A water fast is based on consuming nothing but water, some even use only purified water. This means that the electrolytes needed are absent. This can lead to water intoxication that can cause severe swelling of the brain leading to damage or even death. As English Dave said, the brain needs a supply of glucose. Hypoglycemic shock or seizure can result.

I woke up in a hypoglycemic seizure once after having my wisdom teeth extracted. I had not consumed enough calories for 4 + days and woke up paralyzed with my muscles in contraction. Fortunately, I had room mates who were watching my recovery who called the dentist and were given instruction to carefully pour some sugar (instant breakfast) into my mouth. The sublingual absorption of sugar slowly relieved the muscle contraction.

So, Be careful with such a fast.