Got to agree. Either you are up for it or... not! The "numbers" really don't figure into fasting for the very reasons Pepe mentions. Everything gets shook up! That's the whole idea!
However, if you have serious concerns perhaps you should have your fast supervised by a sympathetic natural health professional with some experience in fasting, as I did on my very long runs (I have fasted 60 days and am here to tell the story).
I am another whose life cleansing has changed, for the radiantly healthy better! I could never have accomplished my personal health goals without juice fasting, professional colonics and the liver flush. I am a poster child!