Fasting During Cancer Treatment

April 5, 2018
Lisa Schulmeister, MN, RN, FAAN

Oncology Nursing News, April 2018, Volume 12, Issue 3

There is not yet enough research to support a medical stance on fasting during cancer treatment despite growing patient interest in the practice.

How is your appetite?” you ask. Your patient receiving chemotherapy responds, “I’m fasting.”

Now what? Are you prepared to engage in a discussion about fasting during cancer treatment? Probably not.

How many patients fast is unknown; however, posts on online blogs and cancer support group websites indicate growing interest in fasting—and suggest that patients are not necessarily informing their healthcare providers that they are fasting during treatment. Patients who disclosed the practice found their oncologists to be dismissive, opposed to fasting, and uninformed.

Fasting is gaining popularity as a way to improve health, lose weight, and even reverse type 2 diabetes. There are different types, including water fasting (drinking only water and water-based beverages, such as tea and coffee with nothing added), dry fasting (abstaining from all food and liquid), and restrictive fasting (limiting certain foods). The duration can be continuous for a defined period (usually 24 hours but could be up to several days), intermittent, with food eaten at prescribed times during a 24-hour period, or whole-day fasting for 1 or 2 days each week.

Why the emerging interest in fasting? In findings from several animal studies, fasting appeared to both reduce toxicity and increase efficacy of chemotherapy. It also may increase the efficacy of targeted kinase inhibitors (TKIs), although its effect on TKI-related adverse effects is still being studied. The key to the protective factor observed in fasting mice may be a genetic signaling pathway called PKA/EGR1. It appears that when normal cells are deprived of nutrients, they are protected from chemotherapy while dividing cancer cells expend energy while “starving” and are then more susceptible to “attack” by chemotherapy. Also, fasting reduces protein kinase A (PKA) activation and increases AMP-activated protein kinase (AMPK) activity. These signal transduction changes and cause the activation of the early growth response protein 1 (EGR1).

Although animal trials are promising, can the results be translated to patients? In a case report of 10 patients being treated for a variety of cancers, those who fasted (water only or with vitamins) for a total of 48 to 140 hours prior to and/or 5 to 56 hours following chemotherapy reported greater tolerance to treatment and less fatigue, weakness, and gastrointestinal symptoms compared with previous nonfasting treatments. Fasting also did not appear to prevent chemotherapy-induced tumor shrinkage or affect tumor markers. Minor complaints during fasting included dizziness, hunger, and headaches at a level that did not interfere with daily activities; weight loss was rapidly recovered.1

In a 2016 study, 20 patients receiving platinum-based regimens fasted for up to 72 hours (48 hours prechemotherapy and 24 hours postchemotherapy infusion). They were instructed to consume no calories, and to drink water and noncaloric beverages; if symptoms related to fasting occurred (eg, feeling faint or weak), they could consume a small amount of juice or food (<200 kcal in a 24-hour period). There were no grade 3 toxicities attributed to fasting, and laboratory studies revealed no evidence of malnutrition. The researchers cautioned that the safety of fasting prior to chemotherapy can be extrapolated to just a selected population of patients because they excluded those with more than 10% recent weight loss, body mass index of less than 20.5, or diabetes mellitus.2

Randomized trials are needed to conclusively determine the impact of fasting on cancer treatment efficacy and adverse effects. Researchers at Mayo Clinic are enrolling patients in a clinical trial to assess the safety and feasibility of short term fasting prior to chemotherapy administration.3 They also are evaluating the effect of fasting on weight changes, determining the longest feasible fasting period prior to chemotherapy, and investigating changes in plasma glucose, insulin, insulin-like growth factor 1 (IGF-1), and IGF-1binding proteins. The trial is expected to conclude in August 2018.

Animal data and clinical findings from 2 small studies suggest that fasting during cancer treatment is not harmful and may be beneficial. Since it is easy to do—and within a person’s control—it’s understandable that patients may want to fast during treatment. But for how long should they fast, and how should fasting be timed in relation to chemotherapy? These are the great unknowns. In the ongoing Mayo Clinic trial, 1 cohort is fasting for 24 hours before day 1 of course 2 of chemotherapy. If the fast is well tolerated, they may escalate fasting by 12 hours for each subsequent course for up to 3 courses in the absence of unacceptable toxicity. A second cohort is following the longest fasting regimen found to be safe and tolerable in cohort I before day 1 of each course of chemotherapy for up to 4 courses in the absence of unacceptable toxicity.

The limited evidence suggesting a beneficial effect of fasting during cancer treatment has spawned unproven claims, such as fasting to treat cancer or prevent recurrence. Claims that fasting for 3 days “renews the entire immune system” also have been made. Unproven products, including liquids and supplements, are being marketed to “ensure” effective fasting during cancer treatment. It is, therefore, imperative that oncology nurses be informed about fasting, maintain a nonjudgmental attitude, and discuss it with their patients. Nurses need to caution patients to not fall prey to unproven claims and products.

Lastly, fasting is not appropriate for all patients receiving cancer treatment. People who have eating disorders, diabetes, or low body mass index (<20.5) or who lost more than 10% of their weight in the preceding year.2

Nurses must know how to identify the patients that may benefit and refer them to reputable resources.


  • Safdie FM, Dorff T, Quinn D, et al. Fasting and cancer treatment in humans: a case series report. Aging (Albany NY). 2009;1(12):988-1007. doi: 10.18632/aging.100114.
  • Dorff TB, Groshen S, Garcia A, et al. Safety and feasibility of fasting in combination with platinum-based chemotherapy. BMC Cancer. 2016;16:360. doi: 10.1186/s12885-016-2370-6.
  • Short-Term Fasting Before Chemotherapy in Treating Patients With Cancer. Updated November 17, 2017. Accessed February 5, 2018.