Here's the information my doctor gave me for pre op diet:
Pre Op diet
In the obese, the liver is a huge, fatty organ. It is “in the way” of the surgical area during surgery, and must be held back with surgical instruments. An enlarged liver is very fragile and bleeds quite easily when touched or manipulated during surgery. This bleeding can be difficult to control, and the extra care required with an enlarged liver increases surgical risk, as well as surgical time. Longer surgical time and extended time under general anesthesia can make both immediate and longer-term recovery much more difficult for you.
For those with a BMI of 40 or over, a special high Protein low-carb pre op diet has been extremely effective in shrinking the liver, so that the risks of bleeding and extended surgery times are greatly decreased. Those with lower BMIs may also choose, if they wish, to follow the liver-shrinking diet to further decrease their surgical risks.
Following this SPECIFIC DIET PLAN is extremely important. The goal is not to lose weight, necessarily, but to enter a temporary period of dietary ketosis, which is not harmful to you at all. However, many do lose a good amount of weight, which will jump-start your weight loss as you begin to regain and improve your health. It is this dietary ketosis that results in liver-shrinking and the normalizing of your metabolism so that you can do well with weight loss surgery.
LIVER-SHRINKING DIET PLAN
There are NO CALORIE LIMITATIONS on the LIVER-SHRINKING diet. You may eat/drink as much as you wish of the allowed things to feel satisfied, although we suggest reasonable limits to assist your weight loss. This does not have to be a liquid diet.
Start a low carb (30 grams or less daily), high protein (minimum 60-70 grams daily), keep a close eye on the fat intake (don't over do it) and at least the minimum required daily amount (64 ounces) of plain Water intake diet prior to surgery.
If you chose to drink a Protein shake. Please read the labels. You should get at least 15 grams of Protein & less than 6 grams of total Carbohydrates per serving. Protein shakes are NOT required, but are very convenient.
You may have low fat yogurt, lean meats, grilled chicken, green vegetables. We recommend checking the carb content on your food. You can find a very useful carb counter on Atkins.com
http://www.atkins.com/Program/FourPhases/CarbCounter.aspx