Hey Skinny, The way my doctor does it is he has you come in for a pre-op either 2 weeks (diabetic) or 1 week (everyone else) before your surgery. At the pre-op, they go over the diet and send you home with everything you are allowed to eat so there is no guess work.. It's about 650 to 800 cals a day and totally no carb. Outside of provided diet, I can have sugar-free Jello, sugar-free Popsicle and various sugar free drinks like crystal light.
Four meals a day are an 8oz shake or bowl of broth. I can have one NutriMed bar a day as well.
The second day was the worse for me, once your body hits ketosis, it's a lot easier. I felt hungry and craved food but the headache and sluggishness have subsided.