Should I Be Fasting After Cancer?

In recent years, fasting has become a hot topic in health circles. From social media influencers to bestselling books, claims are circulating that fasting could help prevent cancer recurrence or improve recovery after treatment. But what does the science actually say—and should you be doing it after cancer?

Where the Fasting Hype Comes From

The buzz around fasting and cancer has been fuelled in part by research from scientists such as Dr. Valter Longo, whose 2025 book Fasting Cancer explores the idea of fasting and “nutritechnology” in cancer prevention and treatment (Longo, 2025). Laboratory animal studies have shown that fasting cycles can slow tumour growth and make certain cancer cells more sensitive to chemotherapy drugs (Lee et al., 2012).

These findings are interesting, but it’s important to note a key detail: most of this research looks at fasting during cancer treatment—particularly chemotherapy—not after cancer treatment has finished. And more often than not, with mice, not people.

What the Research Actually Shows

Human studies so far have been small, short-term, and largely focused on whether fasting alongside chemotherapy could help with treatment side effects, tolerance, or quality of life (Sadeghian et al., 2021; Koppold-Liebscher et al., 2020; Schmidt et al., 2023). These trials are still in the early stages and have not yet led to clear, consistent recommendations.

When it comes to fasting after cancer treatment, there’s even less evidence. One 2016 study in breast cancer survivors found a potential link between prolonged nightly fasting (13+ hours) and a reduced risk of recurrence (Marinac et al., 2016), but this is an isolated finding and not yet confirmed by further large-scale trials.

A review by leading oncology nutrition experts cautioned against embracing fasting as a standard practice in cancer care until more robust research is available (Caccialanza et al., 2019).

What Trusted Cancer Organisations Say

Both Macmillan Cancer Support and Cancer Research UK do not recommend fasting as a proven strategy after cancer. They highlight that while some people may be tempted to follow “alternative cancer diets” promoted online, these are often unsupported by strong scientific evidence and could even be harmful if they lead to nutrient deficiencies (Macmillan, 2023; Cancer Research UK, 2023).

In other words, if you’re thinking about fasting after cancer, it’s best to speak with your oncology or nutrition team before making changes—especially if you’re still recovering from treatment, managing side effects, or aiming to maintain your weight and muscle mass.

The Best Non-Drug Approach We Know Works

While fasting is making headlines, the strongest evidence for reducing the risk of cancer coming back after treatment isn’t about skipping meals—it’s about moving your body.

A major international trial published in The Guardian in 2025 reported that exercise was more effective than drugs at reducing recurrence risk after treatment, particularly in certain cancer types (The Guardian, 2025). Regular physical activity helps improve immune function, maintain a healthy weight, and reduce inflammation—all important for long-term health after cancer.

The Bottom Line

Right now, the evidence for fasting after cancer is limited, unclear, and not strong enough to recommend it as a standard part of recovery. Most research focuses on fasting during chemotherapy, not after treatment, and even then results are mixed.

If you want to take a non-pharmacological step that is proven to help, focus on building regular physical activity into your week. And if you’re curious about fasting, have an open conversation with your healthcare team before trying it—so any changes support, rather than compromise, your recovery.

Check our Rosy's members' webinar talking you though the evidence and myth busting.

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References 

1. Longo, V. (2025). Fasting Cancer: How Fasting and Nutritechnology Are Creating a Revolution in Cancer Prevention and Treatment. New York: Penguin Publishing Group.(Published 4 February 2025; ISBN 978-0593545324)

2. Caccialanza, R., Aprile, G., Cereda, E. & Pedrazzoli, P. (2019) Fasting in oncology: a word of caution. [Online] Available at: https://pubmed.ncbi.nlm.nih.gov/30651606/ [Accessed 8^th August 2025]

3. Lee C, Raffaghello L, Brandhorst S, Safdie FM, Bianchi G, Martin-Montalvo A, Pistoia V, Wei M, Hwang S, Merlino A, Emionite L, de Cabo R, Longo VD. (2012) Fasting Cycles Retard Growth of Tumors and Sensitize a Range of Cancer Cell Types to Chemotherapy. [Online] Available:  https://pmc.ncbi.nlm.nih.gov/articles/PMC3608686/ [Accessed: 15 Aug 2025] 

4. Sadeghian M, Rahmani S, Khalesi S, Hejazi E. (2021) A review of fasting effects on the response of cancer to chemotherapy. [Online] Available: https://pubmed.ncbi.nlm.nih.gov/33153820/ [Accessed: 15 Aug 2025]

5. Koppold-Liebscher, D., Kessler, C.S., Steckhan, N. et al.  (2020) Short-term fasting accompanying chemotherapy as a supportive therapy in gynecological cancer: protocol for a multicenter randomized controlled clinical trial. [Online] Available: https://pubmed.ncbi.nlm.nih.gov/37538111/ [Accessed: 15 Aug 2025]

6. Schmidt MW, Brenner W, Gebhard S, Schmidt M, Singer S, Weidenbach L, Hahn H, Puzankova D, Blau-Schneider B, Lehnert A, Battista MJ, Almstedt K, Lütkemeyer A, Radsak MP, Mähringer-Kunz A, Krajnak S, Linz VC, Schwab R, Gabriel B, Hasenburg A, Anic K. (2023)  Effects of intermittent fasting on quality of life tolerance of chemotherapy in patients with gynecological cancers: study protocol of a randomized-controlled multi-center trial [Online] Available: https://pmc.ncbi.nlm.nih.gov/articles/PMC10396395 [Accessed: 15 Aug 2025]

7. Marinac CR, Nelson SH, Breen CI, et al. (2016) Prolonged Nightly Fasting and Breast Cancer Prognosis [Online] Available: https://jamanetwork.com/journals/jamaoncology/fullarticle/2506710  [Accessed: 15 Aug 2025]

8. MacMillan. (2023) Tackling food myths and cancer risk. [Online] Available: https://www.cancerresearchuk.org/about-cancer/treatment/complementary-alternative-therapies/alternative-cancer-diets [Accessed: 15 Aug 2025]

9. Cancer Research UK. (2023) Alternative cancer diets. [Online] Available: https://www.cancerresearchuk.org/about-cancer/treatment/complementary-alternative-therapies/alternative-cancer-diets [Accessed: 15 Aug 2025]

10. The Guardian (2025) Exercise ‘better than drugs’ to stop cancer returning after treatment, trial finds. [Online] Available: https://www.theguardian.com/society/2025/jun/01/exercise-better-than-drugs-after-cancer-treatment-international-trial-finds  [Accessed: 15 Aug 2025]