Stool color can change for a variety of reasons. The change may reflect substances that are added to stool or changes to substances normally present in the stool. Some stool color changes may suggest an underlying medical condition, and others may be due to the ingestion of certain foods or medications.
Black Stools (Not Sticky, No Odor)
Causes of black stool include iron pills or bismuth-containing medications (such as bismuth subsalicylate or Pepto-Bismol). If the stool color is dark because of any of these medications, it is typically not sticky in texture and is not foul-smelling.
Black Tarry, Sticky Stools
Bleeding in the stomach (from gastritis or an ulcer) or the intestines can change the color of the stool. If bleeding occurs in the stomach or the upper part of the small intestine, the stool may turn black and sticky, and be described medically as black, tarry stool (melena). Generally, the black, tarry stool also is foul-smelling. This change in color and consistency occurs because of chemical reactions to blood within the intestine that are caused by digestive enzymes within the intestines.
Maroon or Red Stools
If the bleeding originates from lower parts of the intestines or the colon, blood may not come into prolonged contact with digestive enzymes because of the short distance from the site of bleeding to the rectum. Moreover, large amounts of blood within the intestines speed up the transit of stool so that there is less time for the changes to take place. The stool in this type of bleeding may be dark red or maroon in color. Beets, other red vegetables, cranberries, and red food dyes also can turn the stool color red or maroon.
Gray or Clay-Colored Stool
The stool can be gray or clay-colored if it contains little or no bile. The pale color may signify a condition (biliary obstruction) where the flow of bile to the intestine is obstructed, such as obstruction of the bile duct from a tumor or gallstone in the duct or nearby pancreas. The change of stool color to gray or clay typically occurs gradually as these medical conditions progress relatively slowly and stool becomes pale over time.
Yellow Stool
Stool that is yellow may suggest the presence of undigested fat in the stool.
This can occur as a result of diseases of the pancreas that reduce delivery of digestive enzymes to the intestines (pancreatic insufficiency), such as:
- cystic fibrosis,
- chronic pancreatitis (long-standing inflammation and destruction of the pancreas usually due to alcohol abuse), or
- obstruction of the pancreatic duct that carries the enzymes to the intestines (most commonly due to pancreatic cancer).
Celiac disease: Another condition that possibly may cause yellow and greasy stool is celiac disease (a malabsorption syndrome).
The digestive enzymes released from the pancreas and into the intestines are necessary to help digest fat and other components of food (proteins, carbohydrates) in the intestines so that they can be absorbed into the body. If the pancreas is not delivering enzymes into the intestines, then components of food, especially the fat, can remain undigested and unabsorbed. The stool containing the undigested fat may appear yellowish in color, greasy, and also may smell foul.
Ingestion of very high-fat foods also can cause yellow, soft, and foul-smelling stools.
Weight loss medications such as orlistat (Xenical, Alli) work by limiting the amount of fat absorbed by the intestines. This can lead to bulky, yellow, and greasy stools.
Green Stool
When stool passes through the intestines rapidly (diarrhea), there may be little time for bilirubin to undergo its usual chemical changes, and stool can appear green in appearance due to rapid transit.
Eating excessive amounts of green foods, foods with green or purple dyes, and vegetables also can cause stool color to turn more green than normal.
