The scientific evidence that fasting can be beneficial to our health is mounting.
The finding, demonstrated both in mice and cancer patients, has dramatic implications for human health, say scientists.
It shows for the first time that a simple natural intervention can trigger the stem cell-based repair of vital systems in the body.
Fasting had the effect of culling old and damaged immune system cells and replacing them with fresh and more effective new ones.
US lead scientist Professor Valter Longo, director of the University of Southern California's Longevity Institute, said: "We could not predict that prolonged fasting would have such a remarkable effect in promoting stem cell-based regeneration.
"When you starve, the system tries to save energy, and one of the things it can do to save energy is to recycle a lot of the immune cells that are not needed, especially those that may be damaged.
"What we started noticing in both our human work and animal work is that the white blood cell count goes down with prolonged fasting. Then when you re-feed, the blood cells come back."
Experiments showed that fasting flipped a switch in bone marrow stem cells causing them to start regenerating themselves.
The "haematopoietic" stem cells develop both into oxygen-carrying red blood cells and the wide range of white blood cells that act as the immune system's foot soldiers.
But the effect only occurs after going without food for a prolonged period, between two and four days at a time.
After a certain period, fasting forces the body to go into "survival mode", using up stores of glucose, fat and organic compounds called ketones, and also breaking down excess numbers of "old" white blood cells.
At the same time, a signal is sent to bone marrow stem cells.
"It gives the 'ok' for stem cells to go ahead and begin proliferating and rebuild the entire system," said Prof Longo. "And the good news is that the body got rid of the parts of the system that might be damaged or old, the inefficient parts, during the fasting.
"If you start with a system heavily damaged by chemotherapy or ageing, fasting cycles can generate, literally, a new immune system."
Both natural ageing and anti-cancer chemotherapy drugs weaken the immune system.
The scientists found that multiple fasting sessions reduced immune system weakening and death in mice exposed a chemotherapy drug. In addition, they rejuvenated the immune systems of ageing mice.
Protection against white blood cell loss was also seen in human cancer patients taking part in a small pilot trial. The patients were asked to fast for a single 72-hour period before undergoing chemotherapy.
Dr Tanya Dorff, also from the University of Southern California, a co-author of the research published in the journal Cell Stem Cell, said: "While chemotherapy saves lives, it causes significant collateral damage to the immune system.
The results of this study suggest that fasting may mitigate some of the harmful effects of chemotherapy."
The scientists are now exploring the possibility that the same effects of fasting might apply across many biological systems and organs.
A key discovery was that prolonged fasting lowered circulating levels of a growth-promoting protein, Insulin-like Growth Factor 1 (IGF-1) , which has been linked to ageing, tumour progression and cancer risk.