For most of history, people have fasted when they were sick. Today, short periods of fasting around chemotherapy are being tested in clinical trials in Germany, the Netherlands, Italy and the United States, with two goals: to protect healthy cells from the side effects of chemotherapy, and to make cancer cells more vulnerable to it. This article explains how fasting may work, what the trials show, who should not fast, and how it is done safely.
How Fasting May Help
When food is withheld, healthy cells slow their growth and switch into a protective, repair-focused state. Cancer cells usually cannot, because the mutations that drive them keep their growth signals switched on. Researchers at the University of Southern California called this differential stress resistance: in their laboratory studies, fasting protected normal cells, but not cancer cells, from high-dose chemotherapy.1 Fasting also lowers blood sugar, insulin and the growth hormone IGF-1, which many cancers depend on. See our article on the Warburg effect.
Because a complete water fast is hard to sustain, researchers developed the fasting-mimicking diet: a low-calorie, low-protein, plant-based diet of vegetable soups, olives, nuts and herbal teas, eaten for a few days, that produces many of the same effects in the body.
What the Clinical Trials Show
Germany: better quality of life. At the Charité university hospital in Berlin, 34 women with breast or ovarian cancer fasted for about 60 hours around some chemotherapy cycles and ate normally around others. During the fasting cycles, they reported better quality of life and less fatigue.2
The Netherlands: stronger tumor response. In the Dutch DIRECT trial of 131 women receiving chemotherapy before breast cancer surgery, those who followed a fasting-mimicking diet for three days before and on the day of each treatment were about three times more likely to have their tumors shrink on scans and about four times more likely to have a strong response in the tissue removed at surgery. Side effects were not reduced, and many women found it hard to follow the diet for every cycle.3
Italy: safe, and a boost to immunity. At the National Cancer Institute in Milan, Claudio Vernieri’s team gave a five-day fasting-mimicking diet to 101 patients receiving many kinds of cancer treatment. It was safe, lowered blood sugar by 18.6 percent, insulin by 50.7 percent and IGF-1 by 30.3 percent, and increased the immune cells that attack tumors.4 In a later analysis of women with advanced triple-negative breast cancer, those who added the diet to chemotherapy lived a median of 30.3 months, compared with 17.2 months, although that comparison was not randomized.5
Hormone therapy. In laboratory and early clinical studies, the fasting-mimicking diet made hormone therapy for breast cancer more effective.6
The limits. A U.S. randomized trial of 121 patients presented in 2026 could not reach a firm conclusion, partly because fewer patients enrolled than planned.7 Most fasting studies so far are small, and larger trials are under way, including a German trial of daily time-restricted eating during chemotherapy.
Who Should Not Fast
Fasting is not for everyone. It is not used by people who are underweight, losing weight, or losing muscle; people with diabetes treated with insulin or other drugs that lower blood sugar, because of the risk of dangerously low blood sugar; women who are pregnant; and anyone with a history of an eating disorder. Weight and blood sugar should be monitored throughout.
How It Is Done in the Trials
In most trials, the fast or fasting-mimicking diet begins one to three days before each chemotherapy treatment and continues until about a day after it. Water, herbal teas and clear vegetable broth are allowed. Between treatments, patients return to normal eating, with enough protein to protect muscle. See our article on the ketogenic diet and cancer and our article on integrative breast cancer treatment.
How Cancer Expert MD Helps
Cancer Expert MD provides education and consultation for people with every type and stage of cancer, and specializes in Stage IV and metastatic cancer. Our book, The New Reality of Cancer, covers fasting, the ketogenic diet and the other metabolic approaches in depth, including what to eat and how to combine them with treatment.
Frequently Asked Questions
Can fasting reduce chemotherapy side effects?
In a German trial, women reported better quality of life and less fatigue during cycles when they fasted. In the larger Dutch DIRECT trial, side effects were not reduced, but tumors responded better.
What is a fasting-mimicking diet?
A low-calorie, low-protein, plant-based diet of soups, olives, nuts and herbal teas, eaten for a few days, that produces many of the effects of a water fast while being easier to follow.
Does fasting make chemotherapy work better?
In the Dutch DIRECT trial, breast tumors were about three times more likely to shrink on scans with a fasting-mimicking diet. Larger trials are needed to confirm the effect on survival.
Who should not fast during chemotherapy?
People who are underweight or losing weight, people with diabetes on insulin or other blood-sugar-lowering drugs, pregnant women, and anyone with a history of an eating disorder.
About the author. Dr. Silver, MD, graduated from Temple University School of Medicine, trained as an interventional cardiologist, and has practiced medicine for nearly fifty years. A lymphoma survivor of about twenty-five years, he is co-author, with Dr. Lindmeier, DHSc, of The New Reality of Cancer: The Science of Recovery. Read more about Dr. Silver, MD.
Research by Dr. Lindmeier, DHSc, cancer educator and co-author of The New Reality of Cancer.
This article is for general education only and is not individual medical advice. Always consult your own doctor before making any changes to your care.
© 2026 Cancer Expert MD, Dr. Silver, MD, and Dr. Lindmeier, DHSc. All rights reserved. This article draws on the research in The New Reality of Cancer and may not be copied, republished or distributed without written permission.
For more in-depth information on this topic and many others, get our book, The New Reality of Cancer: The Science of Recovery.
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References
- Raffaghello L, Lee C, Safdie FM, et al. Starvation-dependent differential stress resistance protects normal but not cancer cells against high-dose chemotherapy. Proceedings of the National Academy of Sciences. 2008;105(24):8215–8220.
- Bauersfeld SP, Kessler CS, Wischnewsky M, et al. The effects of short-term fasting on quality of life and tolerance to chemotherapy in patients with breast and ovarian cancer: a randomized cross-over pilot study. BMC Cancer. 2018;18:476.
- de Groot S, Lugtenberg RT, Cohen D, et al. Fasting mimicking diet as an adjunct to neoadjuvant chemotherapy for breast cancer in the multicentre randomized phase 2 DIRECT trial. Nature Communications. 2020;11:3083.
- Vernieri C, Fucà G, Ligorio F, et al. Fasting-mimicking diet is safe and reshapes metabolism and antitumor immunity in patients with cancer. Cancer Discovery. 2022;12(1):90–107.
- Ligorio F, et al. Adding fasting-mimicking diet to first-line carboplatin-based chemotherapy is associated with better overall survival in advanced triple-negative breast cancer patients. International Journal of Cancer. 2024.
- Caffa I, Spagnolo V, Vernieri C, et al. Fasting-mimicking diet and hormone therapy induce breast cancer regression. Nature. 2020;583(7817):620–624.
- A randomized phase II clinical trial of a fasting-mimic diet prior to chemotherapy to evaluate the impact on toxicity and efficacy. Journal of Clinical Oncology. 2026;44(16 suppl):e12522.
This article is for education only. It is not medical advice and does not diagnose or treat any condition. Cancer Expert MD has no financial relationship with any laboratory, drug maker, supplement company, clinic or hospital.
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