Study Suggests Prolonged Fasting May Trigger Immune System Regeneration, But Claims of 'Complete Rebuild' Are Overstated
A 2014 study led by Dr. Valter Longo at the University of Southern California (USC) found that prolonged fasting—typically cycles of 2 to 4 days without food—can promote the regeneration of the immune system by activating stem cells in the hematopoietic (blood-forming) system
In both mouse models and a small Phase 1 human trial involving chemotherapy patients, extended fasting periods lowered white blood cell counts, depleted older or damaged immune cells, and shifted stem cells from a dormant state to one of self-renewal. Upon resuming eating, the body produced new, healthier white blood cells, effectively "rebooting" parts of the immune system. This process was linked to reduced activity in pathways like IGF-1 and PKA, which Longo identified as key signals that, when suppressed during fasting, allow stem cells to regenerate the system.
The research highlighted potential benefits, particularly for those with compromised immunity, such as chemotherapy patients who experienced protection from treatment toxicity and improved immune recovery. In mice, multiple fasting cycles reversed age-related immune decline and mitigated chemotherapy-induced immunosuppression. A pilot human trial showed patients fasting for around 72 hours before chemo had fewer side effects and signs of immune rejuvenation. Longo noted that the body enters a conservation mode during fasting, recycling inefficient or damaged cells to save energy, then ramps up regeneration when nutrients return. However, the study emphasized cycles of fasting over single prolonged fasts, and the human data was preliminary with a small sample size.
While the findings have fueled interest in fasting as a natural, drug-free approach to enhance immunity—especially for aging or disease-related immune weakening—experts caution that it does not "completely rebuild" the entire immune system in healthy individuals after just one 72-hour fast. The effects are more pronounced in damaged systems, and broader claims often exaggerate the original results. More research is needed to confirm long-term benefits, safety across populations, and applicability beyond specific contexts like cancer treatment. Prolonged fasting should not be attempted without medical supervision due to risks like nutrient deficiencies or complications for certain health conditions.
