Gut health: what science says about the microbiota

You have a second brain. It lives in your gut, contains more than 100 trillion microorganisms, produces 90% of your body’s serotonin, and communicates with the brain in your head in real time — influencing your mood, your memory, your immunity, and even your risk of developing Alzheimer’s.

This is not speculation. It is the fastest-growing field of research in medicine over the last 10 years — and the results of 2025 and 2026 are fundamentally changing what we think about health, disease, and well-being.

What is the gut microbiota — and why is it called the “second genome”

Inside your gut live approximately 100 trillion microorganisms — bacteria, viruses, fungi, and archaea — collectively called the gut microbiota. In number, they outnumber the human cells in the body. In genetic diversity, the total genes of these microorganisms are 150 times greater than the complete human genome — hence the nickname “second genome.”

This microbial community is not merely passive or irrelevant. It performs functions that the human body simply cannot do alone:

• Digests fiber and produces short-chain fatty acids (SCFAs) that feed intestinal cells and reduce inflammation
• Produces essential vitamins — including B12, K2, and folate
• Trains and regulates the immune system — 70 to 80% of the body’s immune cells reside in the gut
• Produces neurotransmitters — including more than 90% of all serotonin in the body
• Protects against pathogens by competing for space and resources
• Communicates with the brain through the vagus nerve, the immune system, and metabolites that reach the bloodstream

When this ecosystem is balanced — with good species diversity and a predominance of beneficial bacteria — the result is health. When it is imbalanced — a state called dysbiosis — the consequences go far beyond the gut.

Global numbers: a silent gut health crisis

The comparison: USA, Europe, and Brazil

🇺🇸 United States: the Western world’s worst diet and its gut consequences

The US combines the highest absolute prevalence of gut diseases with the world’s largest digestive health expenditures. About 10–15% of American adults have IBS symptoms, making it one of the most common functional GI disorders. The main culprit, according to experts, is the “Standard American Diet” (SAD) — rich in refined carbohydrates, sugar, trans fats, and ultra-processed foods, and poor in fiber and fermented foods. Researchers at the American Gastroenterological Association identified that chronic stress — widely prevalent in the US — also disrupts the gut-brain axis balance, altering mood, decision-making, and hunger signals.

🇪🇺 Europe: country diversity and research leadership

Europe shows great internal variation. Canada and Germany record the highest DALY rates from IBD among G20 countries, according to a study published in PMC in January 2026. IBS prevalence in Europe varies between 12% and 19% depending on the country. The UK and Spain are among the countries with the greatest burden of functional gastrointestinal disease. University College Cork in Ireland is the world’s leading institution in gut microbiota publications, and Sweden’s University of Gothenburg is a global reference center for IBS research.

🇧🇷 Brazil: among the most affected in South America

Brazil appears among the countries with the highest IBS prevalence in the world, with data placing prevalence between 15% and 20% of the adult population — above the global average. Contributing factors include accelerated nutritional transition — abandoning the traditional fiber-rich diet in favor of ultra-processed foods — chronic stress, frequent antibiotic use, and unequal access to digestive health services.

The gut-brain axis: the discovery that changed everything

The most transformative discovery of modern gastroenterology is that the gut and brain communicate in a bidirectional, continuous, and profound way — through a system called the microbiota-gut-brain axis.

This communication happens through multiple pathways simultaneously:

• Vagus nerve — the main “cable” of communication between gut and brain, transmitting signals in both directions
• Immune system — cytokines and other immune mediators produced in the gut reach the brain and influence mood and cognition
• Microbial metabolites — short-chain fatty acids, tryptophan, and bile acids produced by gut bacteria enter the bloodstream and cross the blood-brain barrier
• Enteric nervous system — the gut has its own nervous system with over 500 million neurons — more than the spinal cord

In January 2026, researchers at Northwestern University published in ScienceDaily the first direct experimental evidence that the gut microbiota helps shape differences in brain function across primate species. By transplanting microbes from different primates into mice, the animals’ brains began to function differently. As researcher Katherine Amato stated: “This study provides more evidence that microbes may causally contribute to these disorders — specifically, that the gut microbiome is shaping brain function during development.”

At Harvard, researchers identified that the bacterium Morganella morganii — found in excess in people with major depression — produces an inflammatory molecule when it interacts with common pollutants. This mechanism helps explain how gut bacteria can directly influence mood and mental health.

Microbiota and disease: the connections science has confirmed

What destroys the microbiota: the enemies of the modern gut

• Ultra-processed foods — poor in fiber and rich in additives that destroy microbial diversity
• Antibiotics — necessary in infections, but highly destructive to the microbiota. A single course can eliminate hundreds of species and take months to recover
• Chronic stress — cortisol and stress hormones alter microbiota composition and increase intestinal permeability (“leaky gut”)
• Sleep deprivation — research shows that even one night of poor sleep measurably alters microbiota composition
• Lack of dietary diversity — each type of plant you eat feeds a different set of bacteria. Eating the same things consistently impoverishes the gut ecosystem
• C-section birth and lack of breastfeeding — initial microbial colonization happens at birth. Cesarean-born, non-breastfed babies have a microbiota with lower diversity

The latest discoveries of 2025–2026

Coffee and microbiota: a good surprise

In May 2026, researchers published in ScienceDaily that both caffeinated and decaffeinated coffee altered gut bacteria in ways linked to better mood and lower stress. Coffee — independently of caffeine — appears to act as a prebiotic for certain beneficial bacteria.

Microbiota and intelligence: the evolutionary connection

Northwestern University researchers published in 2026 the first direct evidence that the gut microbiota may have contributed to the evolution of human brain size — the largest among primates relative to body size.

Gut serotonin and IBS

In March 2026, researchers identified two gut bacteria capable of producing serotonin directly. In mice without serotonin, the microbes boosted levels of the neurotransmitter and improved bowel movements — opening a new therapeutic window for IBS.

Prebiotics and cognition

A twin cohort study published in 2025 showed that a prebiotic intervention of just three months produced measurable improvements in brain function. Since twins share genetics, results point directly to diet’s effect on the microbiota — and the microbiota’s effect on the brain.

1. Dietary diversity — the rule of 30

Researchers from the American Gut Project found that people who consume 30 or more different types of plants per week have significantly more diverse and healthy microbiota than those eating fewer than 10. “Plants” includes fruits, vegetables, greens, grains, legumes, nuts, seeds, and herbs.

2. Fiber — the food of good bacteria

Beneficial gut bacteria feed mainly on fiber — especially fermentable fibers like inulin, pectin, and resistant starch. Oats, unripe banana, garlic, onion, leek, asparagus, apple with skin, and legumes are excellent sources. The short-chain fatty acids produced by fiber fermentation — especially butyrate — feed colon cells, reduce intestinal inflammation, and reach the brain with neuroprotective effects.

3. Fermented foods

Natural yogurt with live cultures, kefir, sauerkraut, kimchi, miso, kombucha, and other fermented foods introduce beneficial bacteria directly into the gut. A Stanford study published in Cell in 2021 showed that a diet rich in fermented foods increased microbiota diversity and reduced inflammation markers in just 10 weeks.

4. Probiotics with scientific evidence

Not all probiotics work for all people. Strains with the strongest evidence for gut health include Lactobacillus rhamnosus, Bifidobacterium longum, Lactobacillus acidophilus, and Bifidobacterium lactis. For IBS, a combination of strains published in the International Journal of Molecular Sciences in 2025 showed a protective effect on intestinal permeability.

5. Reduce ultra-processed foods and sugar

The Mediterranean diet — validated in a randomized clinical trial published in Annals of Internal Medicine in 2025 — is the most studied for IBS and general gut health, with improvements in both gastrointestinal and psychological symptoms.

6. Manage stress

The gut-brain axis works in both directions. Chronic stress harms the microbiota — and an imbalanced microbiota amplifies stress response. Meditation, diaphragmatic breathing, and physical exercise have measurable impact on microbiota composition.

7. Use antibiotics responsibly

Antibiotics are essential when needed — but every use has a real cost to the microbiota. Avoiding unnecessary antibiotics, completing cycles when prescribed, and using probiotics after treatment are evidence-based practices.

The future: fecal transplants and personalized medicine

The most promising field of microbiome medicine in 2026 is fecal microbiota transplantation (FMT) — transferring the microbiota from a healthy donor to a patient with dysbiosis. Approved by the FDA for recurrent Clostridioides difficile infections, FMT is being studied for dozens of other conditions, including IBS, IBD, obesity, depression, and autism.

Another exciting frontier is microbiome-based precision nutrition: the idea that each person responds differently to the same foods — and that knowing your microbiota can enable dietary recommendations with much greater effectiveness than general guidelines.

The information in this article is based on publications from the New England Journal of Medicine, Nature, Frontiers in Psychiatry, Frontiers in Microbiology, The Lancet Gastroenterology & Endocrinology, Gastroenterology, Annals of Internal Medicine, International Journal of Molecular Sciences, Critical Reviews in Oncology, ScienceDaily, Cambridge University Press, and data from the World Health Organization. Referenced research centers include Harvard Medical School, Northwestern University, McMaster University, Mayo Clinic, University College Cork, and the University of Gothenburg. This content is for informational and educational purposes only and does not replace medical consultation or gastroenterology specialist consultation.