MIT Study Reveals Keto Diet's Hidden Cancer Risk in Small Intestine

Researchers at MIT have discovered something counterintuitive about the ketogenic diet: it appears to increase cancer risk in one part of the intestine while decreasing it in another, a finding that upends some of the popular health claims surrounding the diet and reveals how the same metabolic shift can produce opposite effects in neighboring tissues.

The study, published in Nature, examined mice genetically predisposed to intestinal cancer. Some were fed a ketogenic diet—high in fat, very low in carbohydrates—while others received standard diets or high-fat, high-calorie control diets. The results were striking. Mice on the keto diet developed small intestinal tumors at higher rates than the control group. Their tumor burden matched or exceeded that of mice eating the obesogenic high-fat diet, despite the fact that the keto-fed animals did not become obese. The finding contradicted what researchers expected based on earlier work suggesting ketogenic diets might protect against cancer throughout the gut.

What made the discovery particularly significant was what the researchers found when they dug into the mechanism. The culprit was not ketone bodies—the compounds the body produces when it burns fat for fuel in the absence of carbohydrates. Instead, the increased tumor growth traced directly to how intestinal stem cells processed dietary fat. When the body burns fat for energy, it activates a family of proteins called PPARs. These proteins signal intestinal stem cells to divide more rapidly. More cell division means more opportunities for mutations to accumulate and for cancer to develop. Omer Yilmaz, director of the MIT Stem Cell Initiative and senior author of the work, explained the trade-off: rapid stem cell growth helps the intestinal lining repair itself after injury, but excessive division also raises the risk of malignant transformation.

The colon, however, told a different story. In that tissue, the ketogenic diet suppressed tumor formation—consistent with a 2022 study that had attributed the protective effect to ketone bodies themselves. But the new experiments challenged that conclusion. The same fat-burning mechanism that accelerated tumors in the small intestine appeared to suppress them in the colon. The researchers found that ketone bodies were not the direct drivers of either effect. Instead, the tissue-specific outcomes depended on how each region's stem cells responded to the influx of dietary fat. Two adjacent parts of the digestive tract, exposed to the same diet, produced opposite cancer outcomes.

The implications extend beyond the laboratory. Ketogenic diets have surged in popularity for weight loss and longevity claims. The study suggests that broad generalizations about their safety or benefit are premature. More immediately, the findings have consequences for commercial ketone supplements and drinks, which have become a growing market. Because the tumor effects—both the increased risk in the small intestine and the decreased risk in the colon—stem from fat metabolism rather than ketone bodies themselves, these supplements would not be expected to reproduce either outcome. That distinction matters because small intestinal cancer rates have risen in recent decades, and the risk is particularly acute for people with inherited conditions like familial adenomatous polyposis that predispose them to intestinal tumors.

The research team, which included postdoctoral researchers Jessica Shay and Fangtao Chi as lead authors, now faces a deeper question: why does the same diet produce opposite consequences in two neighboring tissues? Understanding that mechanism could reshape how clinicians counsel patients considering ketogenic diets, particularly those at elevated risk for intestinal cancer. For now, the study serves as a cautionary note about the limits of dietary interventions—what protects one tissue may harm another, and the popular narrative around any single diet requires scrutiny at the tissue level.