Thanks! I can well believe that 10-12 pounds of fat takes up/constitutes a gallon of water, but my question really was whether, when absolute fasting, the body has the "wisdom" to go to that (if it exists in someone's body to spare) and break it down for use as water. I just came off of a 3-day DF, not well-hydrated to begin with, and could easily do another one, but at the end of my next one, I don't think I'd have lost 10 pounds of fat (let's say in three days my body needed to access a gallon of waterfor its physiological processes).
If the body COULD break down fat during a DF, we would not be looking for other signs that it is time to break one; all we'd have to say is, "You can DF until you have very little fat on you" because we could not get dehydrated UNTIL then.