Diets in Oncology
Evidence-based dietary patterns relevant to cancer prevention and care — what research actually supports, without miracle claims. Diet supports treatment; it never replaces it.
Fasting and fasting-mimicking diets during chemotherapy
In carefully selected patients, fasting during chemotherapy has proved safe and feasible, but it has not been shown to improve treatment outcomes or reduce toxicity. A 2025 systematic review identified only 9 studies with 354 patients in total (258 of them with breast cancer) and confirmed no effect on either treatment efficacy or side effects. One phase 2 trial in breast cancer (DIRECT, n=131) observed a more frequent tumour response to preoperative chemotherapy (OR 3.168; p=0.039), but this is a preliminary result from a small study, not a basis for changing practice. Outside breast cancer the data are almost non-existent, and malnutrition, weight loss and diabetes were exclusion criteria in these trials.
Read more — evidence, cautions, sources
THE UNDERLYING IDEA
In animal models, short-term starvation protected healthy cells from chemotherapy toxicity while increasing its effect on cancer cells [1]. Two variants have been tested in humans: true fasting (several dozen hours on water only, around chemotherapy administration) and the fasting-mimicking diet, a several-day menu very low in calories and protein, designed to make the body respond as it would to a fast [1][2].
WHAT THE LARGEST RANDOMISED TRIAL SHOWED
The DIRECT trial (phase 2, 131 patients with HER2-negative stage II/III breast cancer) compared a fasting-mimicking diet used for 3 days before and during neoadjuvant chemotherapy with a regular diet [1]. Results:
- no difference in toxicity between the groups, despite dexamethasone being omitted in the fasting arm [1];
- a radiologically complete or partial response occurred more often in the diet group (OR 3.168; p=0.039) [1];
- in the per-protocol analysis, a Miller-Payne 4/5 pathological response, meaning 90-100% tumour-cell loss, was more likely (OR 4.109; p=0.016) [1];
- the diet reduced chemotherapy-induced DNA damage in T-lymphocytes [1].
WHY THIS IS STILL NOT ENOUGH
The key pathological-response result comes from a per-protocol analysis rather than from all randomised patients, meaning it counted only those who actually kept to the diet [1]. Such an analysis inherently favours the tested method, because patients able to tolerate several days of food restriction are usually in better general condition. An independent 2025 systematic review covering 9 studies and 354 patients concluded plainly that fasting is safe and feasible, but that no effect on treatment outcomes or chemotherapy-related toxicity has been demonstrated [2]. The only consistent finding was a fall in insulin and IGF-1, a change in a biological marker rather than in the patient's health [2]. The authors explicitly call for large randomised trials before fasting is regarded as an adjunct to therapy [2].
WHAT THESE STUDIES DID NOT COVER
DIRECT excluded people with diabetes and those with a BMI below 18 [1]. The available studies were dominated by breast cancer patients (258 of 354), generally in good condition and not losing weight [2]. This means the data say nothing about fasting in people who are malnourished, losing weight, in cancer cachexia, or with gastrointestinal cancers - and these are precisely the groups most likely to consider restricting food after reading reports about fasting. Absence of data is not the same as evidence of safety.
WHAT THIS MEANS FOR THE READER
Fasting and fasting-mimicking diets remain an investigational approach, not part of standard cancer treatment [2]. Maintaining body weight and protein intake during chemotherapy is supported by strong evidence (see the entries on nutritional support and on cachexia), whereas deliberate food restriction is not. Any change to nutrition during cancer treatment is a decision to be made together with the treating team.
Sources
- de Groot S, 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: europepmc.org ↗
- Maes J, et al. — Impact of intermittent fasting on patients with cancer undergoing chemotherapy and/or targeted therapies: a systematic review of the literature, Supportive Care in Cancer 2025: europepmc.org ↗
This page is educational — it is not medical advice and does not replace consultation with an oncologist. Diagnostic and treatment decisions are made solely by specialist physicians.