The only true way is water and urine only. But it is very beneficial to do a juice fast, or fast with light soup instead of eating.
If you want to cure yourself you will do it faster if you take no other foods. A fast reserves energy and heals rapidly, a urine fast can renew organs and systems too.
If you take in small amounts of food then the body does not know whether to go into ketosis or normal metabolism and you can go into starvation mode where the body does not automatically protect itself from losing essential nutrients. I got that from the fasting forum, it is not my own knowledge.
Regarding energy, I could walk a kilometre with a gentle slope both ways and be OK, need a small rest when I returned, but when I tried to walk 100 metres up a steeper street I lost most of my energy and had to lie down for 30 minutes.
So you have plenty of energy on a complete fast at a certain level of exertion, not as much as normal of course, but if you go over that limit with extra exertion you become exhausted for a while.
I could do my work on the computer, clean up the kitchen, water the gardens, pack some items into boxes and light work like that. The misconception is that the longer you fast the less energy you have.
From about day 5 it is much the same until the end although as you heal your energy will actually increase, not decrease. You go up and down slightly as your body adjusts and heals itself from day to day, then when you have healed you start to feel more energy. John Armstrong liked to fast people an extra four days to make sure.