An internal process that may change the color of stool but that is less common than bleeding is pancreatic disease. The pancreas secretes enzymes into the intestines that help with the digestion of fat, protein, and carbohydrates. When pancreatic enzymes are not present in the intestine, fat is not fully digested. If the content of undigested fat is high enough, stool may become a yellowish color. The stool also appears "greasy" and smells bad. The most common pancreatic disease that causes this are pancreatic tumors that block the pancreatic duct through which the enzymes reach the intestines. Another cause is chronic pancreatitis, usually due to damage from excessive ingestion of alcohol, that destroys the ability of the pancreas to make enzymes. If there is both blood and undigested fat in the stool, the stool may become silver.
Several ingested substances can change the color of stool. Iron and bismuth-containing medications, for example, Pepto-Bismol, turn the stool black. Beets and possibly some other red vegetables and fruits can turn stool a reddish color. Food dyes used for coloring foods also may color the stool.
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