The electrolytes I use are some kind of salt combined with the elements you mentioned; without proper electrolytes you might die during longer fasts. I avoid any kind of juice to make sure that part of my digestive tract can rest and do some internal cleanup. Autophagy should start at around day 5 and immunity renewal at around day 3.

If I do sauna (many hot/cold cycles), I feel great for a day, if I do a 7-day fasting, I feel like that for a week. Sauna probably forces some mild cleanup through sweating, fasting through autophagy and burning off fat that stores toxins body didn't want to deal with earlier.