@star0210 is right, it's for your liver. The LAST 2 days of liquid only is to clear your bowels. I did 5 days protein shakes but they also allowed me as many vegetables as I wanted (except corn, peas and potatoes) until the last 2 days, then only protein shakes. It wasn't pleaseant to start out but I found if I could find something to occupy myself with I really didn't think about it. As for the last 2 days, you'll be so close to surgery that you won't mind it at all.