Intermittent Fasting

Many integrative oncology clinicians advise following this circadian rhythm practice of fasting from after an early-evening dinner until breakfast.

Fasting every day is part of our normal circadian rhythm, occurring  naturally when we sleep. The first meal of the day is called breakfast (break fast) for this reason.

Many integrative oncology clinicians—including BCCT advisors Keith Block, Lise Alschuler and Dwight McKee—advise following this circadian rhythm practice of fasting from after an early-evening dinner until breakfast. A fast between 13 and 16 hours is considered optimal. An investigation of whether duration of nightly fasting predicted recurrence and mortality among women with early-stage breast cancer found that those who fasted less than 13 hours each night had an increased risk for breast cancer recurrence compared with fasting 13 or more hours per night.1

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What is eaten when not fasting is also important and can impact your resilience, your body terrain and tumor microenvironment, your wellness and health, your response to challenging treatments and your quality of life. See Eating Well.

Treating the Cancer

Fasting Terms

Fasting: ingesting no or minimal foods or caloric beverages, typically lasting 12 hours to three weeks

Fasting-mimicking diet (FMD): a plant-based diet designed to achieve many of the biological effects of fasting while minimizing the burden and adverse effects of fasting.

Short-term starvation (STS): a fast consisting of ingesting only water for 24 to 72 hours

Starvation: a chronic nutritional insufficiency or extreme form of fasting

Differential stress resistance: protecting normal cells but not cancerous cells from chemotherapy and other stressors

Differential stress sensitization: killing cancer and other damaged cells but not normal cells

Adapted from Buono R, Longo VD. Starvation, stress resistance, and cancer. Trends in Endocrinology and Metabolism. 2018;29(4):271–280.

Working against cancer growth or spread, improving survival, or working with other treatments or therapies to improve their anticancer action

Clinical Evidence

Intermittent fasting may provide many of the same benefits as calorie restriction without the high level of challenge.

Research suggests that periodic fasting around the time of chemotherapy may sensitize cancer cells to chemotherapy, while protecting normal cells in patients with HER2-negative, stage 2/3 breast cancer.4 A further investigation found that a fasting mimicking diet (FMD) for three days prior to and during neoadjuvant chemotherapy led to radiologically complete or partial response more often than a regular diet. Moreover, no difference in toxicity was seen between both groups, even though dexamethasone was omitted in the FMD group. FMD significantly curtailed chemotherapy-induced DNA damage in T-lymphocytes.5

A number of clinical trials are underway to see if these early findings from small studies are replicated.6

Highlighted Video

BCCT advisor Brian Bouch, MD, explains the benefits of fasting before chemotherapy.

Lab and Animal Evidence

In animal studies, intermittent fasting has been as effective as chemotherapy in delaying the progression of a wide range of cancers and can protect normal cells from the toxic effects of chemotherapy drugs while sensitizing cancer cells to the treatment.7

  • Short-term starvation protected mice but not injected neuroblastoma cells against a high dose of the pro-oxidant chemotherapy drug etoposide.8
  • Fasting-mimicking diet showed a synergistic effect with vitamin C in delaying tumor progression in mice with colorectal cancer with the KRAS gene mutation.9
  • Reduced tumor progression in mice with complete fasts of one to two days or alternating fasting and non-fasting days.10
  • Starvation enhanced the effect of virus-mediated cell killing in colorectal cancer cells while protecting normal colon cells.11
  • Alternate-day fasting inhibited tumor growth in mice without causing weight loss.12

Managing Side Effects and Promoting Wellness

Managing or relieving side effects or symptoms, reducing treatment toxicity, supporting quality of life or promoting general well-being

Clinical Evidence

  • Periodic fasting around the time of chemotherapy may reduce chemotherapy side effects (vomiting, diarrhea, fatigue and weakness).13
  • Short-term fasting in patients with breast and ovarian cancer improved quality of life and fatigue during chemotherapy.14
  • Increased protection against stressors including toxics in patients who fasted for 48 hours or longer around the time of platinum-based chemotherapy.15
  • Short-term fasting limited weight loss and toxicity to the heart and cardiovascular system related to chemotherapy.16
  • Reduced DNA damage in white blood cells (leukocytes) in patients who fasted for 48 hours or longer around the time of platinum-based chemotherapy17
  • Preliminary evidence (not yet peer-reviewed) with ovarian cancer patients found that a 48-hour water-only fast at the time of each chemotherapy cycle was well tolerated without increasing weight loss, hospital admissions or chemotherapy dose reduction or delays. Patients did not see an improvement in quality of life, but the fasting patients needed only half as many treatment modifications as the nonfasting group.18

Lab and Animal Evidence

  • Protected mice against irinotecan side effects19
  • Mice pretreated with short-term starvation showed no visible signs of stress or pain after chemotherapy treatment with etoposide. Treated mice also regained most of the fasting weight loss during chemotherapy, while untreated mice lost weight during the chemotherapy treatment.20
  • Short-term fasting protected normal cells from the toxic effects of chemotherapy drugs while sensitizing cancer cells to the treatment.21
  • Multiple cycles of fasting reduced the immunosuppression and mortality caused by chemotherapy and promoted regenerative effects on stem cells in cell and animal studies.22

Reducing Risk

Reducing the risk of developing cancer or the risk of recurrence

Women with early-stage breast cancer who fasted less than 13 hours each night had an increased risk for breast cancer recurrence compared with fasting 13 or more hours per night.23

Optimizing Your Terrain

Creating an environment within your body that does not support cancer development, growth or spread

  • Antioxidant and anti-inflammatory24
  • Altered growth factors and metabolite levels, reducing the capability of cancer cells to adapt and survive;25 similar effects can be achieved with a fasting-mimicking diet (FMD)26
  • Promoted cell self-clearing (autophagy); similar effects can be achieved with a fasting-mimicking diet (FMD)27  
  • Women with early-stage breast cancer who fasted 13 hours or more each night showed longer sleep duration and lower hemoglobin A1c levels, both of which are associated with better outcomes.28
  • A small study of people with metabolic syndrome with a baseline 14-hour or more eating window each day switched to a consistent self-selected 10-hour window, with 14 straight hours of fasting each day. Outcomes potentially related to cancer:29
    • Reduced weight
    • Reduced waist circumference
    • Reduced blood pressure
    • Reduced LDL and non-HDL cholesterol
    • Reduced hemoglobin A1c
    • Increased restful sleep


In a case series, limited intermittent or periodic fasting was used safely in patients undergoing chemotherapy to reduce side effects of treatment and did not prevent the chemotherapy-induced reduction of tumor volume or tumor markers.30

Some researchers advise that the risk of malnutrition and sarcopenia (loss of muscle tissue) during fasts warrant caution.31

Intermittent fasting has not been well studied for use by people with type 2 diabetes. The few completed studies indicate that some intermittent fasting regimens cause wide fluctuations in blood sugar, which is suspected of leading to greater risk of problems such as retina damage or heart disease in patients with type 2 diabetes.32 Those with type 2 should involve your physician before undertaking any fasting regimen.

Naturopathic oncologist and BCCT advisor Lise Alschuler notes that "a prolonged fast is not an appropriate detox program for someone just completing chemo." 


Naturopathic oncologist and BCCT advisor Lise Alschuler, ND, recommends overnight fasting for 13 hours, as this has been associated with improved survival after a diagnosis of breast cancer. For instance, you could finish dinner at 7pm and eat nothing else until 8am the next morning when you "break fast.” In addition, for people having significant side effects, especially gastrointestinal, from chemo, Dr. Alschuler may also recommend fasting for 48 hours—from after dinner on the day before chemo, through the day of chemo and the day following chemotherapy. The chemo fast can be a water fast (which includes coconut water and vegetable broths), or you can eat up to 600 calories per day of vegetable soup and/or low-carb vegetables. She stresses the importance of your being motivated to fast, and also that fasting during chemotherapy should be cleared with your treating oncologist. You should modify or stop the fast if you become dizzy or weak (try adding boiled eggs or nuts), or if you feel worse than if you had eaten.

Integrative oncologist and BCCT advisor Dwight McKee, MD, has advised some of his chemotherapy patients to follow intermittent fasting during chemotherapy. “I have had some patients who nearly died from their first cycle of treatment, but then fasted for three days with their second cycle and breezed through it.”33

Integrative oncologist and BCCT advisor Keith Block, MD, and colleagues wrote, “Patients who have low BMI, who have lost more than 10 percent of body weight, or who do not regain at least 25 percent of weight loss between treatment cycles should not fast.”34

Written by Laura Pole, RN, MSN, OCNS, and Nancy Hepp, MS; most recent update on November 5, 2020.

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