03-12-2012, 10:59 PM
Diet Coke, coffee, tea, etc. are ~99% water. There's no "turning into" anything. Your body has to absorb the water from these drinks like any other. The caffeine may serve as a diuretic and increase your kidneys' water output but you don't have some valve in your digestive system which takes caffeinated drinks and shunts them directly from the GI tract to the kidneys. They all take a ride through your bloodstream after they are absorbed in the GI tract.