Stool color reflects what a person consumes and how quickly material moves through the digestive system. The typical brown color comes from bile, a fluid produced by the liver that aids in digestion. A change toward a darker shade, often brownish-black, is frequently benign and temporary, resulting from undigested pigments or minerals. True black stool, however, can signal a more serious internal issue, making visual and textural differences important to note.
Dietary Causes of Darkened Stool
Many deeply colored foods contain natural pigments that pass through the digestive tract largely intact, causing a noticeable change in stool color. Foods with dark blue, purple, or black hues are the most common culprits for this harmless darkening. Consuming large quantities of blueberries, blackberries, or black licorice can cause stool to take on a very dark, sometimes near-black, appearance.
Pigments like anthocyanins, found in dark berries, are not fully broken down or absorbed, allowing them to retain their color through excretion. This process of transit and elimination means the food coloring is responsible for the temporary change. Similarly, beets, which contain the strong red pigment betacyanin, sometimes result in stool that appears very dark brown or reddish-black.
Animal products with high iron content, such as blood sausage, also influence stool color. The iron in these foods is not completely absorbed, and the unabsorbed mineral interacts with other compounds in the gut. This process of mineral oxidation causes the stool to appear significantly darker, often a greenish-black shade.
Artificial food colorings, particularly dark blue, purple, or black dyes used in frostings, candies, and certain dark beverages, are powerful agents of color change. These synthetic pigments are stable, meaning they resist degradation by digestive enzymes. When consumed, these dyes travel through the gastrointestinal system and exit, staining the stool a dark shade until cleared from the system.
Non-Food Triggers: Medications and Supplements
Certain over-the-counter medications and dietary supplements cause a temporary, harmless darkening of stool that can mimic a more serious concern. Iron supplements are the most common non-food cause, frequently resulting in a dark greenish-black stool. This color change occurs because the body only absorbs a fraction of the iron dosage, and the unabsorbed compounds blacken during passage through the intestines due to oxidation.
Bismuth subsalicylate, the active ingredient in some popular anti-diarrhea and upset stomach remedies, is another frequent cause. When this compound enters the digestive tract, it reacts with trace amounts of sulfur present in the gut. This chemical reaction produces bismuth sulfide, a highly insoluble black salt that stains the stool a distinctive dark color.
Activated charcoal, sometimes used to treat poisoning or digestive issues, is a powerful darkening agent. As a pure carbon substance, it binds to substances in the digestive tract and is not absorbed. This binding action, combined with the charcoal’s inherent black color, results in a jet-black stool that persists until fully eliminated.
When Dark Stool Signals a Medical Concern
The most important distinction is between dark brown or black-stained stool and melena, the medical term for black, tarry stool resulting from digested blood. Melena is typically jet-black, sticky, and has a strong, foul odor due to the breakdown of hemoglobin by digestive enzymes and intestinal bacteria. This appearance signals bleeding in the upper gastrointestinal (GI) tract, such as the stomach or upper small intestine, where blood has time to be digested before excretion.
If the dark stool is truly melena, it is often accompanied by other symptoms indicating blood loss or an underlying GI issue. People may experience unexplained fatigue, weakness, or lightheadedness, which are signs of anemia. Abdominal pain, vomiting blood, or sudden dizziness are associated symptoms that warrant immediate medical attention.
Conditions like peptic ulcers, severe gastritis, or esophageal varices are common causes of upper GI bleeding that lead to melena. Unlike food-induced darkening, which is temporary and uniform, melena persists and relates to an active internal process. If dark stool is accompanied by a sticky, tar-like texture and a noticeably offensive smell, professional medical evaluation is necessary.