It was found fasting kills off old immune cells and activates stem cells to make new ones.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7474734/
https://pubmed.ncbi.nlm.nih.gov/31442401/
https://www.nejm.org/doi/10.1056/NEJMra1905136
http://www.cell.com/cell-stem-cell/abstract/S1934-5909(14)00...
In plain English
https://www.forbes.com/sites/stevensalzberg/2014/12/30/can-a...
https://www.medicalnewstoday.com/articles/277860#In-mice,-pr...