Need an immune system boost? Try a three-day fast.

University of Southern California (USC) researchers published a study last week suggesting that fasting may put the immune system into overdrive by activating stem cells.

Studying the effects of fasting on cancer patients undergoing chemotherapy produced the unexpected result. Fasting not only protected healthy tissue, but also allowed the body to recycle old and damaged white cells. When the patients resumed eating, their white counts returned to normal. Where were the new white cells coming from?

That was the question Valter Longoand his team at USC set out to answer. Longo, director of the USC longevity institute and a member of the USC Norris Cancer Center, compares the recycling of old white cells to throwing excess cargo overboard during a crisis. Prolonged fasting triggered the body to regenerate the lost cells. The process reduces an enzyme called PKA and stimulates stem cells to shift into a regenerative mode. Fasting also lowered IGF-1, a growth hormone which Longo had previously shown to be linked with aging, tumor growth, and cancer risk.

“We could not predict that prolonged fasting would have such a remarkable effect in promoting stem-cell based regeneration of the hematopoietic system,” Longo said in a statement. The prolonged fast in this study lasted three days and limited participants to between 400 and 1,000 calories a day.

Chemotherapy can save cancer patients’ lives, but it takes a toll on normal tissue as well. This collateral damage is thought to be significantly lessened by pre-treatment fasting, which causes normal cells go into a survival state and stop multiplying. Chemotherapy attacks actively dividing cells. Fasting has no effect on the cancer cells, which are already dividing at an unnatural rate. The end result: Cancer cells are killed and the normal cells are spared.

During chemotherapy, patients’ white cells, red cells, and platelets drop after treatment. This new information suggests fasting not only protects normal tissue but cycles patients’ immune cells in a way that causes them to be replenished with new ones after the patients begin eating again.

Calorie restriction has long been known to promote longevity in lab animals, and some humans have tried it in hopes of extending their lives. The underlying mechanism remains unknown, but Longo’s research could begin to provide the answer. The effect may not depend on constant calorie restriction, but on periodic 72-hour fasts.

If fasting has such a profound effect on the immune system, what might be going on in the rest of the body? “We are investigating the possibility that these effects are applicable to many different systems and organs, not just the immune system,” Longo said.

Fasting has long been a part of many religious traditions and considered spiritually beneficial. This new study suggests that it may have a more holistic effect. Medical experts advise fasting should never be done without consulting a doctor first.