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Fasting in Integrative Oncology

November 3, 2023
Kevin Brown, MD

Fasting: Ancient Practice, Growing Awareness

Fasting is an ancient practice that has been part of various cultural and religious traditions for millennia. Many around the world believe fasting is a valuable tool for cleansing the body and promoting health. Over the past several years, the concept of fasting has gained attention across social and mainstream media, and the term “intermittent fasting” has worked its way into the lexicon of many health-conscious people in our society. The excitement around fasting has extended into the world of integrative oncology. Fasting has become an important tool for integrative practitioners and patients as we work to optimize the body’s terrain and gain every advantage against cancer.

Fasting for Cancer

The current buzz around fasting builds on decades of scientific research showing an association between lower caloric intake and longer lifespan when studied in several different animal species, as well as clinical research in humans which has revealed numerous health benefits. In addition to improving metabolic health, reducing the risk of cardiovascular disease, and facilitating weight loss for those trying to lose weight, studies have shown important cancer-related effects. Fasting interferes with cancer cell growth and function through lowering blood sugar, insulin, insulin-like growth factors, and other hormone levels, in addition to affecting other metabolic processes. It also allows normal cells to protect themselves against the toxic effects of chemotherapy and radiation therapy by downregulating certain growth factor pathways including the mTOR (mammalian target of rapamycin) pathway. Because cancer cells are sensitive to the metabolic changes induced by fasting, and because they are not able to downregulate their growth factor pathways such as mTOR in response to fasting, they become more sensitive to the effects of chemotherapy and radiation therapy in the fasted state.

Extending the Overnight Fast

The most accessible way for most people to begin to incorporate fasting into their routine is through extending the duration of the “overnight fast”, the time between finishing dinner and starting breakfast. Many people use the term “intermittent fasting” to refer to the extension of the overnight fast, but this term can refer to various types of fasting. “Time-restricted eating” may be a more useful term. A study in JAMA Oncology 1 showed that patients with a history of early-stage breast cancer who reported a regular overnight fast of at least 13 hours had a significantly lower risk of cancer relapse than those whose overnight fast was shorter than 13 hours, and there is good reason to believe that these benefits may apply to other types of cancer as well. There is no consensus among experts about the optimal duration of the overnight fast, and ongoing studies are exploring the question of whether longer overnight fasting periods may have more benefit. However, it is important to maintain a long enough eating window during the day to be able to consume adequate calories, protein, and other key nutrients.

Longer Fasting

Beyond the overnight fast, more prolonged fasting protocols ranging from one to 4-5 days are sometimes utilized in integrative oncology practice. While there are several methods of longer fasting which are practiced by different populations around the world, the best-studied protocols in the cancer care setting are water-only fasting, which allows only water and other non-caloric beverages such as black coffee and unsweetened tea, and fasting-mimicking diets (“FMD’s”), which are commercial products that provide a small number of calories daily along with important nutrients and which have been shown to mimic some of the physiological effects of fasting in the body. The most commercially available FMD product, marketed under the brand name ProLon™, consists of a 5-day low-calorie fasting-mimicking program.

Fasting and Chemotherapy

Several clinical studies have shown beneficial effects of combining fasting with chemotherapy, and there is some early evidence of benefit in combination with radiation therapy as well. Small studies of water-only fasting around the day of chemotherapy 2,3 have shown a decreased incidence of common chemotherapy side effects including low blood cell counts, mouth sores, nausea, and vomiting. A study of an FMD along with preoperative chemotherapy for breast cancer showed a statistically significant improvement in chemotherapy response in addition to reduced DNA damage to immune cells.4 While laboratory research indicates that the benefits of fasting likely apply to radiation therapy as well, studies looking to confirm this benefit in patients remain ongoing. Different fasting schedules around the day of chemotherapy have been studied. The most appropriate duration of fasting depends on the chemotherapy regimen being given as well as individual patient factors. Particularly for chemotherapy regimens that involve relatively frequent treatment schedules such as every other week or weekly, it is important to make sure that any fasting protocol being considered will allow a patient to maintain adequate nutrition.

Who Should Not Fast?

While almost everyone with cancer can benefit from prolonging the duration of the nightly fast, longer fasting is not appropriate for all patients. Those 65 years of age and older should discuss with their physician or other practitioner whether fasting is appropriate for them. Patients under 18 years of age should not fast. Others who should not fast include anyone who is pregnant or nursing, anyone who is underweight with a body mass index less than 18-19, and anyone with a current or past diagnosis of an eating disorder. Anyone taking prescription medications should discuss with their physician whether fasting is appropriate for them, as certain medications need to be taken along with food for optimal absorption. Patients with diabetes, especially those who take insulin, should discuss with their physician whether they are good candidates for fasting. Patients diagnosed with disorders of energy metabolism or energy storage (for example, glycogen storage disease, which is a rare condition) should discuss with their physician whether they can safely fast. Fasting may theoretically increase the risk of developing gallstones, although this has not been commonly seen in clinical studies. Those with a history of gallstones should discuss this risk with their physician. Staying well-hydrated may reduce the risk of developing gallstones. Some patients develop lightheadedness which tends to be mild and which often resolves with rest and hydration. Due to the risk of electrolyte imbalances, prolonged fasting should be done under the guidance of an experienced practitioner, particularly for those attempting a water-only fast of longer than 24-36 hours. Electrolyte supplementation can be a useful support for water-only fasting. FMD programs generally include some type of electrolyte replacement.


While we continue to learn more about its potential benefits in integrative oncology, fasting is clearly a powerful tool that can help optimize the body’s terrain and that can boost the effectiveness of some common conventional cancer treatments. With proper guidance, many patients can benefit from adding some form of fasting to their cancer treatment regimen.

Dr. Brown has no relevant financial conflicts of interest to report.


1 Marinac CR et al. JAMA Oncology 2016; 2(8):1049-1055.
2 Safdie FM et al. Aging 2009; 1(12):988-1007.
3 Bauersfeld SP et al. BMC Cancer 2018; 18:476.
4 de Groot S et al. Nature Communications 2020; 11:3083.

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