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Medical Author: Jay W. Marks, MD
Medical Editor: Melissa Conrad Stöppler, MD
Stool (feces) is most commonly brown in color, and many people become curious or concerned when the color of their stool changes. Most stool-to-stool changes in color have little meaning; however, some changes, particularly if the changes are consistent from stool-to-stool and not present in only one stool, can be important.
The color of stool is normally due to the presence of bile, specifically, the bilirubin in bile. Bilirubin is formed from hemoglobin after hemoglobin is released from red blood cells during their destruction, a part of the normal process of replacing the red blood cells in blood. The released hemoglobin is modified chemically and removed from the blood by the liver. In the liver the chemically changed hemoglobin (called bilirubin) is attached to other chemicals and secreted from the cells of the liver into bile. Depending on the concentration of bilirubin, bile can vary from almost black to light yellow in color.
Bile travels through the bile ducts (and gallbladder) and into the intestines. As the bilirubin travels through the intestines, some of it undergoes further chemical changes, and some of these changes can have an effect on the color of stool. These changes depend primarily on the speed with which the intestinal contents traverse the intestines.
If the intestinal contents travel at a normal speed, stool is light to dark brown. If the intestinal contents travel more rapidly, chemical changes to bilirubin - and/or the lack of them - may turn the stool green. This is not by itself an important color change. If there is no bilirubin (bile) in the stool, the stool is a gray, clay-like color, an important change in color since it suggests that the flow of bile into the intestine is blocked. The most common causes of blockage are tumors of the bile ducts or pancreas.
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