Why are people losing weight from UC?

First, I dont want this to rub anyone the wrong way. I am just curious and maybe I can learn something here to be more healthy myself.

I've been hearing many people here say they lost a lot of weight over time because of UC. I am currently gaining weight because I love eating. Are people losing weight because UC is directly causing them to lose weight or they cut out a lot of high calorie foods from their diet?