Keto Diet May Fuel Small Intestine Tumors, Study Finds

Put simply: the same high-fat, low-carb diet that seemed to protect one part of the gut can spur tumor growth in another. Curious? So were the scientists.

Researchers at MIT, led by biologist and pathologist Omer Yilmaz, fed mice genetically predisposed to intestinal tumors three distinct diets: a standard control diet, a ketogenic diet, and a high-fat, high-calorie regimen used to induce obesity. The result was not a faint gradient of change but a stark, anatomical split—keto increased tumor formation in the small intestine while continuing to suppress tumors in the colon.

That divergence overturned a tidy hypothesis. For years, ketone bodies—molecules like beta-hydroxybutyrate (BHB) that rise during ketosis—have been proposed as the protective agents behind keto’s reported anti-cancer effects in the colon. The new Nature paper upends that view. Instead of ketones steering the outcome, it was how intestinal cells burned dietary fat that flipped the switch.

The experiments showed that fatty acid oxidation in small intestinal cells activated PPAR proteins, which in turn drove intestinal stem cells to divide more rapidly—fueling a higher risk of malignant change.

Think of it this way: more active stem cells are double-edged. They speed repair when tissue is injured. They also multiply the pool of cells that might acquire cancer-causing mutations. On the ketogenic menu, small intestinal cells lean heavily on fat as fuel. That metabolic preference lights up signaling pathways that encourage stem cell proliferation. The colon, meanwhile, responds differently to the same dietary inputs, which helps explain why tumor rates there fell under keto in these animals.

Mice fed a keto diet (KD) had more intestinal tumors (circled in red in these images) and died more quickly than animals on the control diet (CD). 

Crucially, tweaking ketone production up or down did not change tumor growth in this mouse model. In other words, ketone supplements that raise circulating BHB would probably not mimic the complex consequences of eating a high-fat ketogenic diet. Diet and metabolite are related—but not interchangeable.

These findings come with important caveats. The mice used in the study carry a genetic predisposition to intestinal tumors that best mirrors familial adenomatous polyposis, a rare inherited syndrome in humans. That means the results do not translate directly to the general population. Still, the work prompts deeper questions about how different regions of the gut interpret the same metabolic signals.

Methodologically, the study compared tumor incidence and survival across diets and probed cellular metabolism, signaling molecules, and stem cell behavior. The team included molecular biologists Fangtao Chi and Jessica Shay among its co-first authors, and the work appears in Nature (2026). Their analysis suggests that commercially available ketone supplements are unlikely to reproduce either the protective effects observed in the colon or the tumor-promoting effects seen in the small intestine, because those outcomes were tied to dietary fat metabolism rather than to ketone levels themselves.

Why do neighboring tissues in the intestine respond so differently? The researchers are now pursuing that question. They want to know which local environmental cues and molecular circuits cause the small intestine to favor fatty acid oxidation and why the colon does not follow suit under the same dietary conditions.

For clinicians and people considering ketogenic diets, the takeaway is nuanced. In specific genetic contexts, high dietary fat may alter stem cell dynamics and elevate cancer risk in some parts of the gut while reducing it in others. That complexity argues for targeted research and personalized guidance rather than broad prescriptions.

So where does this leave the broader public? With more questions than answers—and with a clear message from the lab bench: metabolism matters, but it matters differently across tissues. The next chapter will need human studies and a closer look at how diet, not just ketones, reshapes the cellular landscape of our intestines.