I estimate the fat loss on estimated bmr where 1400 calories will yield about 1/3 pound of fat loss and about 1/10 to 1/8 that in protein-type cells and tissue.
That leaves water for the rest (the majority) and the fact that i gain back a pound per day for 21 days with light eating re-affirms for me that it can be nothing else. In many 7-8 day fasts over the years this has generally been about 10 pounds of water very consistently gained back within about 2 weeks.
My theory (and unfortunately i haven't seen this anywhere) is that the fasting body requires less water than the feeding body (and actually thrives on less), so a great amount of water is temporarily eliminated in a fast only to be restored once we are eating again. Maybe Chrisb1 has some information on this.