You know I’m not a calories-in, calories-out guy.
I spent years at 300 pounds counting every calorie and working out like crazy, and I stayed fat. On paper, the math worked. My body clearly didn’t read the math.
There’s more science on why the math doesn’t check out. And the reason lives in your gut.
The calorie number is a 130-year-old estimate
The calorie counts on food labels come from a system a chemist named Wilbur Atwater worked out in the 1890s. He burned food, measured the energy, and then estimated how much of it a human body absorbs. Food companies still use versions of his numbers today.
The problem is that Atwater assumed everybody’s gut absorbs food the same way, when they actually don’t. Your gut has trillions of bacteria in it, and they want a cut.
The poop study
In 2023, researchers at Arizona State and AdventHealth in Orlando conducted one of the most controlled diet experiments you’ll ever see. They checked 17 healthy young adults into a metabolic ward, a hospital-style unit where they weigh every bite of food and measure every calorie burned in a sealed room.
Each person ate two diets for about three weeks each, in random order. Both diets had the same calories and the same amounts of fat, protein, and carbs.
The Western diet: highly processed, low in fiber, soft, finely ground foods. What a lot of people eat every day.
The “microbiome enhancer” diet: more fiber, more resistant starch, foods in bigger pieces, and very little processed food. The goal was to get more food all the way down to the bacteria in your colon.
Then the researchers did something heroic. They collected and measured all the poop.
On the microbiome diet, people lost an average of 116 more calories a day in their stool than on the Western diet, without eating less or feeling any hungrier. And people varied a lot. On the microbiome diet, the total calories each person lost to stool ranged from about 73 to 390 a day. On the Western diet, the range was much tighter, because everybody absorbed nearly everything.
So was it just undigested food?
Partly. The extra calories in people’s stool came from two places.
Some of it was food they didn’t fully digest. Fiber and bigger food pieces are harder to break down, so more of them make it down to your colon.
The rest came from the bacteria themselves. Once that food reaches your colon, your bacteria eat it and multiply. Your gut sheds bacteria all the time, so a bigger, better-fed crowd means more of them leave with every bathroom trip, along with the energy they took from your food. Many of them are still alive when they go. By the researchers’ estimate, bacteria made up more than a quarter of the energy in people’s stool.
Everybody ate the exact same diet, so if it were only undigested food, they’d all lose about the same number of calories. But they didn’t. The people with more gut bacteria and more fermentation byproducts in their stool absorbed the fewest calories.
This year, the Arizona State team published a new model of how your gut absorbs calories, one that accounts for what your bacteria eat and make. It predicted how many calories people absorbed more accurately than the system behind food labels.
You can use this to your advantage
The label says one number. Your gut decides another. Two people can eat the exact same plate and absorb different amounts of energy from it, and a lot of that difference comes down to what they feed their microbes.
You can change that. The researchers used four specific levers, and you can pull all four this week without counting anything.
Here’s what’s coming up:
Four ways to send more food to your bacteria, including a kitchen hack I’ve used for years
Tricks to get more resistant starch in your diet
How food texture can make a difference
How to ramp up without spending a week bloated
How to tell your gut is on board, no lab required
