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 energy metabolism is a fundamental cause of cancer and that cancer should be viewed primarily as a metabolic disease rather than a genetic disease. He emphasizes the dependence of many cancer cells on glucose and glutamine as fuels.
This theory has led to interest in treatments designed to place cancer cells under metabolic stress, including ketogenic diets, fasting, calorie restriction, and approaches that target glucose or glutamine metabolism.
Cancer metabolism is an important and active field of research. However, Seyfried's broader theory that cancer is primarily a metabolic disease remains controversial, and metabolic therapies have not been established as replacements for surgery, radiation, chemotherapy, hormone therapy, or targeted therapy.
For breast cancer specifically, research on ketogenic diets and other metabolic approaches is intriguing but still preliminary. At present, there is not sufficient clinical evidence to show that a ketogenic diet can treat breast cancer or improve survival.
In the 1920s, German scientist Otto Warburg observed that many cancer cells consume unusually large amounts of glucose and convert much of it to lactate even when oxygen is available. This phenomenon became known as the Warburg effect, or aerobic glycolysis.
Normally, cells can use mitochondria to extract large amounts of energy from glucose through oxidative phosphorylation. Warburg believed that cancer cells relied heavily on fermentation because their mitochondrial respiration had been damaged, and he proposed that this metabolic dysfunction was central to the development of cancer.
The Warburg effect itself is now well established and remains an important area of cancer research. It also helps explain why PET scans can detect many cancers: tumors often take up unusually large amounts of a radioactive form of glucose.
What the Warburg effect means, however, remains more complicated. Modern research shows that many cancer cells displaying the Warburg effect still have functioning mitochondria and use both fermentation and mitochondrial metabolism.
The scientific question is therefore not whether cancer metabolism is abnormal—it clearly is—but whether metabolic dysfunction is primarily a cause of cancer, a consequence of genetic and other changes, or part of a complex interaction between the two.
Thomas Seyfried, Ph.D., a professor of biology at Boston College, argues that cancer begins primarily with damage to mitochondrial energy metabolism, rather than with mutations in nuclear DNA.
According to Seyfried, when normal mitochondrial energy production becomes impaired, cells compensate by relying increasingly on fermentation to produce energy. Over time, this metabolic disturbance can disrupt normal cellular control mechanisms and contribute to the uncontrolled growth characteristic of cancer.
Seyfried argues that many of the genetic mutations found in cancer are therefore consequences of metabolic dysfunction rather than its original cause. This differs from the conventional view that cancer generally begins when mutations and other genetic changes disrupt the mechanisms controlling cell growth.
A central part of Seyfried's theory is that many cancer cells depend heavily on two fuels: glucose and glutamine. Glucose supports glycolysis and other pathways needed for growth, while glutamine can provide both energy and building materials for rapidly dividing cells.
This leads to his therapeutic hypothesis: cancer cells might be placed under metabolic stress by lowering glucose availability, raising ketone levels, and simultaneously targeting glutamine metabolism. Seyfried refers to this general strategy as ketogenic metabolic therapy.
He proposes using metabolic approaches primarily as a way to exploit differences between cancer cells and normal cells, which may be better able to adapt to using ketones and other fuels.
Modern cancer research strongly supports the idea that altered metabolism is an important feature of cancer. In fact, reprogramming cellular metabolism is now considered one of the core hallmarks of cancer.
Researchers actively study the Warburg effect, glucose metabolism, glutamine dependence, mitochondrial function, fatty-acid metabolism, and other metabolic pathways as possible ways to understand and attack cancer.
Where mainstream cancer biology differs from Seyfried is primarily in explaining what causes cancer. The conventional view does not regard cancer as simply a genetic disease or simply a metabolic disease. Cancer develops through complex interactions among genetic mutations, changes in gene regulation, abnormal cell signaling, the immune system, the tumor environment, and altered metabolism.
Genetic changes can cause cancer cells to reprogram their metabolism. At the same time, metabolic changes can influence gene expression and cell behavior. The relationship can therefore work in both directions.
Seyfried's theory goes further by proposing that mitochondrial metabolic dysfunction is the fundamental cause of cancer and that many genetic abnormalities arise later. That remains a minority view.
The important point is that the disagreement does not concern whether metabolism matters in cancer. It concerns how central metabolic dysfunction is to the origin of cancer and how effectively it can be exploited for treatment.
Even if Seyfried's theory about the origin of cancer remains controversial, the idea of targeting cancer metabolism is being actively investigated.
Researchers are studying several approaches, including ketogenic diets, fasting and time-restricted eating, lowering insulin and glucose levels, and drugs that interfere with metabolic pathways used by cancer cells.
Breast cancer is part of this research. Clinical trials have investigated ketogenic diets alongside chemotherapy or hormone therapy, and other trials are studying time-restricted eating or fasting during chemotherapy. Researchers are asking whether these approaches might improve treatment response, reduce side effects, or favorably alter insulin and other metabolic signals.
There is also growing interest in combining metabolic interventions with conventional cancer treatments rather than viewing them as replacements.
However, an important distinction remains: a plausible mechanism and encouraging laboratory results do not prove that a treatment improves survival in people. Human studies of ketogenic diets and fasting in breast cancer are still limited, and these approaches are not established treatments for breast cancer.
For now, metabolic therapies are best viewed as an important and promising area of research, particularly when used alongside established cancer treatments. The evidence is still developing, but I believe the metabolic approach is worth serious consideration and further study.
Research on ketogenic diets in breast cancer is still relatively small, but some of the human results are encouraging.
One randomized clinical trial studied 80 women with locally advanced or metastatic breast cancer receiving chemotherapy. Women assigned to a ketogenic diet for 12 weeks had lower insulin levels and greater reductions in tumor size than the control group. The researchers concluded that a ketogenic diet used alongside chemotherapy showed potentially beneficial effects and deserved further study.
Other research has found that ketogenic diets can be feasible for at least some breast cancer patients, while producing favorable changes in glucose, insulin, body weight, and other metabolic measures.
These findings are particularly interesting because insulin, glucose, and cancer metabolism are themselves active areas of cancer research. They provide some human evidence supporting the idea that changing the metabolic environment may influence breast cancer or its response to treatment.
The evidence is not yet strong enough to conclude that ketogenic diets reduce recurrence or improve survival. The studies have generally been small, and larger independent trials with longer follow-up are needed.
Research is continuing. A 2026 clinical trial at UT Health San Antonio, with the National Cancer Institute as a collaborator, is studying postmenopausal women with Stage I or II breast cancer who follow a ketogenic diet for three weeks before surgery. Researchers will examine tumor tissue to see whether the diet changes the immune environment surrounding the tumor.
This is a small exploratory study of only 12 participants rather than a trial designed to determine whether a ketogenic diet reduces recurrence or improves survival. But together with the earlier human studies, it suggests that metabolic approaches to breast cancer are a legitimate and potentially promising area of clinical research.
Fasting and fasting-mimicking diets are other ways researchers are exploring whether changing the metabolic environment can affect breast cancer and its response to treatment.
The DIRECT trial studied 131 women with Stage II or III HER2-negative breast cancer receiving chemotherapy before surgery. Women assigned to a fasting-mimicking diet around their chemotherapy treatments were more likely to show tumor shrinkage on imaging. Among women who adhered to the diet, a strong pathological response was also more likely.
A more recent randomized study found that a fasting-mimicking diet during chemotherapy was associated with fewer severe side effects, including vomiting and low white-blood-cell counts, and greater tumor response.
These studies are relatively small and do not establish that fasting improves long-term survival. But they provide additional human evidence that metabolic interventions used alongside conventional treatment may influence both treatment response and tolerability.
Early clinical studies provide some reason for optimism, while also raising important questions that larger trials need to answer.
In a randomized trial of 80 women with locally advanced or metastatic breast cancer receiving chemotherapy, a ketogenic diet lowered insulin levels and was associated with a greater reduction in tumor size than the control diet.
The DIRECT trial, involving 131 women with HER2-negative Stage II or III breast cancer, found that a fasting-mimicking diet given around chemotherapy was associated with a greater likelihood of radiologic response. Among women who followed the diet as planned, major pathological responses were also more frequent.
Other randomized research has reported encouraging results with fasting-mimicking diets during breast-cancer chemotherapy, including favorable effects on treatment response and some chemotherapy side effects.
Meanwhile, clinical trials are continuing to investigate ketogenic diets, fasting, and time-restricted eating, including studies combining these approaches with chemotherapy or hormone therapy.
The major unanswered questions are now:
Which breast cancers are most likely to respond to metabolic interventions?
Can reducing glucose and insulin make conventional treatments more effective?
Can targeting both glucose and glutamine metabolism produce stronger effects than dietary changes alone?
And most importantly, can these approaches ultimately reduce recurrence or improve survival, rather than simply changing metabolic markers or short-term tumor response?
The early clinical evidence does not yet answer those questions, but it provides enough encouraging results to justify larger and more definitive trials.
Does my breast cancer have any characteristics that suggest metabolic approaches might be particularly relevant?
Do my glucose, insulin levels, or metabolic health have any implications for my treatment?
Is there evidence that a ketogenic diet, fasting, or time-restricted eating could help alongside my treatment?
Could any of these approaches interfere with chemotherapy, hormone therapy, or other medications?
Would fasting around chemotherapy be reasonable in my situation?
How important is it for me to maintain weight and muscle during treatment?
Are there clinical trials of metabolic therapies for which I might be eligible?
If I wanted to try a metabolic approach, is there a dietitian or other nutrition professional familiar with both cancer treatment and ketogenic diets or fasting who could help me do it safely?
A recent detailed presentation of the argument that impaired mitochondrial energy metabolism plays a fundamental role in cancer and that glucose and glutamine metabolism may provide therapeutic targets.
A randomized clinical trial of 80 women with locally advanced or metastatic breast cancer receiving chemotherapy. The ketogenic-diet group had lower insulin levels and greater reductions in tumor size, providing encouraging evidence for further study.
A randomized Phase II trial of 131 women with HER2-negative Stage II or III breast cancer found that women following a fasting-mimicking diet during chemotherapy were more likely to have their tumors shrink. Among women who closely followed the diet, there was also more evidence of the cancer disappearing or substantially shrinking by the time of surgery.
A more recent randomized study found that women following a fasting-mimicking diet during chemotherapy had fewer serious side effects and were more likely to have their tumors shrink or show little remaining cancer at surgery, including fewer cases of severe vomiting and low white-blood-cell counts.
Describes current NCI-supported research examining how diet and metabolism interact with tumor biology and how these relationships might eventually be used to improve cancer treatment.