Purpose of pre-op fast?

I was wondering if anyone knows the purpose of the pre-op fast. I noticed that there is a huge variation in what different doctors require. Everything from 2 weeks to a month and shakes only to additional food choices! So I was wondering what the actual purpose of it is since the requirements are so vaired.

I am ending day 2 of my 2 week pre-op fast and I am only allowed 3-6 shakes per day and water I guess although was not told that. I have had a little ice tea as well and don't know if that is ok either. I was told I needed to use Optifast which is extremely expensive and tastes awful after a few glasses of it. So I am alternating it with Premier Protein drinks. I would SO love to have a little bit of jello or cream of wheat or even some yogurt which I don't even like.

So if anyone knows WHY we are required to do this, I would love to hear it. Thanks.

I am just finishing my two week pre-op fasting. My doctor had given explained to me that the purpose of the liquid only diet was to help shrink your liver before surgery as well as prepare your body for the change since you will be on liquids only for the first few weeks following surgery. So far I have found the only protein powder I can honestly say isn't so bad is Body Fortress Super Advanced Whey Protein. It can be purchased at Walmart, CVS, and I believe some grocery stores have it as well. For one scoop I mix it with 8 oz. of skim milk. I have 4-5 protein shakes a day plus 64 oz. of water. Sometimes I had a propel flavor packet to the water since they have 0 calories.

They did also tell me that sugar free pop-sickles and sugar free jello or pudding was okay.