Fasting and Fasting-Mimicking Diets During Chemotherapy: Promise, Mechanisms, and Caution

How fasting may be beneficial

Scope and key definitions

Interest in short-term fasting (STF) and fasting-mimicking diets (FMDs) during chemotherapy has surged because of compelling preclinical biology and early human feasibility signals. The clinical question is whether deliberately inducing a fasting-like metabolic state around chemotherapy cycles can:

1. reduce toxicity and improve tolerability (support dose intensity),

2. enhance antitumor efficacy (response, pCR ), and/or

3. improve patient-reported outcomes (fatigue, QoL).

Short-term fasting (STF) typically means 24–72+ hours of water (or near-zero calorie) intake before chemotherapy, sometimes continued for 12–48+ hours after.

Fasting-mimicking diet (FMD) is a structured, multi-day, very low-calorie, low-protein, low-carbohydrate regimen designed to reproduce key metabolic effects of fasting while still providing small amounts of food; the best-studied oncology approach uses ~3 days pre-chemo through the day of chemo. (1)

STF and FMD must be distinguished from “intermittent fasting” or “time restricted eating” (see chapter entitled “Dietary Interventions in Cancer”. Time-restricted eating is a type of fasting where food intake is limited to a short window during the day (1 to 8 hours), with only fluids such as water, tea, or coffee for the rest of the day. It is important to emphasize that intermittent fasting is not synonymous with starvation; people/patients who undertake intermittent fasting eat nutrient-dense food. Intermittent fasting does not activate starvation metabolic pathways and if caloric intake is adequate patients do not lose weight. Intermittent fasting has a profound effect on promoting immune system homeostasis, partly by stimulating the clearing of damaged cells (autophagy), damaged mitochondria (mitophagy), and misfolded and foreign proteins. Fasting improves mitochondrial health and increases stem cell production. Intermittent fasting is the most effective therapy for the treatment of insulin resistance, metabolic syndrome, and type II diabetes. (2-7)

Biological rationale: why fasting might matter during chemotherapy

Differential stress resistance (DSR) and differential stress sensitization (DSS)

A central hypothesis is that fasting triggers a protective program in normal cells (“maintenance mode”) while cancer cells—locked into growth signaling—fail to adequately downshift, making them more vulnerable to chemotherapy. This concept is often summarized as:

• Normal tissues: shift toward repair, autophagy, stress resistance

• Tumor cells: remain proliferative/metabolically stressed → heightened chemo sensitivity

Human-focused reviews describe this as “normal cell protection + tumor sensitization,” with circulating metabolic signals (glucose/insulin/IGF-1) as candidate mediators. (8)

Metabolic and endocrine changes relevant to cytotoxic therapy

Across models and early trials, fasting/FMD commonly induces:

• lower glucose, insulin, IGF-1 signaling (growth pathways) (8)

• increased lipolysis/ketone metabolism (context-dependent)

• changes in oxidative stress handling, DNA damage responses, and immune cell energetics

These shifts are mechanistically plausible for toxicity modulation (e.g., hematologic and GI toxicity) and for enhancing tumor response in some contexts.

Immunologic and microenvironment hypotheses (emerging)

More recent translational work argues that cyclical fasting/FMD may influence:

• antitumor immunity (T-cell function, myeloid phenotype)

• tumor stroma and therapy resistance programs
  A 2024 review in Cell Metabolism frames FMD as a metabolic intervention that may modulate treatment resistance and immune contexture but emphasizes that clinical confirmation is incomplete. (9)

Human evidence during chemotherapy: what has been shown

The clinical literature is best understood as (A) feasibility/safety, (B) toxicity/tolerability, and (C) efficacy signals, with the highest-quality randomized data concentrated in breast cancer.

A) Feasibility and adherence: the recurring constraint

Across trials, the most consistent result is: selected patients can complete STF/FMD, but adherence declines over repeated cycles—especially with nausea, fatigue, taste changes, steroid use, and social factors. Major randomized work explicitly notes adherence as a key limitation. (1)

Breast cancer: strongest randomized evidence base

1) DIRECT trial (multicenter randomized phase II, early breast cancer)

The DIRECT trial evaluated FMD cycles as an adjunct to neoadjuvant chemotherapy in women with early breast cancer.

Main takeaways:

• Safety/feasibility: FMD cycles were generally feasible in a subset, but adherence was imperfect across cycles.

• Toxicity: The trial did not demonstrate a clean, consistent reduction in overall chemotherapy toxicity as a primary clinical win.

• Efficacy signals: The study reported signals suggesting improved tumor response among those who adhered better, and it is frequently cited as “first randomized evidence” supporting possible efficacy—while still being phase II and not definitive for long-term outcomes. (1)

A notable mechanistic finding in this trial was reduced DNA damage in circulating immune cells (T-lymphocytes) in the FMD arm, consistent with “normal tissue protection” hypotheses. (1)

2) Additional breast cancer RCTs and feasibility studies (2024–2025)

• A 2024 randomized study of FMD during neoadjuvant chemotherapy reported the intervention as well-tolerated with improvements in selected toxicity and metabolic/hematologic parameters, though external generalizability depends on patient selection, regimen details, and monitoring. (10)

• A 2025 feasibility study of a recipe-based FMD in breast cancer focused on real-world implement ability and metabolic/body composition outcomes rather than hard oncologic endpoints. (11)

Interpretation: Breast cancer data increasingly support “possible benefit in some endpoints” and “generally feasible in selected patients,” but the field still lacks phase III confirmation and consistent demonstrations of clinically meaningful reductions in major toxicity outcomes.

Mixed solid tumors: feasibility with limited definitive efficacy

Short-term fasting peri-chemotherapy (pilot studies)

Pilot work in patients receiving cytotoxic chemotherapy across tumor types has suggested STF can be feasible and may alter metabolic biomarkers (e.g., IGF-1) and some tolerance measures, but these studies are generally underpowered for definitive toxicity or response endpoints. Reviews summarizing this literature emphasize heterogeneity in fasting duration and supportive care. (12)

Quality of life (QoL), fatigue, and symptom burden

A randomized cross-over pilot in breast/ovarian/gynecologic contexts reported STF was tolerated and suggested improvements in fatigue/QoL measures in some analyses, but the design and sample size limit certainty and generalizability. (13)

Interpretation: QoL signals exist, but they are inconsistent and not yet strong enough to justify broad clinical recommendation.

Systematic reviews and what they conclude (high-level synthesis)

2022–2023 systematic review evidence

A systematic review of fasting during cancer treatment (8 studies, 379 participants) concluded:(14)

• evidence that fasting reduces chemotherapy toxicities is missing or insufficient,

• QoL benefits are not reliably demonstrated, and

• because unintentional weight loss and malnutrition are linked to worse oncological outcomes, fasting is not indicated as a general strategy during treatment.

Reviews focused on intermittent fasting in breast cancer

A breast-cancer-focused systematic review (2023) describes potential effects on toxicity/QoL/endocrine outcomes but similarly highlights heterogeneity and limited high-certainty data. (15)

Practice-facing summaries

Clinical nutrition guidance documents emphasize preventing and treating cancer-associated malnutrition and generally caution against practices that risk weight loss or reduced intake during active therapy—supporting the “selective, supervised only” approach to fasting/FMD in oncology. (16)

Safety considerations: the central clinical tension

Why fasting can be risky in oncology

Chemotherapy already challenges intake and metabolism through nausea, mucositis, altered taste, diarrhea/constipation, early satiety, fatigue, and psychosocial stress. Many patients develop or already have:

• weight loss,

• sarcopenia, and/or

• inflammatory catabolism.

In that context, purposeful caloric restriction can worsen:

• malnutrition and frailty

• dehydration/electrolyte disturbances

• treatment delays or dose reductions (if toxicity or deconditioning worsens)

Hence the repeated conclusion across reviews: do not generalize fasting to all patients. (14)

Patients generally considered poor candidates (high caution/avoid)

While exclusion criteria vary by trial, the highest-risk categories in routine practice typically include:

• underweight or significant unintentional weight loss

• clinical cachexia or significant sarcopenia/low muscle mass

• dehydration risk, uncontrolled vomiting/diarrhea, renal vulnerability

• brittle diabetes or high hypoglycemia risk (insulin/sulfonylureas)

• inability to maintain adequate protein/energy intake between cycles

Professional supportive guidance on fasting during treatment (even when framed around religious fasting) stresses individualized risk assessment and careful monitoring. (17)

Practical implementation (when used): what protocols generally look like

Clinical trials that attempt to operationalize fasting physiology while limiting harm usually share these features:

STF “water fast” patterns (research setting)

• ~36–72 hours around infusion (varies widely)

• emphasis on hydration, electrolyte monitoring when appropriate

• stopping rules for intolerance (dizziness, hypotension, severe nausea, etc.)

FMD patterns (most studied)

• ~3 days pre-chemo through chemo day (sometimes extending 1 day after)

• very low calorie with macronutrient composition designed to keep protein and sugars low

• structured meal plans improve feasibility vs pure water fasting (12)

A practical clinical reality: steroids and antiemetics

Many chemotherapy regimens include dexamethasone, which:

• increases appetite (sometimes), but also

• raises glucose/insulin transiently and could blunt some metabolic aims.

Some studies explore minimizing steroid use in selected settings, but in everyday oncology care, antiemetic protection is often essential. This creates a gap between “metabolic purity” and pragmatic supportive care that future trials need to address explicitly. (This issue is discussed indirectly in trial designs and reviews emphasizing heterogeneity and adherence barriers.) (12)

Where the evidence is strongest vs weakest

Strongest

• Feasibility in selected patients (especially breast cancer cohorts) (12)

• Biological plausibility supported by translational markers and extensive preclinical evidence

Moderate/uncertain

• Selective toxicity reductions (some trials report differences in specific toxicities; findings are inconsistent) (10)

• Tumor response signals (notably in neoadjuvant breast cancer, particularly per-protocol/adherent analyses) (12)

Weakest

• Hard outcomes: DFS/OS improvements are not established.(14)

• Generalizability beyond carefully selected patients and specific regimens is uncertain.

• Standardization of fasting duration/composition and supportive care is lacking.

Clinical take-home synthesis

• Fasting/FMD during chemotherapy is biologically plausible and has early human signals suggesting potential for tissue protection and possibly enhanced tumor response in certain settings, most notably neoadjuvant breast cancer. (12)

• The best systematic evidence still does not support routine recommendation because benefits on global toxicity and QoL are not consistently demonstrated, trials are heterogeneous, and the malnutrition/weight-loss risk is clinically important.(14)

• The most defensible current stance is: consider only within clinical trials or with careful selection and close nutrition/oncology supervision, prioritizing maintenance of lean mass, hydration, and overall treatment delivery.

Clinical implementation framework

The central clinical challenge is to capture potential peri-chemotherapy metabolic benefits of fasting/FMD while not worsening malnutrition, sarcopenia, dehydration, or treatment delivery. Contemporary evidence supports feasibility in selected patients (e.g., DIRECT breast cancer FMD trial and Dorff’s short-term fasting feasibility study), but systematic reviews and nutrition guidelines caution against strategies that could precipitate weight loss or inadequate intake during active treatment—so implementation should be trial-like, supervised, and reversible.(12)

Candidate selection checklist (who is most plausible to benefit, with lowest foreseeable harm)

A. Nutritional robustness (highest priority)

• No current malnutrition and no clinically meaningful unintentional weight loss

• Adequate oral intake between cycles and ability to “catch up” nutritionally after the fasting/FMD window

• Low suspicion for clinically significant sarcopenia (or, if present, strong mitigation plan with dietitian oversight)

ESPEN guidance centers cancer nutrition care on early identification and treatment of malnutrition, because malnutrition is associated with worse outcomes and reduced tolerance of therapy. Fasting/FMD should not undermine this principle. (16)

B. Metabolic safety

• No diabetes treated with insulin or sulfonylureas unless there is a clinician-supervised plan to prevent hypoglycemia

• Ability to self-monitor symptoms (and glucose when relevant)

Notably, the DIRECT trial enrolled patients without diabetes and with BMI >18 kg/m², reflecting conservative safety selection.

C. Hydration reliability

• Can maintain adequate fluid intake during the fasting/FMD window

• No recent severe vomiting/diarrhea, orthostasis, or recurrent dehydration episodes

D. Treatment context

• Curative-intent regimens where maintaining dose intensity matters, and where the team can monitor closely (common in neoadjuvant breast cancer trials)

• Access to dietitian support and clear stop rules

E. Patient factors (pragmatic feasibility)

• High motivation and understanding that adherence tends to drop across cycles (a consistent limitation in trials)

• No history of eating disorder, severe anxiety around food, or situations where fasting could destabilize mental health or social functioning (clinical judgment)

Clinical Algorithm

Fasting / Fasting-Mimicking Diet (FMD) During Chemotherapy

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STEP 1 — SCREEN (Pre-Cycle 1 Eligibility)

A. Nutritional Safety (MANDATORY)

✔ BMI ≥ 18.5
✔ No clinically significant unintentional weight loss
✔ No cachexia or active malnutrition
✔ Adequate oral intake between cycles
✔ Access to dietitian (preferred)

✘ If malnourished / ongoing weight loss / sarcopenic decline → DO NOT PROCEED

(ESPEN nutrition guidance emphasizes prioritizing nutritional stability during cancer therapy.)

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B. Metabolic & Medical Safety

✔ No unstable diabetes
✔ Not on insulin or sulfonylureas (unless specialist-managed)
✔ No recurrent hypoglycemia
✔ Adequate renal function & hydration status
✔ No active severe vomiting/diarrhea
✔ No history of eating disorder

✘ If any high-risk metabolic instability → AVOID outside clinical trial

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C. Treatment Context

✔ Curative-intent or closely monitored setting
✔ Patient motivated and understands limited evidence
✔ Oncology team agrees on structured monitoring plan

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If ALL above satisfied → Proceed to Step 2
If ANY major safety concern → Standard nutrition support only

STEP 2 — SELECT APPROACH

Option A: Short-Term Fasting (STF)

Typical research pattern:
• Start ~48 hours before chemo
• Continue until ~24 hours after chemo
• Total ≈ 72 hours
• Water, non-caloric beverages allowed
• Rescue calories permitted if symptomatic

Best suited for:
• Highly motivated patients
• Strong hydration reliability
• Lower GI toxicity risk regimens

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Option B: Fasting-Mimicking Diet (FMD)

Typical breast cancer trial model (DIRECT):
• Start 3 days before chemo
• Continue through chemo day
• Very low calorie, low protein, low carbohydrate

Best suited for:
• Patients preferring structured intake over complete fasting
• Those with lower tolerance for complete caloric abstinence

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STEP 3 — PRE-CYCLE BASELINE

Before each fasting/FMD attempt:

• Weight & % change from baseline
• Symptom review (orthostasis, nausea, weakness)
• Medication review (especially glucose-lowering drugs)
• Routine chemo labs (CBC, CMP, renal function)
• Hydration plan confirmed
• Written stop rules reviewed with patient

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STEP 4 — DURING FASTING / FMD WINDOW

Monitor (daily if possible):

✔ Hydration (urine output, dizziness)
✔ GI tolerance (vomiting, inability to drink)
✔ Weakness / near-syncope
✔ Glucose (if diabetic or high-risk)

Allow rescue calories if:
• Symptomatic hypotension
• Persistent weakness
• Hypoglycemia
• Severe nausea preventing fluids

Safety overrides protocol.

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STEP 5 — POST-CHEMO / BETWEEN CYCLES

• Resume full caloric intake promptly
• Prioritize protein repletion
• Reassess weight trajectory
• Monitor for cumulative decline

If weight drops across cycles → STOP strategy.

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STEP 6 — STOP RULES (Terminate Strategy)

Immediately discontinue fasting/FMD if any of the following occur:

• >2–3% ongoing weight loss across cycles
• Emerging malnutrition or sarcopenia risk
• Dehydration requiring IV fluids
• Recurrent orthostatic symptoms
• Hypoglycemia episodes
• Treatment delay or dose reduction plausibly linked to intake restriction
• Patient intolerance or worsening QoL

Nutrition preservation > theoretical metabolic benefit.

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STEP 7 — CONTINUE ONLY IF

• Weight stable
• No treatment delays
• No dehydration events
• Patient tolerates well
• Oncology team comfortable proceeding

Reassess eligibility each cycle.

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References

1. de Groot S, Lugtenberg RT, Cohen D, Welters MJP, Ehsan I, Vreeswijk MPG, et al. Fasting mimicking diet as an adjunct to neoadjuvant chemotherapy for breast cancer in the multicentre randomized phase 2 DIRECT trial. Nat Commun. 2020;11(1):3083.

2. Guo L, Xi Y, Jin W, Yuan H, Qin G, Chen S, et al. A 5:2 Intermittent Fasting Meal Replacement Diet and Glycemic Control for Adults With Diabetes: The EARLY Randomized Clinical Trial. JAMA Netw Open. 2024;7(6):e2416786.

3. Marinac CR, Nelson SH, Breen CI, Hartman SJ, Natarajan L, Pierce JP, et al. Prolonged Nightly Fasting and Breast Cancer Prognosis. JAMA Oncol. 2016;2(8):1049–55.

4. Manoogian ENC, Chow LS, Taub PR, Laferrère B, Panda S. Time-restricted Eating for the Prevention and Management of Metabolic Diseases. Endocr Rev. 2022;43(2):405–36.

5. Kotarsky CJ, Johnson NR, Mahoney SJ, Mitchell SL, Schimek RL, Stastny SN, et al. Time-restricted eating and concurrent exercise training reduces fat mass and increases lean mass in overweight and obese adults. Physiol Rep. 2021;9(10):e14868.

6. Yuan X, Wang J, Yang S, Gao M, Cao L, Li X, et al. Effect of Intermittent Fasting Diet on Glucose and Lipid Metabolism and Insulin Resistance in Patients with Impaired Glucose and Lipid Metabolism: A Systematic Review and Meta-Analysis. Int. J Endocrinol. 2022;2022:6999907.

7. Ahmed N, Farooq J, Siddiqi HS, Meo SA, Kulsoom B, Laghari AH, et al. Impact of Intermittent Fasting on Lipid Profile-A Quasi-Randomized Clinical Trial. Front Nutr. 2020;7:596787.

8. Caccialanza R, Cereda E, De Lorenzo F, Farina G, Pedrazzoli P. To fast, or not to fast before chemotherapy, that is the question. BMC Cancer. 2018;18(1):337.

9. Vernieri C, Ligorio F, Tripathy D, Longo VD. Cyclic fasting-mimicking diet in cancer treatment: Preclinical and clinical evidence. Cell Metab. 2024;36(8):1644–67.

10. Bahrami A, Haghighi S, Moghani MM, Khodakarim N, Hejazi E. Fasting mimicking diet during neo-adjuvant chemotherapy in breast cancer patients: a randomized controlled trial study. Front Nutr. 2024;11:1483707.

11. Xue M, Zhang N, Dong X, Tian Q, Shen K, Wu B. The feasibility and safety of fasting-mimicking diet in breast cancer patients with chemotherapy in China. Breast Cancer Res Treat. 2025;213(3):313–23.

12. de Groot S, Pijl H, van der Hoeven JJM, Kroep JR. Effects of short-term fasting on cancer treatment. J Exp Clin Cancer Res. 2019;38(1):209.

13. Bauersfeld SP, Kessler CS, Wischnewsky M, Jaensch A, Steckhan N, Stange R, 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(1):476.

14. Drexler U, Dörfler J, von Grundherr J, Erickson N, Hübner J. Fasting during cancer treatment: a systematic review. Qual Life Res. 2023;32(5):1427–46.

15. Anemoulis M, Vlastos A, Kachtsidis V, Karras SN. Intermittent Fasting in Breast Cancer: A Systematic Review and Critical Update of Available Studies. Nutrients. 2023;15(3).

16. Muscaritoli M, Arends J, Bachmann P, Baracos V, Barthelemy N, Bertz H, et al. ESPEN practical guideline: Clinical Nutrition in cancer. Clin Nutr. 2021;40(5):2898–913.

17. Thompson S, Madsen LT, Bazzell A. Impact of Fasting on Patients With Cancer: An Integrative Review. J Adv Pract Oncol. 2023;14(7):608–19.