You have probably seen keto bread, keto bars, and keto meal plans in every supermarket.
Millions of people use the low-carb, high-fat diet to drop weight or steady their blood sugar.
A new finding from MIT asks a harder question about what all that fat does inside the body.
A new study published in Nature found that a ketogenic diet sharply raised tumor growth in the small intestine of cancer-prone mice.
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The same diet lowered tumors in the colon just a short stretch away.
The culprit, the researchers say, is dietary fat itself, not the ketones keto fans track.
This is a mouse study, not a human trial. But it was published in one of the most selective journals in science, and it rewrites a story many keto supporters have taken for granted.
Why dietary fat and not ketones drove the tumors
Keto works by starving the body of carbohydrates.
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The liver then breaks down fat into ketone bodies, small molecules the body burns for energy.
Many trainers and supplement makers treat those ketones as the active ingredient behind keto's benefits.
The MIT team tested that idea directly.
They used mice bred to develop intestinal tumors and fed them either a ketogenic diet, a normal diet, or a high-fat, high-calorie diet.
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According to MIT News, the keto mice developed small intestine tumors at least as often as the mice on the fattening diet.
When the researchers blocked ketone production, tumors kept forming.
When they ramped ketones up, nothing changed either.
The ketones were, in the words of lead author Fangtao Chi, metabolic bystanders.
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What did matter was how intestinal stem cells handled the heavy stream of dietary fat.
The cells cranked up fatty acid oxidation, the process of burning fat for fuel.
That activated a family of proteins called PPARs, which told the stem cells to divide faster.
In a gut that is already genetically primed for tumors, faster division means more chances for a cancer to take hold.
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What the opposite effect on the colon tells us
The small intestine and the colon sit side by side, but they responded in reverse.
In the colon, the same keto diet suppressed tumors, in line with earlier work from the same lab.
Omer Yilmaz, director of the MIT Stem Cell Initiative and senior author of the study, warned against reading the result as a simple yes or no on keto.
He said ketogenic diets have distinct effects on different tissues even within the gastrointestinal tract, and that what might help one tissue can hurt another.
The mice were genetically engineered to grow intestinal tumors, which models a rare inherited condition in humans called familial adenomatous polyposis.
People without that genetic setup may respond differently.
And no clinical trial has yet shown that a keto diet raises small intestine cancer in humans.
What the study does offer is a clear mechanism.
Ketone supplements, which skip the heavy fat intake, would not be expected to trigger the same pathway.
The problem the researchers found sits specifically with the river of dietary fat that defines a true ketogenic diet.
People who follow keto long term should talk to their doctor before deciding whether to stay on the diet.
That is especially true for those with a family history of gastrointestinal cancer or an inherited polyposis condition.
The next step for the MIT team is to work out why two neighboring parts of the gut react in opposite ways to the same food.
This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.