I’m going to start another 7 day fast this Friday and I might push it to 10 days. Last year I did a 7 day fast (water only) and I felt refreshed physically/mentally.
I really have to psych myself up because it isn’t easy to do but once you get the train rolling it gets more tolerable. Especially after day 3. Day 4-5 I felt a big burst in energy and all my senses were heightened.
Russia and Germany have fasting centers at some hospitals. Here is a fasting documentary I found interesting
https://m.youtube.com/watch?v=t1b08X-GvRs&t=1432s