© GETTYFasting for two days can regenerate the immune system
The finding has dramatic implications for health, say scientists.
It shows for the first time that natural intervention can trigger repair of vital systems in the body.
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."
Experiments showed that fasting flipped a switch in bone marrow stem cells which caused them to begin regenerating themselves.
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. Both natural ageing and chemotherapy drugs weaken the immune system.
The scientists found that multiple fasting sessions reduced immune system weakening and death in mice exposed to a chemotherapy drug.
Dr Tanya Dorff, 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."
Source: Cell Stem Cell
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