I had my gall bladdar out last year, and my doctor told me to stick to a low-fat diet. I guess fat cannot be processed as well without the gall bladdar. I didn't really follow his advice, but you might find it worth trying.
DX: Pancolitis as of 9/08 (Proctitis as of 1/08) and IBS
What Worked: Spinach/Sunflower Seed Diet & IBS Diet (www.helpforibs.com)
What Didn't Work: Canasa suppositories, Cortifoam, Prednisone, Colazal, Symex DuoTabs, Rowasa, Bentyl, Cortenema, Asacol, Iron, Zoloft, Acidophilus.