Professor of Stanford University Valter Longo, who studies aging and longevity, published a study, in which he described remarkable metabolic changes that occurred as a result of prolonged fasting. Longo and his colleauges found that fasting for 3 days or longer–drinking only water and eating less than 200 calories per day–can truly “reset” some components of your immune system.

Study  shows that cycles of prolonged fasting not only protect against immune system damage — a major side effect of chemotherapy — but also induce immune system regeneration, shifting stem cells from a dormant state to a state of self-renewal.

The research looked at both mice and humans. In both species, fasting lowered white blood cell counts, which in turn triggered the immune system to start producing new white blood cells. White blood cells (or lymphocytes) are a key component of your body’s immune system.

Longo’s hypothesis is that fasting forces your body to “recycle a lot of the immune cells that are not needed” which explains the drop in the white blood cell count. Two of the key mechanisms are an enzyme called PKA and a hormone called IGF-1, both of which are reduced by fasting. Once you start eating again, your stem cells kick back into high gear to replenish the cells that were recycled.

The study has major implications for healthier aging, in which immune system decline contributes to increased susceptibility to disease as people age. By outlining how prolonged fasting cycles kill older and damaged immune cells and generate new ones, the research also has implications for chemotherapy tolerance and for those with a wide range of immune system deficiencies, including autoimmunity disorders.

On the other hand, Valter Longo has compared the effects of periodic fasting to long-term caloric restriction, which has been shown to prolong lifespan in mouse and other animals. In a separate review article, Longo wrote:

“Fasting has the potential to delay aging and help prevent and treat diseases while minimizing the side effects caused by chronic dietary interventions.”

A key finding in this research is that you have to fast for several days to get any benefit: basically, you have to fully deplete your energy reserves (in the form of glycogen), and it takes your body at least 24 hours, and probably 48 hours or more, to do this. That is why fasting should be at least 3 days long.

Caloric restriction is extremely difficult to achieve for humans. 3-5 day fast is much harder than a temporary caloric restriction or a 1-day fast, which many people do at home routinely. Specialized fasting centers are there to support people in completing a 3-7 day fasting program. Those who fast are under medical observation and obtain a variety of supporting treatments such as massages, physiotherapy, breathing exercises etc.

So, does a 3-day fast truly reset your immune system? The science suggests that, if you can do it, a prolonged fast for 3 days or longer may induce your body to clean out some old immune cells and switch on production of new ones. Stay tuned.