Keto Diet’s Cancer Paradox: Protects the Colon, Fuels Small-Intestine Tumors

A diet that starves colon tumors can, at the same time, feed tumors just a few feet away in the small intestine. That is the surprising conclusion of a new study published in Nature on July 15, 2026, by researchers at MIT and the Whitehead Institute. In mice bred to be prone to intestinal cancer, a ketogenic diet suppressed colon tumor growth just as an earlier study from the same lab had shown, but it pushed small-intestine tumors to grow at rates similar to or even higher than those seen with a high-fat, high-calorie "obesogenic" diet. The twist deepens an already complicated picture of how popular low-carb, high-fat eating patterns interact with cancer risk.

Ketogenic Diet Splits Colon and Small-Intestine Tumor Growth

The core finding, reported by lead authors Jessica Shay and Fangtao Chi and senior author Omer Yilmaz of MIT's Koch Institute, is that the same ketogenic diet has opposite effects depending on which section of the gut it reaches. As MIT News reported, mice on the ketogenic diet developed small-intestine tumors at rates matching or exceeding those on the obesogenic diet, even though the keto-fed mice did not become obese. Meanwhile, the identical diet suppressed tumor formation in the colon, a result that confirms an earlier 2022 Nature paper from the Yilmaz lab. Yilmaz summarized the tension directly: ketogenic diets have distinct effects on different tissues, even within the same digestive tract.

Stakes for Familial Adenomatous Polyposis Patients and Keto Dieters

The findings matter most for people who already carry an elevated genetic risk of intestinal cancer, particularly those with familial adenomatous polyposis, an inherited condition that drives the growth of numerous polyps in the gut, and for whom small-intestine cancer rates have been climbing in recent decades according to coverage of the study on SciTechDaily. For the millions of people who use ketogenic diets for weight loss, epilepsy management, or general health, the study is not a reason to panic, but it is a reason for caution: a diet marketed as broadly beneficial for metabolic health may not be neutral, or even helpful, for every part of the gastrointestinal tract. The researchers stressed that people should not assume a diet's effect on one organ generalizes to another.

Three-Diet Mouse Model Traces Effect to Fatty-Acid Oxidation

To pin down what was actually driving the divergence, the team compared three dietary arms in a genetically predisposed mouse model of intestinal cancer: a standard control diet, a high-fat/high-calorie obesogenic diet, and a ketogenic diet. The Nature paper describes how the researchers then used genetic tools to isolate the mechanism. They manipulated HMGCS2, an enzyme central to producing ketone bodies such as beta-hydroxybutyrate, and found that neither boosting nor blocking ketone production changed tumor outcomes, ruling out ketones as the driver despite being the diet's namesake molecule. Instead, blocking CPT1A, an enzyme that shuttles fat into cells for fatty-acid oxidation, limited tumor (adenoma) formation under the ketogenic diet. Combined loss of three PPAR proteins – PPAR-alpha, PPAR-delta and PPAR-gamma – blunted the diet-driven expansion and proliferation of intestinal stem cells. The mechanism, in other words, runs through how intestinal cells metabolize dietary fat itself, not through the ketone levels typically used to define a "ketogenic" diet.

Open Question: Why Same Diet Splits Two Gut Tissues

The study does not fully explain why fatty-acid oxidation and PPAR signaling would spur stem cell proliferation in the small intestine while apparently doing the reverse, or at least not doing the same thing, in the colon. As the researchers described it, more active intestinal stem cells help the small intestine repair itself after injury, but that same heightened proliferation raises the odds that some of those dividing cells turn cancerous. Why the colon's stem cell population does not respond the same way to the identical circulating fat and PPAR signals remains an open biological question. It is also worth being clear about the limits of the evidence: this is a mouse study using a model genetically engineered to develop intestinal cancer, not a clinical trial or human cohort, so how directly the tissue-specific effect translates to people, and over what duration or dose, is unconfirmed.

What Comes Next for Diet-Based Cancer Prevention Research

One practical takeaway already flagged by the researchers: commercial ketone supplements, which raise blood ketone levels without changing how much dietary fat a person eats, would not be expected to reproduce either the protective colon effect or the harmful small-intestine effect, since the mechanism runs through fat metabolism rather than ketone body levels. Coverage of the study, including from EurekAlert, notes that the work was supported by funding sources including the NIH and the Koch Institute, and involved additional MIT contributors Alex Shalek and Matthew Vander Heiden. The next steps for the field will likely involve testing whether the same fatty-acid-oxidation and PPAR pathway holds up in other genetic cancer models, and eventually in human tissue or patient cohorts, particularly those with familial adenomatous polyposis, before any dietary guidance for at-risk patients could reasonably change.

Evidence rating

INVESTIGATIONAL — 75/100

Caveats:

• Mouse study only – human relevance and dose/duration effects unconfirmed
• Mechanism is fatty-acid oxidation/PPAR signaling in intestinal stem cells, not ketone bodies as commonly assumed
• Effect is tissue-specific and not fully explained: same diet suppressed colon tumors while accelerating small-intestine tumors

Findings come from a genetically predisposed mouse model of intestinal cancer; the tissue-specific diet effect has not been confirmed in humans and the underlying biology is still being worked out.

References

1. (2026). Popular weight loss diet linked to cancer in the small intestine – but only for some people. https://www.gbnews.com/health/cancer-risk-keto-diet-small-intestine
2. Ketogenic diets may increase cancer risk in the small intestine | MIT News. news.mit.edu. https://news.mit.edu/2026/ketogenic-diets-may-increase-cancer-risk-small-intestine-0715
3. Jessica E. S. Shay, Fangtao Chi, Constantine N. Tzouanas, et al. (2026). Ketogenic diet mediates intestinal tumorigenesis through lipids not ketones. Nature. https://doi.org/10.1038/s41586-026-10779-y