When you have UC, there's a risk you could get malnourished because you don't eat enough food. When you get flares, you can get cramping pain and severe diarrhea, which can sometimes alternate with hard bowel movements and constipation.
Certain foods may make your symptoms worse. So on top of a lack of appetite that goes along with flare-ups, you may be tempted to cut back on how much you eat in hopes of easing your symptoms.