The organ that never grows old: a short history of your gut

Twenty-five centuries ago, the Greek physician Hippocrates is said to have declared that all disease begins in the gut. Whether he truly spoke those words, no one knows for certain, the saying is attributed to him, but a firm source is missing. And yet it touches something. The gut has always puzzled us. We have by turns revered it as the seat of the soul and dismissed it as a simple tube that food passes through. The truth, it turns out, is far more beautiful than either.

Because your gut is not a factory. It is a border, a living, breathing borderline between the outside world and you. Let me take you through the centuries, and then show you something most people never learn.

Two thousand years of confusion

In antiquity, people believed health depended on a balance of bodily fluids. Too much ‘black bile’ made you low-spirited. Melancholic, the word we still use for it, literally means ‘black bile’. The belly was seen as the place where moods and illness arose. It was speculation, but not foolish speculation: somehow those ancient physicians sensed a link between what happens in the belly and how we feel.

For centuries it stayed there. The gut was too dark, too deep, too inaccessible to truly understand. People kept choosing one of two extremes: to mystify it or to oversimplify it. Only when we could actually look inside did things begin to shift.

From mystery to microscope

The first breakthrough was anatomical. In 1543 the Flemish anatomist Andreas Vesalius published his famous atlas of the human body and drew the organs more precisely than ever before.¹ But drawing is not yet understanding: how digestion actually worked remained a riddle.

That riddle was cracked open in a bizarre way. In 1822, in North America, a young fur trapper named Alexis St. Martin was accidentally shot in the stomach at close range. He survived, but the wound in his stomach never fully closed. An opening remained, a window straight into his stomach. The army surgeon William Beaumont realised what he had, and between 1825 and 1833 he carried out hundreds of experiments: he lowered pieces of food on a string into the stomach and pulled them out again after a while. For the first time in history, a person could literally watch digestion happen.²

And then the Dutch hero of this story. In 1683 Antonie van Leeuwenhoek of Delft scraped some plaque from his own teeth, placed it under his handmade lens, and saw, to his astonishment, tiny ‘little animals’ moving. Dierkens, he called them. They were bacteria, the creatures he had been the first human ever to see a few years earlier in rainwater and in water that had stood on pepper.³ Now he was looking at them in his own mouth. A charming detail: he later noticed that the little creatures had vanished from his front teeth. The cause? He had taken to drinking scalding-hot coffee, which had killed them.

Yet for a long time afterwards science fell into the other extreme: the gut as a dull ‘food tube’, no more than a conduit. Only very recently have we begun to see its true richness.

A wall one cell thick

The wall of your gut, the border between the outside world inside your gut (all that food, all those bacteria) and your blood within, is, at the most important place, just one cell thick. One. Above it lies a thin layer of protective mucus, and beneath it your immune system stands guard. But the wall itself is a membrane as thin as a soap bubble, separating your interior from a world teeming with microbes.⁴

Just imagine it: everything you eat, everything that lives in your gut, is separated from your bloodstream by a layer one cell thick. It is at once astonishingly fragile and astonishingly cleverly built.

The gut never grows old, but it does fall ill

How does such a wafer-thin wall keep going, year after year, under constant wear? The answer is as elegant as it is surprising: it renews itself continuously. The cells of your gut wall are completely replaced roughly every four to five days. It is one of the fastest-renewing tissues in your entire body.⁴

There is a lovely thought in that. In a sense, your gut never grows old, the wall you have today was not even born a few days ago. And yet that same, forever-young gut can still fall ill. How can that be?

Why guts fall ill anyway

Put simply: an organ that is so thin, so alive and so densely populated is also vulnerable at exactly those points. Three things can go wrong: the mucus grows too thin, the one-cell wall starts to falter, or the balance among the microbes is disturbed. Often it is a combination.

One theme deserves honest attention here, because so much is claimed about it: the so-called ‘leaky gut’. The core is true: the permeability of your gut wall is real, and measurable. Under certain strains the wall can let a little more through for a while, think of extreme endurance sport, some painkillers (NSAIDs such as ibuprofen), excessive alcohol, and emulsifiers: the binding and thickening agents found in many processed foods. So far, the science.⁵

But then comes the nuance that is nearly always missing from popular articles. A thorough review in the journal Gut shows that the reverse is not proven: there is no disease you cure simply by ‘sealing’ the gut wall. Illnesses can make the gut leaky, but closing the leak alone does not cure those illnesses. The idea of ‘leaky gut’ as an explanation-for-everything, to be repaired with expensive supplements, goes far beyond the evidence. Real permeability: yes. Miracle cure: no.⁵

Antibiotics: the ally that also harms your friends

Few inventions have saved as many lives as antibiotics. But they make no distinction between villains and allies: they also hit the good bacteria in your gut.

In one study, twelve healthy men were given a short, heavy course of antibiotics. Their gut flora largely recovered within about a month and a half, a reassuring sign of resilience. But there was a shadow side too: nine bacterial species that had been present in everyone before the course had still not returned in most of the men after six months. You might compare it to a forest fire: the forest grows back, but not quite the same forest. So the rule stands: antibiotics are precious and sometimes life-saving, but only when they are truly needed, not for an ordinary cold.⁶

And what can you do yourself to help your gut flora recover? The honest answer: there is no miracle cure. What is best supported is simple and patient: time, and a varied, fibre-rich, largely plant-based diet that feeds precisely those returning bacteria. Fermented foods such as yoghurt, kefir or sauerkraut are fine as food, but they are not medicine. And probiotic pills? They are popular, but the evidence is mixed, one striking study even found that, after a course of antibiotics, they can slow the natural recovery rather than speed it up.⁷ If you have lingering complaints after a course, do discuss them with your GP.

How we and our microbes grew up together

Why do we have these microbes at all? Because we never evolved alone. Our ancestors shared their bodies with bacteria for millions of years, and they came to need one another. Our gut dwellers help us digest fibres we cannot manage ourselves, and in doing so they make substances that feed our gut wall and help calm inflammation, such as butyrate, produced by friendly bacteria like Faecalibacterium prausnitzii, a species that is notably reduced in people with certain gut inflammations.⁸

So what determines which bacteria you have? Not mainly your genes, it turns out. A large study found that your environment weighs more heavily: your diet, your medication, even the people you share a household with. People who live together come to resemble one another, right down to their guts.⁹

A living window onto how it was ‘meant’ to be is offered by the Hadza in Tanzania, one of the last communities still living as hunter-gatherers. They eat enormous amounts of plant fibre and have a far richer, more diverse gut flora than we do in the West. Their flora even shifts with the seasons: some species disappear from view entirely in the dry season and simply return later.¹⁰ The lesson is not ‘go back to the Stone Age’, but something simpler and more hopeful: variety and plant fibres feed exactly those bacteria that do us good.

A small wonder from Groningen

And sometimes science reveals something that leaves you quietly amazed. In 2024, researchers at the UMCG in Groningen showed that your body, through your genes, feeds certain good gut bacteria itself.¹¹

Here is how. In the mucus layer along your gut wall, your body lays down tiny sugar ‘name tags’ that match your blood type. And some helpful bacteria turn out to be able to eat exactly those sugars as fuel. In people with blood type A, though it also depends on a second gene, this goes together with a somewhat richer, healthier bacterial community, and with slightly better heart and metabolic health.¹¹

One thing is important to remember here, because this is where the media so often go wrong: this is not about the sugar on your plate. These are your body’s own sugars in your mucus lining, not biscuits or fizzy drinks. And it is an association, not a recipe. So it says nothing in favour of a ‘blood-type diet’. When researchers set that diet against the actual blood group of more than fourteen hundred people, the link with health turned out to be entirely independent of blood group itself.¹² What the Groningen work does show is something far more beautiful: how quietly and how cleverly your body tends a garden from which your health benefits.

In closing

Perhaps that is the real lesson of twenty-five centuries of gut history. The gut is neither the mystical seat of all disease nor the dull tube we took it for, for so long. It is something in between, and something far more interesting: a forever-young, wafer-thin border, populated by tens of trillions of allies, that we are only just beginning to understand.¹³

And that is exactly why humility is fitting. In few subjects are correlation and cause so easily confused as with the gut. That two things go together does not yet mean that one causes the other, a simple truth all too often forgotten in the hype around ‘gut health’.

This article is meant to inform, not to give medical advice. If you have complaints or questions about your gut, your diet or your health, please discuss them with your GP or a treating specialist.

Which ‘quiet border’ in your own body would you like to get to know better?

Related reading

• Glutamine: your gut’s favourite food
• The Living Kitchen: what fermented foods really do
• Reheating rice: the bacterium nobody knows

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