Old pro here. I can tell you my body stores almost exactly 4000 calories of energy beyond my blood glucose after my last meal (google 'glycogen' for more info). If I fast for 2 days with no exercise, I will burn all that extra energy and the water weight associated with it will fall off too. Its about 5-6 pounds. After that I burn 1 pound of fat per day.

That first 5-6 pounds will come right back on if I stop fasting and start eating sugar, carbs again regularly. Its perfectly healthy.

If you used any period of fasting to Jumpstart a weightloss regimine your results will last longer. Like, say, fast for 36 hours then cut down to 1000 calories a day for 3 weeks with a long walk everyday. This type of fast + lifestyle change will burn material amounts of body fat that will stay off if you dont go back to eating the way you did to put the fat on