Short-term fasting or fasting-mimicking diets around chemotherapy may reduce some side effects and improve quality of life in selected patients, but data are still early and this should only be done under oncologist supervision. Typical studied fasting windows range from about 24 to 72 hours around each chemotherapy cycle, or very-low-calorie “fasting-mimicking” diets for 3–4 days.

Potential benefits (evidence so far):

Reduced chemo toxicity to normal tissues: Small human studies anPotential benefits (evidence so far):d multiple animal models show less hematologic toxicity (less leukopenia, better WBC lineage balance), and less organ damage (heart, liver, kidney) when fasting 24–72 hours before and after certain chemotherapy regimens.
Better tolerated side-effect profile: Narrative reviews and early clinical trials report lower global toxicity scores, fewer chemotherapy delays, and trends toward fewer hospitalizations and dose reductions in fasting groups.
Improved patient-reported quality of life and fatigue: In an ESMO-presented study in early breast cancer, patients doing very-low-calorie intake (~200 kcal/day) for 2 days before and 1 day after chemo reported better overall quality of life and less fatigue versus a plant-based control diet.
Possible enhancement of anti-tumor effect: Preclinical work suggests “differential stress resistance” (normal cells protected, cancer cells sensitized) and in some models fasting plus chemo slows tumor growth more than chemo alone.
Metabolic and immune effects: Fasting lowers insulin/IGF‑1, activates a metabolic switch to fatty acid/ketone use after about 12–16 hours, and may reprogram immune cells such as NK cells to better recognize tumors (shown so far mainly in animals).

These are promising but not definitive benefits; large phase III trials (for example DIRECT2 using fasting-mimicking diet with neoadjuvant chemotherapy in breast cancer) are ongoing to clarify actual survival and toxicity impacts.

Commonly studied fasting time windows

Different protocols exist; below are the main patterns studied, not clinical recommendations:

Water-only short-term fasting (STF):
- 24 hours before chemo, sometimes extended to 24 hours after (total 24–48 hours) in early-phase trials; tolerance was used to escalate to longer periods.clinicaltrials+1
- 48–72 hours of fasting around chemo (often starting 48–72 hours before infusion and ending shortly after) showed greater protection of hematologic parameters and lower toxicity than 24 hours in some small studies.pmc.ncbi.nlm.nih+2
Extended modified fasting in case series:
- Some patients voluntarily fasted 48–140 hours before and 5–56 hours after chemo (water or very-low-calorie), with feasibility and no major fasting-related adverse events reported, but this is uncontrolled data.pmc.ncbi.nlm.nih
Fasting-mimicking diet (FMD), low-calorie rather than zero calories:
- DIRECT-type protocols in breast cancer: ~600 kcal/day, low-protein, plant-based for 3 days before and on the day of chemo (3–4 days total) each cycle.nature+1
- Other trials: FMD on days −2, −1, 0, and +1 around chemo-immunotherapy, followed by a transition diet, repeated each cycle.frontiersin+1
Very-low-calorie “short-term fasting” with juices/broth:
- About 200 kcal/day for 36–48 hours before chemo and 24 hours after (roughly 3 days total), repeated for the first several cycles; improved quality of life and fatigue in breast cancer patients in one trial.ecancer+1
Intermittent/alternate-day fasting and time-restricted eating:
- Experimental protocols use alternate-day fasting (0–25% of energy needs on fasting days) or fasting windows extended toward 72 hours, based on the idea that a metabolic switch to fat/ketones occurs after 12–16 hours and may need to be repeated around treatments.pmc.ncbi.nlm.nih+2

Key caveats and safety points

NKey caveats and safety points
ot standard of care: Major cancer centers emphasize that fasting during chemotherapy remains investigational; survival benefit is unproven and trials are ongoing.mdanderson+2
Not suitable for everyone: Underweight patients, those with cachexia, uncontrolled diabetes, significant comorbidities, or prior eating disorders are typically excluded from fasting trials.breastcancer+2
Must be individualized: If considering fasting or an FMD, it is essential to discuss with the oncology team, as regimen type, drug schedule, and nutritional status all matter.cedars-sinai+1

If you share the specific chemo regimen and baseline nutritional status (e.g., BMI, any weight loss, diabetes), more tailored discussion of which studied windows are even theoretically relevant can be provided, but any actual implementation still needs oncologist approval.

Omar, Enas M., et al. “Intermittent Fasting during Adjuvant Chemotherapy May Promote Differential Stress Resistance in Breast Cancer Patients.” Journal of the Egyptian National Cancer Institute, vol. 34, no. 1, 2022, p. 38, doi:10.1186/s43046-022-00141-4.

Lee, Changhan, et al. “Reduced Levels of IGF-I Mediate Differential Protection of Normal and Cancer Cells in Response to Fasting and Improve Chemotherapeutic Index.” Cancer Research, vol. 70, no. 4, 2010, pp. 1564–72, doi:10.1158/0008-5472.can-09-3228.

Barradas, Marta, et al. “Fatty Acids Homeostasis during Fasting Predicts Protection from Chemotherapy Toxicity.” Nature Communications, vol. 13, no. 1, 2022, p. 5677, doi:10.1038/s41467-022-33352-3.

Gruil, Nadia de, et al. “Short-Term Fasting Synergizes with Solid Cancer Therapy by Boosting Antitumor Immunity.” Cancers, vol. 14, no. 6, 2022, p. 1390, doi:10.3390/cancers14061390.

Bahrami, Alireza, et al. “Fasting Mimicking Diet during Neo-Adjuvant Chemotherapy in Breast Cancer Patients: A Randomized Controlled Trial Study.” Frontiers in Nutrition, vol. 11, 2024, p. 1483707, doi:10.3389/fnut.2024.1483707.

Fatima G, Mehdi A A, Fedacko J, et al. (March 29, 2025) Fasting as Cancer Treatment: Myth or Breakthrough in Oncology. Cureus 17(3): e81395. doi:10.7759/cureus.81395