“I eat clean. I dropped the sugar. I eat a lot of fat now, like everybody told me to. But I’m gaining fat, Matt.”
I hear that a lot from people who are on low-carb, high-fat diets, and a lot of times they also find over time that their mood kind of gets sort of flattened out too.
A gut problem. And a head problem.
Nobody told these guys that the underlying reason may be the same.
Here is the piece of evidence that started me down this road.
A team in Paris took stomach lining biopsies from 19 obese people and 14 normal-weight people.
They were looking at one particular gene. The gene that switches on the production of a single chemical in the lining of the gut.
In the obese people, that gene was running about four times hotter.
Four times.
Not in their brains. In their stomachs.
And these were living human beings, not mice.
Everyone thinks it’s the fat what done it.
Specifically the saturated fat. The butter, the cream, the fatty red meat, the eggs.
That is the suspect. It has been the suspect since 1961.
And I have to tell you, the case against it looks strong. But I never agreed with it and I’ll show you proof it’s wrong.
It seems obvious. Because when you feed an animal a diet where 60 percent of the calories come from fat, the animal gets fat, gets inflamed, gets a leaky gut, and its behavior changes.
That has been reproduced hundreds of times in labs all over the world.
It looks airtight.
It is not.
I went and read what those animals were actually fed. Not the label on the study. The actual formula, ingredient by ingredient, in grams.
What is in there is not what anybody told me was in it. Not even close.
That same ingredient is in most of the packaged food in an American grocery store and in nearly every restaurant fryer in the country.
Three things I am going to walk you through.
What is actually happening in the lining of your gut in the hours after a fatty meal. It is not digestion and it is not calories.
Why the lining of your intestine comes apart — and why the thing that pulls it apart is not the fat.
And what your head has to do with any of it. Because it turns out it has everything to do with it.
That last one is the part that explains my student. And it is the part I did not believe until I read it three times.
There is a specific number in one of these studies that made me put my coffee down.
Somebody else did this to him. Not the butter.
Let me show you who.
Quick disclaimer: I am not your doctor and this is not medical advice. I am a men’s health educator who reads a lot of research. Talk to your own doctor before you change any medication.
I wrote a whole piece a while back called “Serotonin Is Not the Happy Hormone,” and I got more angry email about that one than almost anything else I have published.
The reason is that almost everybody has the story exactly backwards, and the backwards version has been sold very hard for about 35 years.
So here is the part almost nobody knows.
Roughly 90 to 95 percent of the serotonin in your body is not in your brain at all.
It is in your gut.
It is made in the lining of your intestine, by a specialized cell called an enterochromaffin cell. Those are hormone-producing cells sitting right in the wall of your intestine, facing the food going by.
They taste what you eat. And they respond by squirting out serotonin.
For many, many years, or even decades, they have always said that serotonin made in your gut cannot get into your brain, because it cannot cross the blood-brain barrier.
But are they right? It makes a great deal of difference because gut serotonin is so overwhelmingly large compared to the amount produced in the brain.
If even a small amount of gut serotonin gets in the brain, that changes what people think about serotonin in the brain and what drugs do what.
And let me right now change what you may think about dietary fat and serotonin.
In August of 2025 a group led by Taylor Gray at North Carolina Agricultural and Technical State University published a review in the journal Nutritional Neuroscience.
They had collaborators at Brown and at Cornell. What they set out to do was simple and, honestly, overdue.
They gathered up everything anybody had ever measured about what a long-term high-fat diet does to serotonin, and laid out the gut findings and the brain findings side by side on the same page.
Health research has gotten very siloed. In this case, the gut people were just talking gut stuff. The brain people were talking brain stuff. Nobody’s reading each other’s articles because they were all published in different journals that others weren’t following.
When Gray’s group put the two stacks of paper next to each other, the pattern that fell out was not the one anybody expected.
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