In actuality, this statement comes from the quack health manual Your Body's Many Cries for Water, written by one Dr. F. Batmanghelidj. He starts with the valid premise that Americans are overweight and goes on to claim that hunger is actually the body calling for water,that when one feels thirsty dehydration is already far advanced, and other such claims. Hs assertions are unproven, and it is possible to drink too much water and injure oneself. He actually does not mention prepared foods.
Exposure to this "fact" was provided by a chain email.