fasting is okay every once in a while. something to keep in mind is that hydration is more than just drinking more water. in fact, the goal is to be so hydrated that you don't need to drink as often. hydration is mainly achieved through electrolytes, and it's more important when you're fasting and in ketosis.

when someone only drinks water and nothing else, eventually through a great deal of time or exertion, they will get rhabdomyolysis and let's just say your kidneys will not enjoy it.

i would always get headaches and fatigue fasting, but after supplementing with some seasalt it would go away. i suppose it's obvious but just my experience from personal pains to anybody else that ever tries...