Originally Posted by MoAlpha

You have three fluid compartments to fill. The first is circulating plasma; the second is the interstitial space (in tissue between cells); and the third is intracellular. Transfer from compartment to compartment, particularly from interstitial to intracellular is slow. You can dilute your plasma and piss like a race horse while your cells are still dehydrated.

Then there is the weight of muscle and liver glycogen and the water bound in its crystalline structure. That can take a day to replenish too.