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You have said it to someone. Maybe you have said it to yourself. Somebody laid out a perfectly reasonable case, and something in your middle disagreed before you had a single reason to give them. Trust your gut.
The phrase is older than the microscope. It is older than germ theory, older than anyone knowing that there was anything alive down there at all. And for most of the time people have been saying it, they did not mean it as a figure of speech.
It turns out to be two different claims wearing one coat, and almost nobody separates them.
There really is a sense behind it.
It has a name. Interoception. Your body reports its own condition upward, constantly, and your brain reads the report: heart rate, breathing, temperature, pressure, the tightness or the ease in your middle. Almost none of it reaches your attention. Some of it arrives as a feeling with no argument attached, which is a big part of what a gut feeling actually is.
So the feeling is real and it is worth listening to. Here is where the modern version quietly cheats.
That feeling is not your bacteria talking.
The connection people mean is the gut-brain axis. The two ends are wired together by the vagus nerve, which runs from your brainstem down into your gut, and I spent a whole section on it in Experiment 002, Take a Breath. Ninety percent of your serotonin is made down there, and it is not for your brain. It makes the wall of your gut contract, which is what moves food through you. The connection is real and it is starting to be measured, and more researchers find more of it every year, even though nobody has shown yet that the bacteria in your colon are producing your hunches.
One meaning of the phrase is about a sense. The other is about an organ.
One thing before we start.
This started as one piece. Then it got long, and the section on fermented food became an Experiment of its own. So I split it. This one is the organ: what it does, what wrecks it, what rebuilds it, and where yours came from. Next week is about fermentation and fermented food.
And one thing worth setting straight, because almost nobody does.
The word gut gets used for four different things: the tube food travels through, the wall that lines it, the nervous system that runs it, and the bacteria living inside. They affect each other constantly, which is why they get blurred together. But a finding about the bacteria is not a finding about the wall, and something that changes the wall has not been shown to change how you feel. Keep those four apart and most of what you read about gut health sorts itself out.
Here is the organ, plainly.
It breaks food into pieces small enough to enter you, and it absorbs them. That is the job everything else exists to serve. It carries the largest immune surface in your body, which is not a slogan, it is architecture: the inside of your gut is the biggest border you have with the outside world. It runs its own nervous system, a few hundred million neurons, which is why people call it your second brain. It keeps running while you sleep. It argues with your brain while you are awake.
And it decides. Every hour of every day, something in your intestine is deciding what gets into your blood and what goes straight through you and out. That single wall is the whole difference between food and poison.
Here is something you may not have known, and it changed how I think about all of this: the lining you have right now is not the one you had last week.
The cells lining your gut replace themselves every four to five days. Whatever you are worried you did to it, it is already rebuilding while you read this. Not everything can be undone. But the thing is not a museum piece. It is under construction, permanently, and what it is built out of is what you eat.
While we are here, two things you have probably been told about it are wrong. The gut lining is not the size of a tennis court. It is closer to a studio apartment, which is impressive enough without the exaggeration. And all the bacteria you are carrying weigh about as much as a large orange, not the two kilos people repeat.
Who else lives there.
Very little thrives in your stomach. It runs at about the acidity of battery acid, and that is deliberate: it is a gate. Past it, the further down you go the more crowded it gets, until you reach the colon, which is where essentially all of your bacteria live.
There are trillions of them and they are yours. Not a passing population like the bacteria on a door handle, but a community you have been carrying for decades, most of it assembled before you were three years old. It shifts with what you eat and it takes hits from things we will get to, but the core of it is as much yours as your fingerprints.
They are not there as tenants. They are working.
What fiber actually is, because nobody explains it.
Fiber is the part of a plant you cannot digest. That is the whole definition. Your own enzymes cannot break it down, so it arrives in your colon intact, which is where they live.
That is exactly the point. Fiber is not food for you. It is food for them.
Your bacteria ferment it, and one of the things they make out of it is a short chain fatty acid called butyrate, and butyrate is the main fuel the cells of your colon run on.
Read that again, because everything else in this piece depends on it. The wall that decides what enters your bloodstream is largely powered by bacteria eating your vegetables. You do not feed that wall directly. You feed them, and they feed it.
The part everybody wants to be true.
Something real is being found here. Nobody has agreed on how to measure it yet. So I am going to go in the order the evidence goes, strongest first.
Depression is the furthest along. In over a thousand people, two specific bacteria were consistently lower in the depressed group, and it held up in a second thousand.
And one of the best known trials in the whole field is not about bacteria at all. Sixty seven people with depression, split at random between dietary coaching and social support, and the diet group improved substantially on a proper clinical scale. The intervention was a Mediterranean diet. Not a strain, not a supplement. Dinner.
I have a saying I bring out at the table, that if people ate the way we eat there would be a lot more happy people in the world. That trial is the closest thing to evidence I have found for it.
Then there is ADHD, and I have a stake in this one, so I am going to be careful with it. This is where the feed lives and this is where the money is. Study after study finds differences in the guts of people who have it, but the specific findings rarely line up, and there is still no agreed signature, which is what research this early looks like before somebody works out what to measure. The most striking result in the whole area came out of a trial that was originally about allergies. Thirteen years later, six of the thirty five children in the placebo group had been diagnosed with ADHD or Asperger’s. None of the children who had been given a probiotic had. It was a small trial and that was not the question it was built to answer. It is still very hard to look away from.
And autism is the one place somebody went looking for the direction, and it ran the opposite way to how it gets sold. Autistic children in the study ate a narrower range of foods, often far narrower, because many autistic people are sensitive to texture and taste and end up eating the same few safe things. A narrower diet feeds a narrower set of bacteria. The difference in their gut was mostly explained by the food, not by the autism.
Where else it keeps turning up.
A lot of what we still cannot cure keeps leading back to this organ. Parkinson’s, where a leading theory has the damage starting in the gut and traveling up the vagus nerve years before the first tremor. Alzheimer’s, where the links are real but much earlier. IBS and celiac do not just connect to the gut, they live in it. There is not enough room in this one to go into them properly. What matters is that the link keeps turning out to be real.
Which brings up leaky gut, the phrase everybody has heard. The wall is real and it does become more permeable, and that part is not in question. What no gastroenterology body recognizes is leaky gut as a disease of its own. And two separate labs checked the blood test being sold to diagnose it, and neither found it measuring what the label says.
Now the part that wrecks it.
When I had the nightclubs, I had a system. After a heavy night of drinking, if I remembered to take an Advil before bed, there was no headache in the morning. If I forgot, the day was ruined.
The problem is that remembering is not a skill you have at four in the morning after a night out. I forgot a bunch of times. And when I did remember, halfway through the night, I have woken up girlfriends to hand them one and told them they would thank me in the morning. They always did.
I do not do that any more. I almost never drink now, so the morning after mostly does not happen. Which is its own small mercy, because alcohol damages the gut lining directly, and it does it from one heavy night rather than from years of them. The full cost of alcohol comes up in a future experiment.
I used to take Naproxen sometimes too, an anti inflammatory, after a strain or a gym injury. Two or three days of it, enough to take the swelling down so the thing could heal. And it worked. That is what makes this a real decision and not an easy one.
What an anti inflammatory actually does.
When something in you is damaged, your body makes prostaglandins, the chemicals that produce the swelling, the heat and the pain. That is inflammation, and it is your body calling for repairs. Ibuprofen and naproxen, which is Advil and Aleve, work by stopping your body making it, which is why the swelling goes down and the pain goes with it.
The catch is that the same chemical has a second job. It is also what keeps the lining of your stomach and intestine maintained. Block it for the pain and you block it there too, which switches off the maintenance on the wall that keeps your gut contents out of your blood.
Forty healthy volunteers took a prescription strength anti inflammatory for two weeks. Twenty seven of them came out with new damage in the small intestine, visible on a camera. Two weeks, in people with no gut problems at all.
That tells me nothing about what an occasional tablet does. It tells me enough to take the headache instead, and to tough out the strain. That is my call, not advice about your back pain.
Antibiotics, which I have never had to take.
Not as an adult, and not as a kid that I can remember. I have been lucky. I have never been in a position where I had no choice. There is a cost to a course of antibiotics, and nobody mentions it when they hand it to you.
A study published this year followed nearly fifteen thousand people and found differences linked to antibiotic use still showing up four to eight years afterward. And a smaller one shows you what that looks like up close. Twelve healthy men took four days of three antibiotics at once, the kind you get in a hospital. Six weeks later the overall count looked normal again, which is where most studies stop. But several species that every one of them had carried beforehand were still missing six months later. The population came back. The residents did not.
I am not telling you not to take them. Sometimes they are the only answer, and people used to die of things we now clear up in a week. But think about it the way an athlete thinks about an injury. You do not stop when the injury is fixed. You rebuild the muscle. It is the same after antibiotics. The prescription tells you how to kill the infection. Nobody tells you how to build it back.
Three words that get sold as one thing.
A probiotic is live bacteria. A prebiotic is the food they eat, which is mostly fiber. A postbiotic is a preparation of bacteria that have been killed, or the compounds they made while alive, sold on the idea that you get the effect without needing anything to move in. Three different things, and the words get used as if they were one. The postbiotic idea is the newest and the least settled, and the one name worth watching is Akkermansia.
Now the study that changed my mind about the first one. Twenty one volunteers took a week of antibiotics. Some of them had their own gut bacteria collected and frozen beforehand. Afterward they were split three ways: do nothing, take a standard probiotic, or have their own frozen bacteria put back.
The group who did nothing recovered slowly. The group who got their own bacteria back recovered in days. And the group who took the probiotic did worst of all, their own native bacteria taking months longer to come back than in the group who did nothing. The probiotic did not speed their own bacteria back. It delayed them.
And yet a probiotic taken during antibiotics does cut the odds of diarrhea, and that part is solid across a lot of trials. So there is a protocol here and it is not the one on the label. During the course, it helps with the symptom. Afterward, there is little evidence that anything off a shelf puts your own community back, and the study that looked found the opposite. Same bottle, and the label only tells you about the good half.
And then there is what you eat.
Medicines are not the only thing on the list. Ultra processed food does it, and the clearest evidence is about emulsifiers, the ingredients that keep things smooth and stop them separating. Healthy people were fed two identical diets, one with a common emulsifier added and one without. The emulsifier group came out with less bacterial variety, and the compounds their bacteria were making changed with it.
Then there are the artificial sweeteners. A hundred and twenty people were given one of four of them for two weeks. Aspartame, saccharin, stevia, sucralose. All four changed their gut bacteria, and saccharin and sucralose also changed how those people handled sugar in their blood. Those are the things sitting in diet drinks, sweetener sachets and chewing gum, and plenty of people have them every day.
And stress, which runs the same wiring in reverse.
The gut-brain axis from the opening is not a one way road. Chronic stress damages the gut barrier and shifts the community living behind it. The direction is well established even where the numbers are not, and it is the part of this subject everybody has already felt. You know what a bad week does to your stomach. That is not in your head. Or rather, it starts there and does not stay there.
Which brings me to the most useful sentence in this section, and it is free: the best supported way to fix a damaged gut lining is to stop doing the thing that is damaging it. Not a supplement. Removal.
Now the part that rebuilds it.
Thirty six adults, seventeen weeks, split into two groups. One worked up to about six servings a day of live fermented food: yogurt, kefir, kimchi, fermented vegetables, brine. The other tripled their fiber.
The fermented group’s bacterial variety went up and stayed up, and nineteen of the inflammation markers a blood test uses went down. The fiber group saw neither, in ten weeks.
It is not an argument against fiber. Fiber is food for bacteria. If the right bacteria are thin on the ground, adding more of their food does not do much, not quickly anyway. One possible explanation is that fermented food brings the organisms and the food in the same mouthful. Over a longer run enough fiber is still the foundation. It is just not the quick lever.
Six servings a day is the number in the trial. Not two, not a capsule. Six.
Why the pills mostly do not do it.
Four things have to go right for a probiotic capsule, and a lot of products miss one. It has to still be alive when you buy it, which is why some of them tell you to refrigerate them and why they carry an expiry date. It has to survive your stomach acid, which is the whole point of your stomach acid. It has to find space in a community that is already fully occupied. And it has to stay, which most of them do not. Even the strains that survive the trip vanish once you stop taking them. Surviving is not colonizing. Passing through is not moving in.
I did try Seed, the one that is all over your feed and the one with the most advanced delivery system. It builds a capsule inside a capsule so the bacteria survive the stomach, and I was curious about the mechanism. The subscription did not last, because I get mine from food and I do not feel I need to supplement it. I still have some sitting in my fridge. I even used to recommend it to friends. Now I just tell them to eat kimchi, sauerkraut and kefir instead, and next week I will hand you a much longer list.
What actually seems to matter.
Variety of plants. Vegetables, fruit, nuts, seeds, herbs, spices. That is also where your fiber comes from, and most adults eat about half of what is recommended. The number that gets quoted is thirty different ones a week, and it comes from a very large study. I think it is the most useful eating instruction in this whole field, because you cannot follow it without improving everything else about your diet by accident.
Exercise, which is free. Six weeks of it shifted the gut community in the same people, and the shift partly went back when they stopped. Even a walk after dinner counts.
And leave gaps between meals, because your gut runs a cleaning wave between them that eating switches off.
What gas and bloating actually mean.
Bloating and gas are the first thing anybody notices, and gas on its own is not a fault, it is bacteria eating. One common reason for the bloating and the cramping is something reaching the colon that was supposed to be absorbed further up. Milk is the clearest case: the enzyme that digests milk sugar sits in the wall of your small intestine, plenty of adults stop making much of it, and your bacteria ferment it instead.
Where yours came from.
You were born with essentially none. The first ones come from your mother, and most of them come from her gut. Birth is the handoff. Feeding and skin contact add the rest.
That is why the way you are born matters. Babies born by C-section pick up fewer of their mother’s own strains and more from whatever else is around them. All three of us, my brother, my sister and me, were born that way, so I am not pointing at anybody. It is one influence among several, not a line drawn under a life.
And antibiotics during pregnancy or around the birth change what she has to give you, because they change her own community first.
The thing about breast milk.
Breast milk contains around two hundred different sugar structures that a baby cannot digest at all. Cannot. They pass through the stomach and the small intestine untouched, on purpose. They are there to feed a specific family of bacteria in the baby’s colon, and to keep the wrong ones from getting a grip on the wall. And her milk carries some of her own bacteria too, so she is sending more workers along with their food.
Sit with that. A significant fraction of the milk a mother makes is not food for her child. It is food for her child’s bacteria. Evolution built a feeding system for something that is not even human, inside the first thing any of us ever eats, long before anyone knew bacteria existed.
That is the strongest argument for breastfeeding there is, and nobody puts it on the poster.
Then you go outside.
When we were kids at my grandmother’s, in the village, we would play in the garden all day. If our hands got sticky from eating pomegranate, grapes or oranges straight off the trees, and there was no soap around, she would have us wash with dirt under the garden hose. Rub the soil between your palms and rinse it off, she would say.
There is something real underneath that, and it is not what people repeat. The hygiene hypothesis does not say dirty is better. The claim is narrower: we evolved alongside a set of specific organisms, from soil, plants, animals and other people, and most of us have stopped meeting them, because most of us now live in a concrete jungle.
The cleanest evidence anyone has comes from two farming communities in America who were studied side by side. They share almost the same ancestry. The Amish farm the old way, with the animals close to the house. The Hutterites, a closed community just like them, farm with machinery and keep the animals at a distance. Childhood asthma runs at about five percent in the first and twenty one percent in the second, and the researchers could tell the two apart by how much farm dust was in the house. Similar lives. Different air.
What you own.
You own an organ that rebuilds its own lining every four or five days, decides what enters your blood, and does all of it across a surface the size of a studio apartment. Nobody sat you down and told you that. What you were sold instead was a shelf of capsules whose passengers mostly never move in, riding in a coffin.
Here is what actually held up, and notice how little of it comes in a bottle.
Six servings a day of real fermented food did something measurable in seventeen weeks. Thirty different plants a week is the best eating instruction in the field. The most reliable way to repair a gut lining is to stop doing the thing that is damaging it. And your lining is already rebuilding, right now, out of whatever you last ate.
So before you put this down, do one thing.
Start adding real fermented food to your day. There are far more of them than you think, and next week we will cover a lot of them.
Experiment 005 is complete. You just read it.
Georges
Usually one Experiment on Sunday. Lab Notes in between. Free.