Hey,

don't know how long your intended fast is.
I once attempted a juice fasting program that was supposed to go on for a week.

I gave up Wednesday morning. I felt horrible for 2 days. Cold, tired, unconcentrated, cranky, there was this horrible fermented grain juice I was supposed to down in the mornings; on Wed I sat in front of it, crying, for an hour or two before I chucked it down the drain and made myself some soup.

Honestly, biologically speaking I see no sense in fasting or "cleansing" - but if you want to do it as a "body-mind-spirit" exercise, go ahead...