I've been fasting periodically for most of my life, and I'm a firm believer in its health benefits.
Today, I found this:
Fasting for three days can regenerate entire immune system, study finds - Flowin
My Master advises one day a week fasting. I think that's quite safe (for most people). When you have health issues, fasting can kill you.
My health is too bad for fasting (10 kg underweight). I tried it once, and within 20 hours I went into coma. Was an interesting experience though. I never experienced that before. Luckily my Master helped me to get out of my coma, otherwise I would not be typing now (I was alone in my house).
So, maybe if you are not underweight, and having a good health (not too bad), it will work (without killing you).
I remember reading years ago (ca. 1990), that, when your immune system is not good, and you continue fasting till you almost die, then just before you die, your immune system will kick in. Makes sense to me. So, the fat and relatively healthy, maybe have to fast for weeks, to get to that point