A 72-hour fast can trigger a powerful immune system reset. Scientists call this stem cell regeneration. The process clears old immune cells and grows fresh ones. Research from the University of Southern California proves it works in both animals and humans.
Valter Longo led the landmark 2014 study. His team published the results in Cell Stem Cell. They tested prolonged fasting cycles of 2 to 4 days. The goal is to protect and renew the immune system.
How 72-Hour Fasting Affects Your Immune System Step by Step
Day 1-2: Your white blood cell count drops fast. The body saves energy. It recycles damaged or old immune cells.
Day 3 (72 hours): Stem cells in your bone marrow wake up. These hematopoietic stem cells start to multiply.
When you eat again: Regeneration surges. Fresh, balanced immune cells flood back into your blood.
The switch happens because fasting lowers two signals:
These drops push stem cells from sleep mode into full repair mode. The result brings a younger, stronger immune system.
What the Valter Longo USC Study Found
In mice, repeated 48-hour fasts reversed age-related immune decline. Older mice produced more balanced white blood cells. They fought inflammation better. Their bone marrow worked like it did in youth.
In humans, cancer patients fasted up to 72 hours before chemotherapy. They kept healthier lymphocyte counts. They showed less immune damage. Longo called the effect “remarkable.”
The study proved something new. Prolonged fasting does not just pause the immune system. It rebuilds it.
Real Benefits of 72-Hour Fasting for Immune Health
- Clears damaged immune cells
- Boosts production of new white blood cells
- Helps reverse immune ageing (immunosenescence)
- May protect during chemotherapy or other stresses
- Supports balance between lymphoid and myeloid cells
Many people search for “72 hour fasting immune system reset” because the promise is huge. Early data backs it especially for older adults or those with weakened immunity.
Important Safety Warnings
Do not try a 72-hour water fast without medical supervision. Risks include electrolyte imbalance, low blood sugar, and nutrient loss. People with diabetes, eating disorders, or certain medications should avoid it completely.
The original human trial focused on safety around chemotherapy. It was small. Larger studies are still needed for healthy people.
Safer Option: Fasting-Mimicking Diets
Longo later created plant-based, low-calorie plans that copy fasting effects. You eat small amounts of food for 5 days. No total starvation. These diets show similar immune benefits in bigger trials and feel easier for most people.
References
Cheng, C.-W. et al. (2014) ‘Prolonged Fasting Reduces IGF-1/PKA to Promote Hematopoietic-Stem-Cell-Based Regeneration and Reverse Immunosuppression’, Cell Stem Cell, 14(6), pp. 810–823. Available at: https://www.cell.com/cell-stem-cell/fulltext/S1934-5909(14)00151-9
University of Southern California Stem Cell (2014) ‘Fasting triggers stem cell regeneration of damaged, old immune system’. Available at: https://stemcell.keck.usc.edu/fasting-triggers-stem-cell-regeneration-of-damaged-old-immune-system/
Medical News Today (2014) ‘Prolonged fasting ‘re-boots’ immune system’. Available at: https://www.medicalnewstoday.com/articles/277860
