Not So Fast! Examining the Emerging Research on Fasting and Cancer


Dating back 2,400 years ago to the days of Hippocrates, fasting was proposed as a potential treatment for chronic illnesses. This was based on the idea that along with a fever, loss of appetite during illness may be a natural and beneficial response of the body. In recent years, fasting has gained renewed attention from the medical and scientific communities as a strategy for promoting overall health, longevity, and enhancement of cancer treatment. 

Fasting is when you intentionally avoid food and beverages (other than water) or consume very little food for cultural or religious practices. Recent science has shown that fasting may increase longevity, reduce cardiovascular disease risk, promote weight loss, improve inflammatory markers, and improve mental clarity. Furthermore, research on animal models suggests that fasting may help to reduce tumor growth, increase the effectiveness of chemotherapy treatments, and alleviate cancer-related side effects. However, more long-term human studies are needed to confirm these findings.

What’s the difference between fasting and dieting for weight loss?

The easiest way to understand the difference is this: 

Most diets focus on telling you what to eat, not when to eat. 

A fasting protocol may tell you the time you can eat, but not what you are allowed to eat. 

While fasting has been incorporated into many weight loss diets, not all diets involve fasting.

What are the different types of fasting?

Periodic Fasting: This is a 2 to 21 day fasting period that can be used for dietary or religious purposes for holidays like Ramadan and Yom Kippur. 

Intermittent Fasting: 24-48 hour fasts with days of regular eating in between. This also includes Alternate Day Fasting where one fasts every other day for 24 hours. 

Fasting Mimicking Diet: This is a diet designed to mimic a calorie restricted diet. This entails purchasing meal replacements to eat 5 days a week and is a medically supervised protocol.

Time Restricted Feeding: Prolonged nightly fasts spanning 10-16 hours overnight. For example, if you stop eating at 8 pm, you would consume your first meal between 6 am and 12 pm depending on how long you are fasting. 

When is fasting appropriate?

It’s worth noting that fasting is only appropriate for individuals who maintain a healthy BMI and follow a nutrient-dense diet during their eating windows, as fasting may lead to malnutrition and dangerous weight loss, especially for those who have already experienced cancer-related weight loss. Therefore, it is essential to consult with a registered dietitian or your care team before considering fasting as a lifestyle change during cancer treatment. Reach out to one of our registered dietitians at OncoPower to see if fasting might be right for you. 

What does the research say?

Overweight and obesity is a risk factor linked to at least 13 cancers including colorectal, ovarian, breast, and pancreatic cancers. The exact mechanisms by which obesity increases cancer risk are not fully understood, but it is thought that several factors may contribute including chronic inflammation, hormonal imbalances, changes in metabolism, immune system dysfunction, and increased oxidative stress. Several studies have suggested that fasting may be beneficial for weight loss and, therefore, may reduce the risk of cancer and cancer recurrence by decreasing these pro-cancerous risk factors. 

Additionally, a study conducted out of San Diego examined how prolonged nightly fasting affected cancer recurrence and risk of death from cancer in breast cancer patients. The study found that fasting for 13 hours overnight was associated with a 36% reduction in breast cancer recurrence compared to those who did not fast for longer than 13 hours overnight. However, the results also showed that nightly fasts of less than 13 hours were not significantly associated with a higher risk of death from breast cancer or risk of death from all-cause mortality. This research is promising, but more randomized controlled trial studies are needed to verify the validity of the study findings before these suggestions can be made to everyone. 

Chemotherapy can damage DNA in both cancerous and normal cells, leading to uncomfortable side effects such as vomiting, diarrhea, fatigue, and loss of appetite. In a 2018 study, researchers investigated the effects of short-term fasting on quality of life and tolerance to chemotherapy in patients with breast and ovarian cancer. Specifically, the researchers looked at changes in quality of life, fatigue, and tolerance to chemotherapy in response to a 24-hour fast before chemotherapy compared to a non-fasting control group. The fasting group followed a protocol of a 24-hour fast before each chemotherapy session, while the control group followed their usual diet throughout the study period. The results of the study showed that the fasting group had less fatigue, better physical and emotional well-being, and improved tolerance to chemotherapy compared to the control group. 

In addition to fasting prior to chemotherapy to help with side effects, the fasting mimicking diet (FMD) may also be a promising strategy for improving the efficacy of chemotherapy and enhancing the immune system’s ability to fight cancer. A study published in 2016 found that a FMD was effective in reducing the growth of tumors and increasing the efficacy of chemotherapy in mice with cancer. The FMD was found to reduce the expression of the protein HO-1, which is known to suppress the immune system and promote tumor growth. In addition, the FMD increased the activity of T cells, a type of immune cell that plays a crucial role in destroying cancer cells. The FMD enhanced the ability of T cells to recognize and eliminate cancer cells, leading to improved tumor clearance.

This exciting preliminary research suggests that periodic fasting may have the potential to improve cancer patients’ comfort levels during and after chemotherapy treatment, as well as enhance the effectiveness of treatments. However, further research on human subjects is necessary to confirm these beneficial effects on cancer survivorship and treatment efficacy. Currently, there are ongoing studies (1,2,) that are actively recruiting participants to evaluate the use of short-term fasting as a means of improving chemotherapy and radiation treatments. For more information on open trials currently recruiting, sign up for OncoPower, and we can help match you to a clinical trial, absolutely free! 

Looking to learn more or experience the potential benefits of fasting? Our expert Registered Dietitians at OncoPower can provide you with personalized recommendations tailored to your unique needs and goals. Reach out to us today to explore the potential benefits of fasting and discover how we can support you through treatment and recovery. 

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