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Fasting During Cancer Treatment: What Does the Science Say?

You might have heard of fasting and other forms of dietary restrictions being promoted as a way to address side effects of cancer treatment and boost the body’s immunity. Different fasting regimens and supplements have been sold by websites and companies despite a lack of science supporting their use [i].

We know that some cancers, such as breast cancer, are sensitive to body fat composition and insulin sensitivity; thus, weight loss and fat loss through fasting have been proposed to help combat them [ii]. But is fasting safe, and does it actually support your body while you are undergoing cancer treatment? We review the science behind different types of fasting as they stand right now. 

Fasting: What is it?

Fasting implies a complete restriction of caloric intake [iii]; typically, only water and non-caloric beverages (such as coffee or tea without sugar or milk) would be allowed. 

Other forms of fasting-adjacent diets include:

Intermittent fasting: Refers to any alternating period of fasting and normal eating. There are different variations, such as: alternating eating days and complete fasting days, fasting on 1 or 2 days a week, or restricting eating window by hours in a day [iv]. Here, when you eat matters more than what you eat. 

Fasting-mimicking diets: Refers to very low-calorie diets (around 300-1100 calories per day), typically low in carbohydrates, low in protein, and high in unsaturated fat, designed to give you a modest supply of energy while creating a condition similar to fasting [v].

Night fast: Refers to the period of no caloric intake at night, typically from your last meal the day before to the first meal the next morning. Researchers can quantify this as the number of hours after subtracting the time between your first and last meal of the day from 24 hours [vi].

Fasting: How does it work?

The body has 3 main sources of energy, corresponding to the 3 main macronutrients: carbohydrate, fat, and protein. In simple terms, our body uses these three energy sources in order of preference, as follows: 

Carbohydrate (from bread, rice, oatmeal, sugar, etc.) is the preferred source of energy, and is the only source of energy that our liver and red blood cells can use. 

Once we use up the carbohydrate from our meals and the carbohydrate we store, our body begins to break down fat and convert it to ketones for energy. When your body uses ketones as the main source of energy, you are in ketosis. (If this sounds familiar, you probably have heard about this in regard to the ketogenic diet).

Once we use up our fat stores, muscle begins to be broken down into amino acids, which will then be converted to sugar molecules. This is the last resort, as we also need muscles for our daily activities (our heart is a muscle, so is our diaphragm which helps us breathe, and so are the skeletal muscles that help us get around). Therefore, we want to preserve muscle mass, or lean body mass, especially in times of illness such as cancer. Muscle mass can be used as an indicator of nutrition status. 

Fasting: Why are we interested?

Cancer cells and normal cells use energy differently, a concept scientists call the Warburg effect [iv, vii, viii]. Fasting forces cells to shift their energy usage, and this process may make normal cells more resilient while rendering cancer cells more vulnerable to damage by chemotherapy and radiation [iii, vii, viii]. This is the basis behind the hypotheses that fasting can have a beneficial effect in cancer patients. 

Fasting: What does the science say?

Small-scale human studies found that fasting, especially in the period after chemotherapy, can help reduce symptoms and promote cellular repair. Particularly, fasting in the few days after chemotherapy was observed to reduce fatigue, nausea, vomiting, diarrhea, abdominal pain, and mouth sores. Markers of cellular damage also appeared lower in patients who fasted after chemotherapy [vii]. It is important to note that these associations were observed in small samples of patients, and it is unclear whether it is fasting that causes these favorable outcomes. 

Research into intermittent fasting in cancer is still in its preclinical stages, mostly just in rodents and other models [ii]. Different rodent studies have found conflicting results, some even pointing to negative outcomes with regards to cancer. There is insufficient data to assume that it could be helpful in cancer, aside from being a tool for weight loss [iv].

Regarding fasting-mimicking diets, human studies found that they do not seem to interfere with treatment and can be safe and tolerated when carried out between chemotherapy treatment cycles. Favorable changes in metabolites and hormones are observed, but the data is insufficient to conclude that it can positively impact cancer treatment [ix].

Observational studies into night fasting found that women who are able to have a longer night fast have better control of their blood glucose [x] and a decreased risk of breast cancer recurrence [vi]

Fasting: Risks and side effects

It’s important to note that these restrictions carry risks: dietary restrictions can lead to malnutrition, sarcopenia (loss of muscle mass), or cachexia in cancer patients [i, v]. Early human studies typically exclude participants who are underweight or at risk of malnutrition [i], so the current data regarding these diets’ safety might not apply to them. Some participants in fasting studies have also had weight loss, some regained the weight, and some had to drop out of the study because of the loss [ix].

Compliance is another concern; studies have reported dropouts as participants are unable to follow or tolerate the diet. It can be hard to consistently follow a restrictive diet. Dietitians typically recommend patients to find a dietary pattern that is easy to maintain long-term. 

Fasting: Should we do it?

The short answer: probably not.

There is not enough research to support the use of fasting for cancer patients. Research on fasting, intermittent fasting, and fasting-mimicking diets for individuals with cancer is still in its infancy, and there is heightened concern about its impact on weight and muscle mass and potential interaction with treatment. We need to know what types of cancer, at what stages, and with what individual condition fasting is safe [i].

Cancer patients have increased energy and protein needs. It is important to obtain enough energy and prevent the body from resorting to muscles for energy needs. We should also remember that in addition to providing us with nutrients, food can also be a source of joy and culture, and a way to connect with our family and community. This can be especially helpful during times of increased psychological stress, such as during cancer treatment.

The American Cancer Society recommends eating plenty of food to accommodate the increased needs during treatment, as well as maintaining physical activity to lessen the side effects. For more recommendations, click here from the American Cancer Society.


[i] Caccialanza, R., Cereda, E., De Lorenzo, F. et al. To fast, or not to fast before chemotherapy, that is the question. BMC Cancer (2018) 18(1):337. doi: 10.1186/s12885-018-4245-5

[ii] Piersol, B. (2020). Intermittent fasting and breast cancer: What you need to know. Memorial Sloan Kettering Cancer Center. Retrieved September 16, 2022, from https://www.mskcc.org/news/intermittent-fasting-and-breast-cancer-what-you-need-know

[iii] Cancer and Fasting/Calorie Restriction. UCSF Osher Center for Integrative Health. Retrieved September 16, 2022, from https://osher.ucsf.edu/patient-care/integrative-medicine-resources/cancer-and-nutrition/faq/cancer-and-fasting-calorie-restriction

[iv] Clifton, K.K., Ma, C.X., Fontana, L., Peterson, L.L. Intermittent fasting in the prevention and treatment of cancer. CA Cancer J Clin (2021) 71:527–546. doi: 10.3322/caac.21694

[v] Nencioni, A., Caffa, I., Cortellino, S., Longo, V.D. Fasting and cancer: Molecular mechanisms and clinical application. Nat Rev Cancer (2018) 18(11):707-719. doi: 10.1038/s41568-018-0061-0

[vi] Marinac, C.R., Nelson, S.H., Breen, C.I. et al. Prolonged nightly fasting and breast cancer prognosis. JAMA Oncol (2016) 2(8):1049-55. doi: 10.1001/jamaoncol.2016.0164

[vii] Plotti, F., Terranova, C., Luvero, D. et al. Diet and chemotherapy: The effects of fasting and ketogenic diet on cancer treatment. Chemotherapy (2020) 65(3-4):77-84. doi: 10.1159/000510839.

[viii] de Groot, S., Pijl, H., van der Hoeven, J.J.M. et al. Effects of short-term fasting on cancer treatment. J Exp Clin Cancer Res (2019) 38, 209. doi: 10.1186/s13046-019-1189-9

[ix] Vernieri, C., Fucà, G., Ligorio, F. et al. Fasting-mimicking diet is safe and reshapes metabolism and antitumor immunity in patients with cancer. Cancer Discov (2022) 12 (1): 90–107. doi: 10.1158/2159-8290.CD-21-0030[x] Marinac, C.R., Natarajan, L., Sears, D.D. et al. Prolonged nightly fasting and breast cancer risk: Findings from NHANES (2009-2010). Cancer Epidemiol Biomarkers Prev (2015) 24 (5): 783–789. doi: 10.1158/1055-9965.EPI-14-1292

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