Results of a new study on mice and a phase 1 trial of humans suggest that prolonged cycles of fasting – for 2-4 days at a time – not only protect against toxic effects of chemotherapy, but also trigger stem cell regeneration of new immune cells and clearing out of old, damaged cells.

The study, by researchers from the University of Southern California (USC) in Los Angeles, and published in the journal Cell Stem Cell, is the first to show that a natural intervention can trigger regeneration of an organ or system through stem cells.

The team believes the findings could benefit people with immune system damage, for example if they have received chemotherapy treatment for cancer. It could also benefit the elderly whose immune systems are weakened through aging, making them more susceptible to disease.

The scientists say prolonged fasting appears to shift stem cells of the immune system from a dormant state to an active state of self-renewal.

Results from experiments with mice and a phase 1 human clinical trial showed that long periods of fasting significantly lowered levels of white blood cells. In the mice, it flipped a switch that changed the signaling pathways of hematopoietic stem cells – a group of stem cells that generate blood and immune systems.

“We could not predict that prolonged fasting would have such a remarkable effect in promoting stem cell-based regeneration of the hematopoietic system,” says Valter Longo, a professor of Gerontology and the Biological Sciences at the USC Davis School of Gerontology, and director of the USC Longevity Institute.

He says that when you stop eating, the body uses up stored glucose, fat and ketones, and also recycles worn out and damaged immune cells.

“What we started noticing in both our human work and animal work is that the white blood cell count goes down with prolonged fasting,” he explains. “Then when you re-feed, the blood cells come back. So we started thinking, well, where does it come from?”