Cancer cells often have abnormal metabolism. One of the best-known examples is the Warburg effect: many cancer cells consume large amounts of glucose and produce lactate even when oxygen is available.
Thomas Seyfried, Ph.D., building on the work of Otto Warburg, argues that impaired mitochondrial metabolism is a fundamental cause of cancer and that cancer should be viewed primarily as a metabolic rather than a genetic disease. He emphasizes glucose and glutamine as important fuels for many cancers.
This has led to interest in ketogenic diets, fasting, and other approaches that target cancer metabolism.
Cancer metabolism is an important field of research, but Seyfried's broader theory remains controversial, and metabolic therapies have not been established as replacements for standard cancer treatment.
Modern cancer research recognizes altered metabolism as an important feature of cancer. Researchers study glucose and glutamine metabolism, mitochondrial function, fatty-acid metabolism, and other pathways as possible treatment targets.
Where mainstream cancer biology differs from Seyfried is primarily in explaining what causes cancer. The prevailing view involves interactions among genetic changes, cell signaling, the immune system, the tumor environment, and metabolism.
Seyfried goes further by proposing that mitochondrial dysfunction is the fundamental cause of cancer. That remains a minority view.
The disagreement is therefore not about whether metabolism matters, but how central it is to the origin and treatment of cancer.
Researchers are studying ketogenic diets, fasting, fasting-mimicking diets, and other metabolic interventions, usually alongside conventional treatment rather than as replacements.
A randomized trial of 80 women with locally advanced or metastatic breast cancer receiving chemotherapy found that women assigned to a ketogenic diet had lower insulin levels and greater reductions in tumor size than the control group.
The DIRECT trial studied 131 women with Stage II or III HER2-negative breast cancer receiving chemotherapy. Women assigned to a fasting-mimicking diet were more likely to show tumor shrinkage on imaging. Among women who followed the diet as planned, strong pathological responses were also more frequent.
Another randomized study reported encouraging results with a fasting-mimicking diet during chemotherapy, including fewer severe side effects and greater tumor response.
These studies are relatively small and do not establish that ketogenic diets or fasting reduce recurrence or improve survival. Larger trials are needed.
Research is continuing. A 2026 clinical trial at UT Health San Antonio, with the National Cancer Institute as a collaborator, is studying 12 women with Stage I or II breast cancer who follow a ketogenic diet for three weeks before surgery. Researchers will examine whether the diet changes the immune environment surrounding the tumor.
Researchers still need to determine:
Which breast cancers are most likely to respond?
Can reducing glucose and insulin improve conventional treatment?
Can targeting glucose and glutamine produce stronger effects?
Most importantly, can metabolic approaches reduce recurrence or improve survival?
The early evidence is encouraging enough to justify further research, but these approaches remain investigational.
Could my metabolic health have implications for my treatment?
Could a ketogenic diet or fasting be useful—or interfere with my treatment?
How important is it to maintain weight and muscle during treatment?
Are there clinical trials of metabolic therapies that might be appropriate for me?
Should I work with a nutrition professional experienced in cancer care?
A recent presentation of Seyfried's mitochondrial metabolic theory of cancer.
A randomized clinical trial of 80 women receiving chemotherapy that examined the effects of a ketogenic diet.
A randomized Phase II trial of 131 women with HER2-negative breast cancer.
A randomized study examining treatment response and side effects during chemotherapy.
Research examining how diet and metabolism interact with tumor biology.
A concise introduction to Thomas Seyfried's central argument.
Is Breast Cancer a Metabolic Disease? with Dr. Donese Worden - 27 Minutes
Dr. Donese Worden discusses breast cancer from a metabolic perspective, including cancer-cell metabolism and the possible role of metabolic approaches alongside conventional treatment.
A wide-ranging 2026 interview with Seyfried discussing mitochondria, glucose and glutamine, ketogenic diets, fasting, and metabolic approaches to cancer treatment.