juice fasting (although very beneficial) is still "feeding" the body, so hunger can still persist throughout the length of the fast.
A Water Fast however, produces a state of inanition where hunger is suppressed by a process known as ketosis (a process that transfers the nourishment of the body from glucose to obtain energy to its stored fats instead. As a result of this, the ketone levels in your blood will rise and suppress appetite or what most perceive as hunger).
If you click on this link and look under the heading of "APPETITE" it could go a long way in answering your question.