So I have fasted before, about 10 days, with very strict eating afterwards,lost 9kg, and I gained nothing back- I actually continued to lose. I really believe if you have weight to lose and eat really well then you won't gain anything, or much more than a few pounds.
This fast- I am on day 18. I went from 90 kg, 5 foot 8, to 80kg in the first 7 days. Keep in mind I had the weight to lose. In the last from day 8 until day 18 I lost 8kg, so total weight loss of 18kg.
This is heaps, I am not 100% sure of what makes the loss so large, but it is possible!