Predominately water and intestinal content replacement, otherwise to gain 4 pounds of fat in a day would require taking in 14,000 calories in excess of what she is burning.
Then she did the third fast, came down to 103 lb, and of course, broke this fast with steak and milk!!I am starting to beleive she really had god's help, since she didn't go to the emergency room with gastritis.
In Olivia's case she was taking supplements throughout the fast so she never allowed her digestive system to go deep into slumber as with a true water-only fast. This would account for being able to break with steak and milk and create far less of a shock to the system, though even still not in any way advisable.