastfeeding, diet, and environment. Today, more than 1,000 different bacterial species have been identified in the human gut, but each person hosts only a fraction of them—between 160 and 200 species on average.

The two major dominant phyla are Firmicutes and Bacteroidetes, which together represent more than 90% of the gut bacterial population. The balance between these populations—and their overall diversity—is a key marker of your health.

The gut microbiota is not static: it evolves throughout life, influenced by your dietary choices, stress levels, medications, and lifestyle. It is precisely this plasticity that makes it a major therapeutic and preventative tool.

To learn more:

What is the gut microbiota? A complete guide

Where is the gut microbiota located?

The microbiota is not confined to a single organ. It colonizes the entire digestive tract, from the mouth to the rectum, but its density varies considerably from one segment to another.

The stomach, which is highly acidic (pH between 1 and 3), harbors very few bacteria—primarily Helicobacter pylori in some individuals. The small intestine (duodenum, jejunum, ileum) contains an intermediate population, dominated by lactobacilli and streptococci. But it is in the colon that the concentration reaches its maximum: up to 10¹¹ bacteria per milliliter of intestinal contents are found there.

The colon is therefore the true "headquarters" of the microbiota. It is there that bacteria ferment undigested dietary fibers, produce short-chain fatty acids (butyrate, propionate, acetate), synthesize certain vitamins (K, B12, B9) and constantly interact with the immune system via the intestinal mucosa.

This mucous membrane—covering an area of ​​approximately 400 m² if unfolded—is lined with a layer of mucus that serves both as protection and as a communication interface between bacteria and your immune cells. The quality of this intestinal barrier is directly linked to the composition of your microbiota.

To learn more:

Where is the gut microbiota located? A complete guide

The key roles of the gut microbiota

The gut microbiota is much more than just a digestive aid. Research over the last twenty years has revealed the extent of its functions, which affect virtually all of the body's systems.

Digestion and absorption of nutrients

Gut bacteria break down dietary fiber that your digestive enzymes cannot process on their own. This fermentation produces short-chain fatty acids (SCFAs)—including butyrate—which nourish colon cells, strengthen the intestinal lining, and regulate inflammation. The gut microbiota also facilitates the absorption of calcium, magnesium, and iron.

Immunity

Approximately 70% of your immune cells are located in and around the gut. The microbiota "educates" the immune system from birth, teaching it to distinguish pathogens from harmless substances. A diverse microbiota is associated with a better immune response and a reduced risk of autoimmune diseases, allergies, and recurrent infections [1].

Gut-brain axis

The gut and brain communicate constantly via the vagus nerve, the enteric nervous system, and hormonal pathways. The microbiota produces or influences the production of neurotransmitters such as serotonin (90% of your serotonin is synthesized in the gut), GABA, and dopamine. Imbalances in the microbiota have been linked to anxiety, depression, and autism spectrum disorders [2].

Metabolism and weight

The gut microbiota influences your energy metabolism, blood sugar regulation, and fat storage. Studies have shown that transplanting the microbiota from obese mice to lean mice induces weight gain in the latter, even with the same caloric intake. In humans, an impoverished microbiota is associated with an increased risk of obesity, type 2 diabetes, and metabolic syndrome [3].

Skin health

The gut-skin axis is increasingly well-documented. An imbalance in the gut microbiota is correlated with skin conditions such as acne, eczema, psoriasis, and rosacea. Short-chain fatty acids (SCFAs) produced by gut bacteria modulate systemic inflammation, which manifests itself particularly in the skin.

Vitamin synthesis

Your gut bacteria synthesize vitamin K2 (essential for blood clotting and bone health), vitamin B12, vitamin B9 (folate), and vitamin B7 (biotin). These endogenous sources complement—but do not replace—dietary intake.

Dysbiosis: when the microbiota becomes unbalanced

Dysbiosis occurs when the balance of the gut microbiota is disrupted: reduced bacterial diversity, proliferation of certain pathogenic species at the expense of beneficial species, or impaired intestinal barrier function. Dysbiosis is not a disease in itself, but a condition that promotes the development of numerous pathologies.

The main causes of dysbiosis

• Antibiotics: they eliminate pathogenic bacteria but also beneficial bacteria. A course of antibiotics can reduce the diversity of the microbiota by 30 to 50%, with effects that sometimes persist for several months.
• Ultra-processed food: low in fiber, high in refined sugars and additives, it deprives beneficial bacteria of their fermentation substrate and promotes pro-inflammatory species.
• Chronic stress: via the gut-brain axis, stress alters intestinal motility, mucus composition, and intestinal barrier permeability.
• Lack of sleep: the microbiota follows a circadian rhythm. Nights that are too short or irregular disrupt its composition and diversity.
• Sedentary lifestyle: Regular physical activity is associated with greater microbial diversity. Athletes, on average, have a richer microbiota than sedentary individuals.
• Alcohol and tobacco: both damage the intestinal lining and adversely alter the composition of the microbiota.

The consequences of dysbiosis

Prolonged dysbiosis may promote: irritable bowel syndrome (IBS), chronic inflammatory bowel diseases (IBD — Crohn's, ulcerative colitis), obesity, type 2 diabetes, allergies, cardiovascular diseases, depression and anxiety, and certain colorectal cancers [4].

The good news: the microbiota is adaptable. Dietary and lifestyle changes can reshape it in a few weeks, or even a few days for some parameters.

Diet and the gut microbiota: what to eat

Diet is the most powerful lever for modulating your gut microbiota. What you eat directly determines which bacterial species thrive in your colon.

Fiber: fuel for good bacteria

Dietary fibers—and more specifically prebiotic fibers (inulin, FOS, pectins, beta-glucans)—are the preferred fermentation substrate for beneficial bacteria such as Bifidobacterium and Lactobacillus. Their fermentation produces butyrate, which nourishes colonocytes and reduces inflammation.

Rich sources of prebiotic fiber include garlic, onion, leek, Jerusalem artichoke, chicory, green banana, asparagus, oats, and barley. ANSES recommends 25 to 30 g of fiber per day for an adult — the majority of French people consume only 18 g [5].

Fermented foods: a direct source of live bacteria

Yogurt, kefir, kombucha, raw sauerkraut, kimchi, miso, tempeh—these foods provide live bacteria that temporarily enrich your gut microbiota and boost microbial diversity. A Stanford study (2021) showed that a diet rich in fermented foods increased gut microbiota diversity and reduced markers of systemic inflammation more effectively than a high-fiber diet alone.

Polyphenols: Unsung Allies

Polyphenols (flavonoids, resveratrol, curcumin, lignans) are plant compounds that nourish certain beneficial bacteria and inhibit pathogens. They are found in berries, green tea, cocoa, extra-virgin olive oil, legumes, and spices. Their metabolism by the gut microbiota produces metabolites with anti-inflammatory and antioxidant effects.

What needs to be reduced

• Refined sugars and ultra-processed products (additives, emulsifiers such as carboxymethylcellulose)
• Excess saturated fats (fatty red meats, processed meats)
• Alcohol (even in moderate quantities, it damages the intestinal lining)
• Artificial sweeteners (saccharin, sucralose) — studies suggest they disrupt the composition of the gut microbiota

The Mediterranean diet: the benchmark

The Mediterranean diet—rich in vegetables, legumes, whole grains, olive oil, oily fish, and fruit—is the best-documented dietary model for promoting a diverse and healthy gut microbiota. It combines fiber, polyphenols, omega-3 fatty acids, and fermented foods in optimal proportions.

Probiotics and the gut microbiota

Probiotics are live microorganisms which, when administered in sufficient quantities, exert a beneficial effect on the host's health. They act by temporarily colonizing the gut, competing with pathogens, strengthening the intestinal barrier, and modulating the immune response.

The most studied strains

• Lactobacillus acidophilus: improves lactose digestion, reduces antibiotic-associated diarrhea
• Lactobacillus rhamnosus GG: one of the best-documented strains, effective against infectious diarrhea in children
• Bifidobacterium longum: reduces anxiety, improves sleep quality, supports immunity
• Bifidobacterium infantis: particularly important in infants
• Saccharomyces boulardii: a yeast (not a bacterium) effective against traveler's diarrhea and Clostridioides difficile colitis
• Lactobacillus plantarum: reduces the symptoms of irritable bowel syndrome

Effectiveness: what the research says

The effectiveness of probiotics is strain-specific and indication-specific. What works for post-antibiotic diarrhea will not necessarily work for anxiety or weight loss. The strongest evidence supports their use in: the prevention and treatment of antibiotic-associated diarrhea, irritable bowel syndrome, the prevention of vaginal yeast infections, and immune support during the winter months.

How to choose the right probiotics

Three essential criteria: the strain (genus + species + strain number), the dose (expressed in CFU — colony forming units — generally between 1 and 100 billion per dose), and survival through gastric transit (gastro-resistant capsules or strains naturally resistant to acidity).

To learn more:

Probiotic gut microbiota: a complete guide 2026

Dietary supplements for the gut microbiota

Beyond probiotics, several types of dietary supplements can support your gut microbiota. Here's an overview of the best-documented options.

Prebiotics

Prebiotics are indigestible fibers that selectively nourish beneficial bacteria. The most studied are inulin, fructooligosaccharides (FOS), galactooligosaccharides (GOS), and pectin. As dietary supplements, they are available in powder form (chicory inulin, psyllium) or capsules. They are particularly useful for increasing populations of Bifidobacterium and Lactobacillus.

Symbiotics

A synbiotic combines probiotics and prebiotics in a single formula. The idea is to provide both the beneficial bacteria and their fermentation substrate to maximize their colonization and activity. Symbiotic formulas show promising results in managing irritable bowel syndrome and reducing intestinal inflammation.

Glutamine

L-glutamine is the most abundant amino acid in the blood and the primary fuel for enterocytes (cells of the intestinal wall). Glutamine supplementation may help strengthen the intestinal barrier, reduce intestinal permeability ("leaky gut"), and accelerate recovery after severe dysbiosis or digestive surgery.

Omega-3s

Omega-3 fatty acids (EPA and DHA) have well-documented anti-inflammatory effects and favorably influence the composition of the gut microbiota, notably by increasing Bifidobacterium and reducing pro-inflammatory bacteria. Supplementation with high-quality fish oil can complement a diet insufficient in oily fish.

Berberine

Berberine, extracted from plants such as barberry, has modulating effects on the gut microbiota: it increases butyrate-producing bacteria and reduces pathogens. It is also being studied for its effects on blood glucose and lipid metabolism, partly via its effects on the microbiota.

How to choose a good supplement for the gut microbiota

Choose products with clearly identified strains (genus, species, strain number), a guaranteed CFU dose until the expiration date (not just at the time of manufacture), and published clinical studies. Be wary of proprietary formulas that do not detail their composition.

To learn more:

Gut microbiota dietary supplement: a complete guide

What science says about the human microbiota

Research on the human gut microbiota has accelerated dramatically since the launch of the Human Microbiome Project in 2008. Metagenomic sequencing techniques have made it possible to characterize the composition of the microbiota with unprecedented precision, paving the way for a systemic understanding of its role in human health.

Major recent advances

Fecal microbiota transplantation (FMT): This technique, which involves transferring the microbiota from a healthy donor to a sick recipient, has revolutionized the treatment of recurrent Clostridioides difficile, with cure rates exceeding 90%. Clinical trials are exploring its potential in the treatment of Crohn's disease, obesity, depression, and even some cancers.

Microbiota and cancer immunotherapy: several studies published in Science and Nature have shown that the composition of the microbiota influences the response to anti-cancer immunotherapies (checkpoint inhibitors). Patients with a diverse microbiota respond better to treatments.

Psychobiotics: This term refers to probiotics and prebiotics that exert beneficial effects on mental health via the gut-brain axis. Randomized clinical trials show significant effects of certain strains on anxiety, mild to moderate depression, and cognition.

Microbiota and longevity: studies on healthy centenarians reveal a microbiota particularly rich in butyrate-producing bacteria and rare species. Microbial diversity appears to be a biomarker of healthy aging.

The current limitations of research

Despite these advances, several points of concern remain. Causality is often difficult to establish: is an altered microbiota the cause or the consequence of a disease? Studies are often conducted on small samples, using heterogeneous methodologies. Finally, the interindividual variability of the microbiota is such that universal recommendations remain limited—microbiomic precision medicine is still under development.

To learn more:

Human gut microbiota: what science says

Key takeaways

The gut microbiota is a living, complex ecosystem that is essential to your health. Here are the key points to remember:

• It contains approximately 38 trillion microorganisms, concentrated mainly in the colon.
• It plays a central role in digestion, immunity, mental health, metabolism, and skin health.
• Dysbiosis — an imbalance of the microbiota — can be caused by antibiotics, a diet low in fiber, chronic stress, or lack of sleep.
• Diet is the most powerful lever: increase prebiotic fibers, fermented foods and polyphenols; reduce refined and ultra-processed sugars.
• Probiotics and dietary supplements can support the microbiota, provided that scientifically validated strains and doses are chosen.
• Research is progressing rapidly: fecal transplantation, psychobiotics, microbiota and cancer — the microbiome is at the heart of tomorrow's medicine.

Concrete action: start today by adding a serving of fermented food (plain yogurt, kefir, or raw sauerkraut) to your daily diet, and gradually increase your fiber intake by 5g per week until you reach 25-30g per day. These two simple steps are enough to initiate a positive transformation of your gut microbiota in less than two weeks.

Sources

1. Belkaid Y, Hand TW. Role of the microbiota in immunity and inflammation. Cell. 2014;157(1):121-141. Available at: pubmed.ncbi.nlm.nih.gov/24679531
2. Cryan JF, et al. The Microbiota-Gut-Brain Axis. Physiological Reviews. 2019;99(4):1877-2013. Available at: pubmed.ncbi.nlm.nih.gov/31460832
3. Turnbaugh PJ, et al. An obesity-associated gut microbiome with increased capacity for energy harvest. Nature. 2006;444:1027-1031. Available at: pubmed.ncbi.nlm.nih.gov/17183312
4. Shreiner AB, Kao JY, Young VB. The gut microbiome in health and in disease. Current Opinion in Gastroenterology. 2015;31(1):69-75. Available at: pubmed.ncbi.nlm.nih.gov/25394236
5. ANSES. Update of the PNNS guidelines — Revision of dietary intake guidelines for adults. 2017. Available at: anses.fr