For those that in the pre-op phase, just know that there are lots of different opinions you'll hear about nutrition and getting ready for surgery, and after the surgery. The main thing they want to see in the beginning is that you can stick to a program and show progress before you introduce the element of surgery. Because after that, there's no turning back. Try to find a routine that you can keep up for months and months (maybe years), as this is a life changing decision.
What worked for me- high protein, low carb, kept calories below 1200 pre-op, and drank lots of Gatorade Zero and protein shakes. I had to read lots of labels and try different things to find foods/shakes/protein bars I liked enough to have every day. And I kept a log of everything I ate and drank for three or four months until I got into a solid routine. I stuck with it and am still following the program for the most part, although I have made a few changes as my body has changed.
Remember that everyone is different and you may need something different - That's between you and your doctor and the nutritionist. They should be able to help you come up with a good plan that works and this forum has lots of people that have been down this road, so there is a lot of helpful advice and wisdom to get here.