I had a 12 day liquid Pre-op. The first couple days were rough, but I took Tylenol to get through the headaches. Those were due to withdrawal from sugar, caffeine, etc. After the first couple days it wasn't too bad. I lost 10 pounds.
The preop is to shrink your liver so the surgeon has better access during surgery. It's not necessarily to lose weight, but I was glad I did because I gained 10 pounds from the IV fluids during my hospital stay.
I was very grateful to have gone through the sugar and caffeine withdrawals preop because it would have been awful to go through that in addition to the surgery recovery at the same time!