Black diarrhea is often the sign of blood in the digestive tract that can have multiple causes depending on where the bleeding is coming from. Locations can range from anywhere from the nose or mouth to the colon, though bleeding lower in the GI tract tends to present as blood in the stool more than as black diarrhea. Black diarrhea caused by bleeding is called melena. Other causes include certain foods, drugs and toxins.
Certain foods can cause black diarrhea. Dark colored fruits and vegetables such as blueberries, blackberries an beets are one cause. Deserts such as black licorice and black pudding can also cause black diarrhea, as can bloody meats.
Minerals, Medicines and Toxins
Iron supplements, as given for iron deficiency anemia and other conditions of low iron, can be a source of black diarrhea. Charcoal can cause this symptom too. It is sometimes given to treat toxin ingestion, because it can bind to some drugs in the GI tract. Bismuth is the active ingredient in Pepto Bismol and can also cause black diarrhea. Maalox is another GI medication that may cause black diarrhea. The toxic mineral lead can also be a cause. Lead paint in older homes is the most frequent cause of lead poisoning in the United States according to "Nelson Textbook of Pediatrics" by Robert M. Kliegman, M.D.
Oral, Nasal and Esophageal Bleeding
According to "Harrison's Principles of Internal Medicine" by Anthony S Fauci M.D., brisk bleeding from the oral or nasal cavities, as in a nose bleed, can be swallowed into the GI tract and cause melena. There are several sources of bleeding from the esophagus. Esophageal ulcers or inflammation can be caused by infections or the reflux of acid into the esophagus from the stomach. A Mallory-Weiss tear is a tear in the esophagus from retching and vomiting, generally after drinking excessive amounts of alcohol. Esophageal varices are dilated veins in the esophagus, typically from liver cirrhosis. They are dangerous because they can cause brisk bleeding into the GI tract.
Ulcers in the stomach can also bleed. Infections, stomach acid and cancers are all sources of ulcers. Angiodysplasia is a malformation of blood vessels that can bleed and cause melena. Gastritis, or general inflammation of the stomach, can also be a cause but is a more rare source of bleeding.
Small Intestine Bleeds
The small intestine can also develop ulcers, typically in the duodenum, which is the first part of the small intestine near the stomach. Infection with the bacteria Helicobacter pylori is a the most frequent cause of a duodenal ulcer. A Meckel's diverticulum can form lower in the small intestine. This lesion is a growth of normal, non-intestinal tissue such as stomach tissue in the intestine. A small focus of stomach tissue in the intestine can make stomach acid, dump it into the intestine and cause ulcers or bleeding. Another condition that can cause bleeds in the small intestine leading to black diarrhea is Crohn's disease. This is an inflammatory disease that can be found throughout the GI tract in which lesions erode the tissues of the bowel wall.
Large Intestine Bleeds
Large intestine bleeds are more likely to cause frank blood in the stool rather than black diarrhea but can be a source of black diarrhea, particularly if the bleeding is slow and ongoing. Cancer and angiodysplasia are two possible sources of slow bleeding in the large intestine.