Why is poop brown?

Everybody poops, but have you ever wondered why, out of all the colors, poop is the color brown? 

While this question might seem like a simple biological curiosity, doctors will likely tell you that the color of your poop is no laughing matter. That's because poop isn't always brown, and when it's not, it can tell you a lot about what's happening inside your body. 

Poop is brown because the body is typically very good at making sure that no useful food goes to, well, waste. Your body breaks down nearly all of the energy sources it can from what you eat, and one of the key substances your body uses to break down and absorb nutrients is bile.

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While bile itself is yellow-green, its role in digestion leads to the brown color of poop. "Bile plays a vital role in the digestion and absorption of intestinal nutrients such as cholesterol, fat, and fat-soluble vitamins," Dr. David Q.H. Wang, a professor of medicine at Albert Einstein College of Medicine in New York, told Live Science. "Bile contains bilirubin, which is secreted by the liver and stored in the gallbladder. During a meal, bile is released into the small intestine. In the intestine, bilirubin is converted to urobilinogen and then reduced to stercobilinogen. Both urobilinogen and stercobilinogen are colorless. Finally, stercobilinogen is oxidized to stercobilin that is excreted in the feces."

Stercobilin is responsible for the brown color of human feces. "It should be pointed out that human feces are usually light to dark brown in color and are composed of a combination of the bilirubin derivatives, mainly stercobilin and some urobilin," Wang explained. "The longer stercobilin is oxidized, the darker the stool will be. The color of feces also depends on the freshness of the stool, the concentration of stercobilin, and the stercobilin/urobilin ratio."

David Q. H. Wang, MD, PhD, FAASLD, AGAF

Dr. David Q. H. Wang is a Professor of Medicine and Genetics in the Departments of Medicine and Genetics, and the Director of Molecular Biology and Next Generation Technology Core of Marion Bessin Liver Research Center at Albert Einstein College of Medicine in New York, USA. He is an elected fellow of the American Gastroenterological Association and the American Association for the Study of Liver Diseases. Dr. Wang has an active research interest in the molecular and genetic mechanisms of cholesterol homeostasis and the pathophysiology of cholesterol-related diseases. 

What does it mean if your poop isn't brown?

Doctors are interested in the color of your poop because discolored stools can be a warning sign of gastroenterological problems. However, not all discolored stools are cause for concern. 

"In some cases, certain foods can change the color of feces," Wang said. "For example, feces can be green due to eating licorice candy, as it is typically made with anise oil rather than licorice herb. Feces may turn black if a certain amount of the food containing animal blood (i.e., pig blood) is consumed." 

These colors, if occasional and tied to a specific food you've just eaten, aren't cause for concern. 

What your poop says about your health

Sometimes, medical conditions can affect how your body processes or absorbs various nutrients, which, in turn, can affect the resulting color.

"Under normal physiological conditions, human feces are light brown to dark brown in color," Wang said. "In some pathological conditions, the color of the stool changes. For example, when any type of bile duct is obstructed [which can occur due to a range of conditions], it usually results in a tarnished silver or aluminum paint-like stool color."

Other colors can also be a sign of illness. "Feces can be black in color because red blood cells stay in the intestine long enough to be broken down by digestive enzymes," Wang said. "This is called melena and is usually due to upper gastrointestinal bleeding, such as bleeding from a peptic ulcer in the duodenum [the uppermost part of the small intestine] and/or stomach." 

In these instances, the color of your poop matters to doctors, because it may indicate that something is wrong in your body.

"If you notice a change in the color of your feces, you have to see your doctor as soon as possible," Wang said. 

• Related: Why is my poop green?

Originally published on Live Science.