Quick bowel transit time and an inflamed colon mean you're probably not absorbing all the nutrients of the food you ARE managing to eat and 1,500 cals is really low for a guy (not sure what your height/weight is). I'd definitely supplement with protein or meal replacement shakes. Have your vitamin levels checked and make sure you're not anemic. Alot of folks feel B12 shots are helpful; I never could tell a difference.
I'm a believer in listening to the body. When you're fatigued, you do need to make sure you get plenty of rest. However, you may want to do some weight training to keep your healthy weight up or to replace lost weight. Gentle cardio might help your energy level, but I sure wouldn't try to run any marathons.
Diagnosed with ulcerative colitis spring 1999. Have been in remission for years with only a few minor blips. Scope on 6/22 showed no inflammation at all.
Maintenance dose sulfasalazine. Probiotics, vitamin D, fiber supplement and fish oil caps. George's aloe vera juice and l-glutamine for gut healing. Mostly grain-free and dairy-free low carb diet.