I wondered about my lack of weight loss too from time to time. I wonder why I am not just wasting away with everything coming out of me so fast and in such great quantities.
I think the reason must be because I am a militant food eater. I will pretty much eat through anything, lack of appetite, pain, nausea, etc. I rarely vomit so I keep down what I put in.
For me the worse thing I can do for my body is not eat regularly so I am totally committed to a daily regimen of good quality, nutritional food of many different varieties at least three times a day. I use a joking tone, but I am serious - I have to eat or I get sicker.
Maybe you are like me and your body doesn't give up a calorie without a fight and after your eight months of diarrhea your body has adjusted and holds onto what it can. I think also it is probably true that the diarrhea some of us with UC have doesn't rob us of as much calories as the diarrhea that arises from other reasons.