There is no established guideline: multi-day fasts should be done with medical supervision and good hydration. For gentler and much better-studied approaches, see intermittent fasting or time-restricted eating.
Prospective study (4,730 women, ~10 years): prolonged fasting periods and weight-loss diets were linked to more hospitalizations for gallstones. Observational: association, causation not proven.
View the study (study) →In mice, several cycles of prolonged fasting lowered IGF-1 and PKA activity, activated self-renewal of hematopoietic stem cells, and regenerated the immune system; they reversed the immunosuppression caused by chemotherapy. In patients, there are only preliminary data that fasting protects lymphocytes from chemotherapy.
View the study →In young and old mice, a 24-hour fast enhanced intestinal stem cell function by activating fatty acid oxidation. This has not been confirmed in people.
View the study →In cells and mice, short-term fasting sensitized 15 of 17 cancer lines to chemotherapy and increased programmed cell death (apoptosis, with caspase-3 activation) in breast cancer cells. In mouse neuroblastoma, fasting cycles plus chemotherapy achieved long-term cancer-free survival. It is not a proven therapy in people.
View the study →Short-term fasting protected healthy cells and mice from high-dose chemotherapy, but not cancer cells (differential stress resistance).
View the study →Pilot randomized trial in 13 women: fasting for 24 hours before and after chemotherapy was well tolerated, kept red blood cells and platelets higher at 7 days, and prevented the increase in leukocyte DNA damage that did appear without fasting. Only under oncology supervision.
View the study →Diet · Highest evidence: Low-moderate · 1 finding
Long fasting periods (days), different from intermittent fasting or overnight fasting. Requires medical supervision: avoid it if you have biliary disease, treated diabetes, pregnancy, or low weight.
Much of the interest in prolonged fasting comes from two processes: regeneration from stem cells and apoptosis (programmed cell death). Almost all the evidence comes from cells and animals; in people there are only small studies, mainly in patients with cancer.
Stem cells. In mice, several cycles of prolonged fasting lower IGF-1 and PKA pathway activity, which activates the self-renewal of blood stem cells and regenerates a more balanced immune system, even after chemotherapy (Cheng 2014, preclinical (A)). A 24-hour fast also boosts intestinal stem cells in young and old mice (Mihaylova 2018, preclinical (A)).
Apoptosis. Fasting does not "activate apoptosis" in general: it protects healthy cells, which enter maintenance mode, while tumor cells, which do not know how to slow their growth, become more vulnerable to chemotherapy and die more through apoptosis (Raffaghello 2008; Lee 2012, preclinical (A)). In a pilot trial with 13 women with breast cancer, fasting for 24 h before and after chemotherapy reduced its toxicity in the blood (de Groot 2015, pilot RCT (A)).
In practice. These are promising results but not a proven therapy: there are no studies showing that fasting for several days regenerates organs or eliminates damaged cells in healthy people. In cancer patients it should only be done within a protocol and with their medical team.
Safer alternative. The fasting-mimicking diet seeks the same effects with less risk; in mice it increases stem cells and regeneration after each cycle (Brandhorst 2015, preclinical (A)) and in people it already has a randomized trial on risk markers (Wei 2017, RCT (AA)).
Information, not medical advice. Prescription drugs require a doctor.
Information, not medical advice. Scientific evidence reflects the quality of the studies.