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Fasting for three days can regenerate the entire immune system, even in the elderly, scientists have found in a breakthrough described as ‘‘remarkable’’.

Although fasting diets have been criticised by nutritionists, research suggests that starving the body kick-starts stem cells into producing more white blood cells, which fight off infection.

Scientists at the University of Southern California (USC) say the discovery could be particularly beneficial for those suffering from damaged immune systems, such as cancer patients on chemotherapy. It could also help the elderly whose immune systems become less effective.

The researchers say that fasting ‘‘flips a regenerative switch’’ which prompts stem cells to create white blood cells, essentially restoring the immune system.

‘‘It gives the OK for stem cells to go ahead and begin proliferating and rebuild the entire system,’’ said Valter Longo, professor of gerontology and biological sciences at the university.

‘‘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. Now, if you start with a system heavily damaged by chemotherapy or ageing, fasting cycles can generate, literally, a new immune system.’’

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 induces changes that trigger stem cell-based regeneration of immune system cells.

In trials, volunteers were asked to fast regularly 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 ageing and a hormone which increases cancer risk and tumour growth.

‘‘We could not predict that prolonged fasting would have such a remarkable effect in promoting stem cell-based regeneration of the hematopoietic [formation of stem cells] 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.

‘‘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.’’

Fasting for 72 hours also protected cancer patients against the toxic impact of chemotherapy.

‘‘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.

Referring to the 72-hour fasting period, Chris Mason, professor of regenerative medicine at University College London, said: ‘‘That could be potentially useful because that is not such a long time that it would be terribly harmful to someone with cancer. But I think the most sensible way forward would be to synthesise this effect with drugs. I am not sure fasting is the best idea. People are better eating on a regular basis.’’

Dr Longo added: ‘‘There is no evidence at all that fasting would be dangerous while there is strong evidence that it is beneficial.’’

The Telegraph, London