It is water weight, but as miata just mentioned, some of it is water that was stored in glycogen, rather than water being retained due to sodium. Glycogen combines glucose and water, so when you burn it a bunch of water is released. Fasting burns stored glycogen because your brain needs a certain amount of glucose each day to survive.
Originally Posted by Ariel C. McGlothin
I would go by thirst and energy levels. Your body will retain how much it needs based on glucose and electrolyte levels, unless something is broken.
Eureka5280: M / 38 / 235lbs / Goal: 180lbs
Diet: Currently experimenting with higher carb (Peat-esque) primal with emphasis on beef, dairy, seafood, sugar and a bit of starch on lifting days.
Activities: Started Stronglifts 5x5 on 3/1/14. Adding sprints and hikes soon.
End of Year Working Set Goals: Squats-250, Bench-200, DL-315