Fasting for as little as 72 hours can regenerate the entire immune system, even in older individuals, report scientists from the University of Southern California.

While fasting diets have been criticized by nutritionists for being unhealthy, new research published in the journal Cell Stem Cell suggests holding back nutrients from the body prompts stem cells to produce new white blood cells, which fight off infections.

Scientists at the University of Southern California say the discovery could be particularly beneficial for people suffering from damaged immune systems, such as cancer patients who have received chemotherapy.

The limited-time fasting could also benefit people whose immune systems have becomes less effective in their later years.

"It gives the 'OK' for stem cells to go ahead and begin proliferating and rebuild the entire system," Valter Longo, a professor of gerontology and biological sciences at the university, said in a news release. "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."

Prolonged fasting, explained Longo, forces the body to store glucose and fat and also ends up depleting a notable portion of white blood cells.

As such, during each cycle of fasting, the ongoing loss of the white blood cells trigger stem cell-based regeneration of new immune system cells.

In research trials, subjects were directed to regularly fast between two to four days during a six-month period.

The study team found that, aside from replenishing the immune system, prolonged fasting also reduced the PKA enzyme, which is associated with aging, as well as a hormone believed to increase cancer risk and promote 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," said 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."

In experiments focused on humans and the also on animals, the researchers observed "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?"

Said Longo: "We are investigating the possibility that these effects are applicable to many different systems and organs, not just the immune system."