Why Is Poop Brown? The Science of Stercobilin

The characteristic brown color of feces comes primarily from stercobilin, a pigment derived from the breakdown of hemoglobin.

The Biochemical Pathway

1. Red Blood Cell Turnover: Aged RBCs (lifespan ~120 days) are phagocytosed by macrophages in the spleen, liver, and bone marrow.
2. Hemoglobin Breakdown: Heme is converted to biliverdin (green), then to unconjugated bilirubin (yellow).
3. Liver Processing: Unconjugated bilirubin binds albumin, travels to liver, and is conjugated with glucuronic acid to form conjugated bilirubin (water-soluble).
4. Biliary Excretion: Conjugated bilirubin is secreted into bile, stored in gallbladder, and released into the duodenum.
5. Bacterial Action: Intestinal bacteria (especially in colon) deconjugate and reduce bilirubin to urobilinogen.
6. Final Conversion: Most urobilinogen is oxidized to stercobilin (brown) in the colon, giving stool its color. A small portion is reabsorbed (enterohepatic circulation) and excreted in urine as urobilin (yellow).

Factors Affecting Stool Color

• Diet: Beets (red), spinach (green), artificial dyes.
• Transit Time: Rapid transit (diarrhea) → less bacterial conversion → lighter/yellow stool. Slow transit → darker brown.
• Bile Output: Obstruction (e.g., gallstones) → pale/clay-colored stool (acholic).
• Medications: Iron supplements (black), bismuth (black), antibiotics (alter flora).

Persistent pale, black (tarry), or red stool warrants medical evaluation to rule out biliary obstruction, upper GI bleeding, or lower GI bleeding.