"The researchers say fasting 'flips a regenerative switch' which prompts stem cells to create brand new white blood cells, essentially regenerating the entire immune system," writes Science Correspondent Sarah Knapton.
"It gives the 'OK' for stem cells to go ahead and begin proliferating and rebuild the entire system," said Prof Valter Longo, Professor of Gerontology and the Biological Sciences at the University of California.
In an article in the Telegraph, Knapton writes:
Prolonged fasting forces the body to use stores of glucose and fat but also breaks down a significant portion of white blood cells.
During each cycle of fasting, this depletion of white blood cells induces changes that trigger stem cell-based regeneration of new immune system cells.
In trials humans were asked to regularly fast for between two and four days over a six-month period.
Scientists found that prolonged fasting also reduced the enzyme PKA, which is linked to aging and a hormone which increases cancer risk and tumor growth.
"We could not predict that prolonged fasting would have such a remarkable effect in promoting stem cell-based regeneration of the hematopoietic system," added Prof Longo.
"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," Dr Longo said.
"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. So we started thinking, well, where does it come from?"
Fasting for 72 hours also protected cancer patients against the toxic impact of chemotherapy.
"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," said co-author Tanya Dorff, assistant professor of clinical medicine at the USC Norris Comprehensive Cancer Center and Hospital.