Well it sounds like you may have the common sense to do it. Christians do 21 day fasts since people in the bible did that. The 2 people on this forum that tried 40 day fasts both disappeared into the ether after a few days.
Here is a
webpage on fasting with quotes from 19 MDs about fasting. One of them says that fasting is profoundly simple and simply profound.
Those 2 people mentioned above both said that they were doing water fasts but were adding strange things to their fasts like salt, broth, bat guana and who knows what else. I just read that one country's main export was bird droppings.