Is intermittent fasting safe during chemotherapy?
Yes, multiple studies confirm that intermittent fasting is safe and tolerable for cancer patients undergoing chemotherapy. A 2025 systematic review of nine clinical studies (mostly in breast cancer patients) found that fasting interventions were feasible and did not cause serious harm [2]. In a 2022 trial of 48 breast cancer patients, those who fasted 18 hours a day for three days around each chemotherapy cycle tolerated the regimen well, with no increase in dangerous side effects [4]. A 2021 review in CA: A Cancer Journal for Clinicians also concluded that preliminary studies show prolonged fasting is safe in some cancer patients [3].
Does intermittent fasting reduce chemotherapy side effects?
The evidence suggests intermittent fasting can reduce certain chemotherapy side effects, especially gastrointestinal (GI) toxicity. In the 2022 breast cancer trial, the fasting group had significantly less GI toxicity (nausea, vomiting, diarrhea) compared to the non-fasting group [4]. A 2024 review of the literature noted that some promising clinical studies indicate IF may improve fatigue and reduce GI toxicities in certain patients [5]. However, the 2025 systematic review found that across all nine studies, no consistent reduction in chemotherapy-related toxicities was demonstrated, meaning the effect is not guaranteed for everyone [2].
Does intermittent fasting improve chemotherapy outcomes?
There is currently no strong evidence that intermittent fasting improves chemotherapy outcomes like tumor shrinkage or survival. The 2025 systematic review explicitly states that no impact on treatment outcomes was shown across the nine studies analyzed [2]. The 2021 CA review echoes this, noting that the effects of IF on human cancer prognosis remain unknown due to a lack of high-quality randomized trials [3]. While some metabolic benefits have been observed—such as lower insulin and glucose levels in fasting patients [4]—these have not yet translated into proven better cancer outcomes. Experts recommend that patients undergoing active cancer treatment should not fast outside of a clinical trial [3].
About These Sources
This answer is built on 5 peer-reviewed studies — published from 2021 to 2025, 3 from 2024 or later, 4 in Q1 journals, collectively cited 218 times — selected as the most relevant from 6 studies that passed quality screening, drawn from 48 papers retrieved from a database of over 500 million.
Sources used in this answer
Time-Restricted Eating in Adults With Metabolic Syndrome
In a randomized controlled trial of 108 adults with metabolic syndrome, time-restricted eating (8–10 hour window) modestly improved HbA1c by 0.10% compared to standard care alone, but this study did not involve cancer patients.
Impact of intermittent fasting on patients with cancer undergoing chemotherapy and/or targeted therapies: a systematic review of the literature
This 2025 systematic review of nine studies (mostly breast cancer) found intermittent fasting is safe and feasible during chemotherapy, but no impact on treatment outcomes or chemotherapy toxicities was demonstrated; a trend toward lower insulin and IGF-1 levels was noted.
Intermittent fasting in the prevention and treatment of cancer
This 2021 review concluded that intermittent fasting is safe in some cancer patients and may decrease chemotherapy toxicity, but effects on cancer prognosis are unknown; the authors do not recommend fasting outside clinical trials for patients on active treatment.
Intermittent fasting during adjuvant chemotherapy may promote differential stress resistance in breast cancer patients
In a trial of 48 breast cancer patients, intermittent fasting (18 hours/day for 3 days around chemo) reduced gastrointestinal toxicity and improved metabolic profiles (lower insulin levels) compared to non-fasting, with no difference in hematologic parameters.
Intermittent fasting and its impact on toxicities, symptoms and quality of life in patients on active cancer treatment
This 2024 review found that intermittent fasting may improve fatigue and reduce gastrointestinal toxicities in some cancer patients, and may lower hyperglycemia and fat mass, but larger controlled studies are needed to confirm these benefits.