Brown Color of Mucus in Stool
The brown color of mucus in stool is primarily caused by stercobilin, a breakdown product of bilirubin that results from the normal metabolism of hemoglobin through the hepatobiliary system and bacterial action in the intestines.
Physiological Mechanism of Brown Coloration
Bilirubin Metabolism and Transformation
Bilirubin is secreted into bile as conjugated bilirubin (bilirubin diglucuronide and monoconjugates) by the liver and enters the intestinal tract through the biliary system 1, 2.
Bacterial enzymes in the colon, particularly β-glucuronidase, hydrolyze conjugated bilirubin to produce unconjugated bilirubin 1, 2, 3.
Further bacterial reduction of unconjugated bilirubin in the intestinal lumen converts it to urobilinogen and then to stercobilin, which imparts the characteristic brown color to stool 1, 2.
The Role of Intestinal Bacteria
Enteric bacteria in the colon produce hydrolytic enzymes including β-glucuronidase that are essential for converting bilirubin conjugates into their colored metabolites 2, 3.
The presence of anaerobic bacteria is particularly important for the complete reduction pathway that ultimately produces the brown pigments 3.
Clinical Context: When Mucus Color Matters
Normal vs. Abnormal Mucus Appearance
Colonic, inflammatory, or secretory forms of diarrhea typically present with liquid loose stools with blood or mucous discharge, and inspection of the stool may be helpful in distinguishing pathology 4.
When mucus appears brown in stool, it has been mixed with fecal material and exposed to the normal stercobilin pigmentation process, indicating it originated from or passed through areas where normal bacterial metabolism occurs 4.
Conditions Affecting Bilirubin and Stool Color
Increased unconjugated bilirubin in bile may occur in conditions such as hemolysis, Gilbert's syndrome, or bacterial overgrowth, which can affect the overall pigmentation of intestinal contents 5, 1, 2.
Total bilirubin levels are associated with small intestinal bacterial overgrowth (SIBO) in diarrhea-predominant IBS, suggesting that alterations in bilirubin metabolism may influence stool characteristics 5.
Important Clinical Caveats
When to Investigate Further
If mucus appears with blood, this suggests colonic or inflammatory pathology and warrants endoscopic evaluation 4.
Pale or clay-colored stools indicate biliary obstruction and absence of bilirubin reaching the intestine, which is a red flag requiring immediate investigation 4.
Steatorrhea presents with bulky, malodorous pale stools due to fat malabsorption, distinct from normal brown-colored mucus 4.
Artificial Coloration from Medical Procedures
Methylene blue used in endoscopic procedures may cause a green hue to urine and stool for up to 24 hours, which should not be confused with pathologic changes 6, 7.
Certain laxatives can alter stool color: phenolphthalein and some anthraquinones turn stool red, while bisacodyl turns it purple-blue 4.