It was the 5th century BC when Hippocrates of Cos, father of Western medicine, observing his patients without microscopes or blood tests, came to a conclusion that seemed more philosophy than science: that the origin of many of the body's ailments had to be sought in the womb.

For more than two millennia, the phrase remained a textbook curiosity, an anecdote with which to open popular talks. But in the last decade, and at an accelerated pace during 2025, that intuition has ceased to be a metaphor and has become a field of research with its own name: the microbiota-gut-body axis.

The stomach is often the first “reflex” of disease due to its dense nervous network (called the enteric nervous system) and its connection to the brain. Thus, when the body becomes stressed or sick, it prioritizes survival, altering digestion and causing symptoms such as nausea or pain, as explained in the Gut-Brain Interaction Disorders in the MSD Manuals.

From intuition to molecular mechanism

The change in status of the Hippocratic phrase has a concrete explanation: the mechanisms are now known. These are not just statistical correlations between “poor digestion” and disease, but specific biochemical pathways that scientists can track, measure and, increasingly, intervene on.

A recent example illustrates this well. Researchers have identified that certain intestinal bacteria produce a molecule called imidazole propionate, capable of altering insulin signaling and amplifying inflammatory processes. According to a study published in Nature, this compound not only contributes to the development of atherosclerosis - the hardening of the arteries that precedes heart attacks and strokes - but also functions as an early biomarker capable of detecting cardiovascular risk before the usual clinical indicators appear. In parallel, other teams have described how intestinal microbes regulate the production of bile acids, a mechanism that helps prevent the accumulation of harmful cholesterol and maintain the body's metabolic balance.

That is to say: what happens in a few grams of bacteria housed in the colon may, literally, be deciding the fate of the arteries of the heart.

The intestine that speaks to the heart

If there is an area where the old Hippocratic maxim has gained almost literal relevance, it is that of mental and neurological health. The so-called gut-brain axis—a bidirectional communication path between the central nervous system and the set of microorganisms that inhabit the digestive tract—has become one of the most active areas of current biomedical research.

Recent studies in animal models have shown that certain immune cells in the intestine, known as dendritic cells, are able to migrate to the brain and influence behavior during the early stages of life, when initial colonization by microbes occurs. Other works have described how the microbiota participates in the regulation of circadian rhythms and stress hormones, suggesting that an imbalance in this microbial community could increase vulnerability to anxiety-related disorders. There is even evidence that certain combinations of prebiotic fibers improve cognitive performance in people over 60 years of age, a finding that opens the door to nutritional strategies aimed at preserving brain health during aging.

This bidirectionality—the intestine that informs the brain and the brain that in turn modulates digestive physiology—has led some researchers to describe it as a constant feedback loop, where an alteration at either end has an impact on the other.

Limits of the promise

The science of the microbiome, however, advances with nuances that should not be overlooked, and the field itself has been responsible for putting them on the table. One of the most rigorous clinical trials published last year – double-blind and placebo-controlled – showed that fecal microbiota transplantation failed to induce clinical remission in adult patients with Crohn's disease after eight weeks of treatment, a result that reminds us that transferring bacteria from a healthy intestine to a sick one is not, yet, a magic solution.

Evidence has also emerged that calls for caution in the face of popular enthusiasm for microbiome tests sold directly to consumers: a recent international consensus warned about their limitations and insisted on the need for health professionals to interpret these results with caution, avoiding their indiscriminate use. Similarly, a controlled trial with irritable bowel syndrome patients found that personal expectations and beliefs—not always gluten or wheat themselves—explain much of the symptoms that many people attribute to these foods.

The underlying message is clear: the intestine matters, a lot, but the jump from “the microbiota is involved” to “we know how to cure it by modifying the microbiota” remains, in most cases, a field under construction.

Microbial community with weight

What does seem established is the magnitude of the phenomenon. The human intestine is home to more than a thousand different bacterial species, which together add up to trillions of microorganisms with the capacity to produce signaling molecules, train the immune system and communicate with distant organs. When that ecosystem becomes unbalanced—a phenomenon that scientists call dysbiosis—the documented consequences are no longer limited to the digestive tract: they have been linked to inflammatory bowel diseases, metabolic disorders, nervous system conditions, and even vascular diseases.

In Berlin, a team of basic researchers, clinicians and computer specialists is currently working to build what they call a “spatial atlas” of the intestinal mucosa microbiome, with the aim of specifying where exactly in the intestine inflammation originates and how this affects the function of blood vessels. The ultimate goal, they explain, is to abandon trial-and-error medicine—"why a drug works in some patients and not in others"—to move toward treatments designed based on precise measurements of each person's microbiota.

2,400 years later

Hippocrates did not have genetic sequencers nor did he know about bacteria; He probably spoke of humours, digestion and purges, in the language of a medicine that was just beginning to separate itself from superstition. And yet, the core of his intuition—that the state of the intestine impacts the rest of the body in ways that go far beyond simple digestion—is now remarkably compatible with what contemporary genomics, immunology, and neuroscience show.

The phrase, as it is popularly known, may not even be a verbatim quote from the Greek doctor, something that medical historians themselves debate. But as a summary of a scientific hypothesis under construction, few ancient maxims have proven to have such surprising validity.