I can attest to this. I completed a 7 day fast once, and by the end of the fast I was more alert, I had a boost in mental clarity, I had a lot of energy and I noticed improvement in other physical abilities.
During the fast I drunk 1 gallon of water a day, along with herbs to feed the cells and control the appetite. I would also sip some fruit juice to control the blood sugar.
Prior to the fast I was also experiencing symptoms that were similar to Plantar fasciitis. Those symptoms were gone by the end of the fast.