I am sorry to hear that you have insipidus diabetes. I must say that fasting does not cause this condition.
There are a few things known about this condition. One, it can be caused by weak hypothalamus and pituitary gland, the other, it can be caused by weak kidneys.
For boosting hypothalamus function one should take omega 3, good fats, vitamin Bs and chromium. For kidney function one can do kidney cleanses, drink lemon water 3-4 times a day. Also regular exercise and yoga will help as well.
I am sure the doctors will give you drugs that will mimic hormone, vasopressin.