Black substance in feces may be related to diet, digestive bleeding, constipation and other factors. 1. Diet: black substance in feces may be caused by eating foods with high iron content, such as pig liver, chicken liver, black fungus, etc. You can stop eating these foods for a while, and it will not have any effect if your stools recover. 2. Digestive bleeding: black substance in feces may be caused by digestive bleeding. Gastroscopy, fecal routine and other examination tests should be conducted to clarify. 3. Constipation: The most common type of feces is black, which is mainly caused by constipation. The stool can not be pulled out for several days in the intestines, and the stool changes color due to water absorption and bacterial decomposition of bilirubin. When patients have black material in the feces for a long time, they should go to the hospital in time to find out the cause of the disease, and carry out the correct treatment under the guidance of the doctor. In daily life, you should exercise more and eat more vegetables and fruits to help digestion.