I agree, I lost 9lbs in 10 days on my pre op liquid diet. I was only allowed to drink sugar free carnation instant breakfast drinks with skim milk up to 5 times a day. Chicken/ beef broth, sugar free Jello and sugar free Popsicles.
I could only have the Protein shakes the first 9 days, on the last day, I was only allowed Clear liquids.
The first 3 days are the worst but I actually got to a point where I could not even drink 5 of those protein shakes a day.
I would check with your nutritionist first!