Why Is Healthy Poop Yellowish Brown?
⚠️ This science article comes with a certain aroma!
Why is poop usually yellowish brown no matter what we eat? Does the color come from food, or from an epic journey through the body? By the end of this article, you will know where the familiar color comes from—and what other stool colors may be trying to tell you.
Authors: 阿氯 and 时光
Reviewer: 司徒陌
You probably do not study your poop closely after using the bathroom. Take an occasional look, though, and you will notice that it is usually yellowish brown. Braised pork and leafy greens generally end up producing the same color, although a few foods—notably dragon fruit—can tint the stool. The normal yellow-brown color comes mainly from stercobilin, a product of the reactions that bilirubin undergoes in the body.
So bilirubin appears to be the culprit. Where does it come from?
A healthy adult red blood cell lives for an average of 115 days. Every day, some of the body’s red blood cells age. These old or damaged cells are eventually removed by mononuclear phagocytes in the spleen, liver, bone marrow, and other tissues. When their hemoglobin is dismantled, a component called heme is first converted by heme oxygenase 1 (HO-1) into green biliverdin. Biliverdin reductase (BVR) then catalyzes its conversion into orange-yellow unconjugated bilirubin.
Unconjugated bilirubin does not dissolve readily in water. Once it binds to albumin, however, the resulting bilirubin–albumin complex can circulate in the blood. After entering a liver cell, unconjugated bilirubin is joined to glucuronic acid by glucuronosyltransferase, producing conjugated bilirubin, mainly in the form of bilirubin diglucuronide. Conjugation makes bilirubin more water-soluble, and transport proteins actively secrete it into the bile.
Bile leaves the liver through the bile ducts. Some flows directly into the small intestine, while the rest is stored temporarily in the gallbladder. The gallbladder’s lining absorbs water and some electrolytes. As the water content falls, bile pigments, bile acids, and other components become more concentrated, so bile in the gallbladder is usually darker and more concentrated than freshly secreted liver bile.
After we eat—especially when the meal contains fat—the gallbladder contracts and releases concentrated bile into the duodenum to help digest that fat. Bile acids disperse fat into smaller droplets and promote the digestion and absorption of fats and fat-soluble substances. Bilirubin travels into the intestine along with the bile.
A Bacterial Color Change
Once conjugated bilirubin reaches the intestine, β-glucuronidase produced by gut bacteria first removes the attached glucuronic acid through hydrolysis, turning it back into unconjugated bilirubin. Some gut bacteria then use enzymes including bilirubin reductase to convert bilirubin into colorless urobilinogen.
We are now close to the answer. Some of that urobilinogen is converted in the intestine into stercobilinogen. It travels down the gut with the remaining food matter and, near the end of the colon or after exposure to air outside the body, is oxidized into yellow-brown stercobilin. This is the main source of the color of normal stool. Some urobilinogen is reabsorbed by the intestine, returns to the liver through the portal vein, and is secreted back into the gut with the bile. This process is called the enterohepatic circulation of urobilinogen. Another portion enters the blood, reaches the kidneys, gradually oxidizes into yellow urobilin, and leaves the body in urine. That is one reason urine is pale yellow.
In short, the yellow-brown color of poop is mainly the color of stercobilin.
What Other Colors Can Stool Be?
Clearly, not every bowel movement is yellowish brown. Many readers will have had this experience: eat a large amount of dragon fruit one day, and your stool turns purplish red. You may also have encountered more mysterious colors—green, black, or even white. Diet, artificial coloring, illness, and medication can all change the color of stool. Here are several colors commonly associated with disease:
1. Black
Viewers of Chubbyemu’s videos may often have seen patients pass black, tarry stool. This is common in upper gastrointestinal bleeding, because hemoglobin in the blood is digested and broken down as it passes through the gastrointestinal tract, darkening the stool.
2. Bright Red
Unlike the purplish-red stool caused by dragon fruit, bright-red or dark-red stool often indicates bleeding, commonly from the colon, rectum, or anus. Heavy, rapid bleeding from the upper gastrointestinal tract can also produce dark-red or even bright-red stool.
3. Gray or White
If far less bile and bile pigment enter the intestine—as can happen when a bile duct is obstructed—the stool may lose its normal yellow-brown color and become off-white, pale gray, or clay-colored. Intrahepatic cholestasis and some severe liver or biliary diseases can produce a similar appearance.
4. Green
During diarrhea, or whenever material moves through the intestine too quickly, gut microbes may not have enough time to convert bile pigments and their metabolites fully into yellow-brown stercobilin. The stool may therefore appear yellow-green or green. Iron supplements and artificial colorings can produce green stool as well.
Every stool color can have more than one cause, so leave the specific diagnosis to a doctor. Eat well, everyone, and may your poop be healthy (*•ω•).
References
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[2] Chowdhury JR, Chowdhury NR, Wu G, Shouval R, Arias IM. Bilirubin mono- and diglucuronide formation by human liver in vitro: assay by high-pressure liquid chromatography. Hepatology. 1981;1(6):622–627. DOI: 10.1002/hep.1840010610.
[3] Hall B, Levy S, Dufault-Thompson K, et al. BilR is a gut microbial enzyme that reduces bilirubin to urobilinogen. Nature Microbiology. 2024;9:173–184. DOI: 10.1038/s41564-023-01549-x.
[4] Sanada S, Suzuki T, Nagata A, et al. Intestinal microbial metabolite stercobilin involvement in the chronic inflammation of ob/ob mice. Scientific Reports. 2020;10:6479. DOI: 10.1038/s41598-020-63627-y.
[5] Guerra Ruiz AR, Crespo J, López Martínez RM, et al. Measurement and clinical usefulness of bilirubin in liver disease. Advances in Laboratory Medicine. 2021;2(3):352–372. DOI: 10.1515/almed-2021-0047.
[6] Kim BSM, Li BT, Engel A, et al. Diagnosis of gastrointestinal bleeding: a practical guide for clinicians. World Journal of Gastrointestinal Pathophysiology. 2014;5(4):467–478. DOI: 10.4291/wjgp.v5.i4.467.
[7] Beckingham IJ, Ryder SD. Investigation of liver and biliary disease. BMJ. 2001;322(7277):33–36. DOI: 10.1136/bmj.322.7277.33.
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