Fasting and cancer sits at a sensitive intersection. The short version: promising mechanisms and early human signals, but not a replacement for oncology care. Here’s an honest, research-grounded tour.

Why scientists are interested

Cancer cells often rely on constant growth signals (insulin/IGF-1, mTOR) and have rigid metabolic needs (e.g., high glucose uptake). Fasting lowers insulin/IGF-1, suppresses mTOR, activates AMPK/sirtuins, and ramps autophagy—a milieu that can stress cancer cells while leaving normal cells more resilient. Ketones may not be efficiently used by some tumors, creating a metabolic mismatch.

Differential stress resistance (DSR)

In multiple preclinical models, short fasts protected normal cells from chemotherapy toxicity while sensitizing tumors—the “DSR” concept. Mechanism: healthy cells, sensing a low-nutrient state, enter maintenance mode; cancer cells keep dividing and become more vulnerable.

Fasting-mimicking diet (FMD) and small human studies

Cyclical FMD (very-low-calorie, low-protein, plant-based for 4–5 days/month) reduced IGF-1, insulin, and inflammation and improved risk factor clusters in humans (Brandhorst et al., 2015). Small pilot studies around chemotherapy suggest reduced fatigue, GI side effects, and better tolerance when patients used brief fasting/FMD under medical supervision. Early phase trials are exploring whether fasting/FMD enhances response to chemo, endocrine therapy, or immunotherapy; results are preliminary.

What we cannot claim

• Fasting does not “cure” cancer.

• We lack large RCTs showing fasting improves survival.

• Tumors are heterogeneous; some may adapt to fasting or low-glucose environments.

Practical guidance for patients (with their oncology team)

• Do not self-impose long fasts during active treatment without your oncologist’s approval—malnutrition and sarcopenia worsen outcomes.

• If your team approves, brief fasts (e.g., 24–48 h pre-chemo, 24 h post) or FMD cycles are the usual research-backed patterns.

• Protein sufficiency between cycles is critical; resistance exercise helps preserve lean mass.

• Watch for weight loss, weakness, or poor wound healing—reasons to stop fasting immediately.

Prevention & survivorship

Outside active treatment, IF may lower insulin/IGF-1, reduce visceral fat/inflammation, and improve metabolic health—factors tied to lower risk of several cancers (e.g., colorectal, breast post-menopause). A Mediterranean-style diet, regular exercise, limited alcohol, and healthy weight remain the strongest lifestyle levers; IF can help you keep them.

Bottom line

Fasting and FMD show mechanistic plausibility and early clinical promise for improving tolerance to therapy and possibly enhancing efficacy—but standard care comes first. Any fasting in oncology should be clinician-directed, with vigilant nutrition and strength maintenance.

Selected references

• Brandhorst S et al. FMD improves risk factors. Sci Transl Med. 2015;7:285ra62.

• Longo VD, Mattson MP. Fasting and disease, mechanisms. Cell Metab. 2014;19:181–192.

• de Cabo R, Mattson MP. N Engl J Med. 2019;381:2541–2551.

• Safdie FM et al. Fasting and chemotherapy tolerance (pilot). Sci Transl Med. 2009;1:13ra2.

• Dorff TB et al. Safety of fasting before chemotherapy. BMC Cancer. 2016;16:360.