My best guess is that rapid weight loss is associated with pancolitis and especially backwash ileitis in UC patients. Aside from the weight loss that arises from voluntary dietary restrictions to control symptoms (which -- don't get me wrong -- can be significant), weight loss from a flare is going to come from calories not being absorbed properly in the small intestine. The further away from the small intestine the inflammation, the less likely you are to face that problem.
When I was diagnosed only about
2/3 of the my colon was inflamed (and the scope did go through to the TI), and I hadn't really lost any weight. Colonoscopy about
five months later showed total pancolitis as well as backwash ileitis, and that was when the weight loss became relentless for me -- I lost about
35 lb in a few weeks, went back on prednisone, and continued to lose slowly (a pound a week or so) until I had surgery.
dx'ed UC pancolitis 5/12
past meds: asacol hd, VSL#3, apriso, rowasa, xifaxan, 6mp, cortifoam, pentasa, cimzia, canasa, butyrate, flagyl, cipro, prednisone, remicade, methotrexate, cholestyramine, cortenema
current meds: none!
step one: colectomy, end ileo 1/16/13
step two: j-pouch construction, loop ileo 5/1/13
step three: takedown 7/31/13