Hi, I had gone through your question and understand your concerns. Well this should be investigated in every case. It is hard to say if anorexia and blood in stool are related but some bowel diseases may lead to weight loss+ blood in stool and in first case I mean coeliacia, ulcerative colitis and crohns disease. So you should do colonoscopy definitely. It will also rule out more common causes of blood in stool such as hemorrhoids, anal fissure or rectal polyps. These can then be treated but as I said, more severe issues should be investigated with colonoscopy especially if there is associated anemia in blood picture. Hope this answers your question. If you have additional questions or follow up questions then please do not hesitate in writing to us. I will be happy to answer your questions. Wishing you good health.