Keto Diet May Fuel Small Intestine Tumors Through Fat Not Ketones, MIT Finds

A Surprising Twist in Keto Diet Cancer Research

A high-fat, low-carbohydrate ketogenic diet, while popular for weight loss, may increase the risk of small intestine tumors, according to a study published in Nature by researchers at MIT. The finding is striking not just for what it reveals about cancer risk, but for what it overturns: a widely held assumption about how the diet interacts with the body at a cellular level.

Rather than being driven by ketone bodies — the molecules that have become almost synonymous with ketogenic diets — the effects were linked to how intestinal cells metabolize dietary fat. The findings challenge one of the leading theories about how ketogenic diets influence cancer. A 2022 Nature study had suggested that ketogenic diets have a protective effect against colon cancer and that BHB — the most abundant ketone body — is responsible for this effect. The new research complicates that picture significantly.

What the Researchers Actually Found

The researchers fed mice who were genetically predisposed to developing intestinal cancer either a ketogenic diet, a control diet, or a high-fat/high-calorie diet. They found that mice on a ketogenic diet were more likely to develop tumors of the small intestine than those on a control diet. Mice on the ketogenic diet developed small intestinal tumors at rates comparable to — or even higher than — those fed the obesity-inducing diet, despite remaining lean themselves.

Additional studies revealed that ketone bodies did not play a role in tumor development. Instead, tumor growth was driven by how intestinal cells burn dietary fat for energy — a metabolic pathway called fatty acid oxidation. This pathway activates a family of proteins called PPARs, which signal stem cells to multiply more rapidly, increasing the chance that some become cancerous. The stem cell angle is key: this stem cell proliferation can be beneficial in certain situations, such as when the intestinal lining needs to be repaired after illness or injury. However, too much proliferation can tip cells toward becoming cancerous.

The Gut's Contradictory Response

Surprisingly, the same ketogenic diet that promoted tumors in the small intestine had the opposite effect in the colon. The same diet suppressed colon tumors, also via fat metabolism rather than ketones. Ketone supplements alone are therefore unlikely to reproduce these tissue-specific cancer effects. This means the gut does not respond to the keto diet as a single, uniform system — location within the digestive tract matters enormously.

Lead researcher Omer Yilmaz, director of the MIT Stem Cell Initiative, put it plainly. "I think the message here is that we need to be very careful in generalizing the effects that these diets can have, because what might be beneficial for one tissue may be detrimental for another tissue," he said. His team was equally surprised by where the danger was coming from: "Our experiments in genetically engineered mice revealed that these molecules are essentially metabolic bystanders. The real surprise is that tumor acceleration is driven entirely by how stem cells process and burn the heavy influx of dietary fat itself," Yilmaz said.

What This Means Going Forward

The impact of ketogenic diets, which contain even higher lipid content than standard high-fat diets but reduce circulating insulin and induce ketogenesis, remains poorly understood. This is particularly relevant for patients with familial adenomatous polyposis who face a high risk of small-intestinal tumors. For those individuals, the study's findings carry especially urgent implications.

The researchers now hope to further study why ketogenic diets have such different effects in the colon and the small intestine. For the millions of people who follow keto diets for weight loss, metabolic health, or perceived cancer-preventive benefits, this research is a reminder that dietary science rarely delivers simple answers. The fat you eat doesn't just fuel your body — it actively shapes the cellular environment in ways scientists are only beginning to fully map.