My surgery is scheduled for August 1st and I'm on day 4 of the pre op diet too! My physician has me doing Optifast, 6 times a day, 3 shakes, 2 bars and 1 soup. I too am having diarrhea but like everyone is saying, fluid in, fluid out. Other than that and the occasional stomach rumbling I'm doing pretty good and down 4 pounds. Hang in there and good luck!