Life's Little Mysteries

Why does poop smell bad?

Father holding baby with dirty diaper.
Stools are stinky because they release byproducts of digestion. (Image credit: SelectStock via Getty Images)

Poop, feces, stool — whatever you call it, there's no denying the unpleasant smell. While it's perfectly normal for poo to be pungent, it makes you wonder what it is about our excrement that makes it stink. So why does poop smell bad?

"Stools are generally not a pleasant smell because they are releasing byproducts of your digestion," Shelby Yaceczko, a clinical dietician at UCLA Health, told Live Science.

Skatole, also known as 3-methylindole, is "one of the compounds in feces that gives it its foul smell," Emma Laing, a clinical professor and director of dietetics at the University of Georgia and a spokesperson for the Academy of Nutrition and Dietetics, told Live Science. Bacteria make this compound when they break down the amino acid L-tryptophan in the gastrointestinal tract, she said. (Oddly enough, the same compound in small concentrations gives a pleasant smell to flowers like jasmine, according to the American Chemical Society.)

Related: Why do some men take so long to poop?

There are more than 10,000 microbial species living in humans and more bacterial cells than human ones. These microorganisms are essential to digestion and largely to blame for feces' odor. Different bacteria emit different gases depending on the types of foods and substances they are breaking down, Laing explained; bacteria in the gastrointestinal tract and the mouth contribute to this process, she said.

Because bacteria break down what we eat, factors such as dietary patterns, alcohol intake, dietary supplements and prescription medication can affect the way poop smells. Sugar alcohols, like sorbitol, are often used in candies and can make poop smell particularly bad. And sulfate-containing foods — like eggs, broccoli, cabbage, cauliflower, onions, legumes and meat — can contribute to the production of sulfur gas, which has a rotten egg smell, during digestion.

Highly processed and sugary foods can be difficult to digest, leading bacteria to produce more gases and stinkier poo, Yaceczko said. And consuming large amounts of alcohol produces smelly stools because it wreaks havoc on the intestines and the digestive process, Laing added.

If you notice a change or worsening in the smell of your stool, it's most likely due to a change in your diet or medication, Laing said. The digestive process eventually adjusts, and the worsened smell is usually temporary, she added.

However, an especially foul smell, like a putrid or rotten odor, that doesn't go away could indicate a serious health problem. Malabsorption diseases, like inflammatory bowel disease or celiac disease, can prevent the body from digesting and absorbing nutrients, which can cause consistently foul odors. A viral or bacterial infection in the gut could also be to blame. And so-called motility disorders, which cause a slower-than-normal emptying of the gastrointestinal tract, give poop a longer time to ferment, thus increasing the stink, Yaceczko said.

If an unusually bad smell persists, especially in combination with symptoms such as diarrhea, blood in the stool, abdominal pain or fever, "a prompt visit to your health care provider is warranted," Laing said.

This article is for informational purposes only, and is not meant to offer medical advice.

Donavyn Coffey
Live Science Contributor

Donavyn Coffey is a Kentucky-based health and environment journalist reporting on healthcare, food systems and anything you can CRISPR. Her work has appeared in Scientific American, Wired UK, Popular Science and Youth Today, among others. Donavyn was a Fulbright Fellow to Denmark where she studied  molecular nutrition and food policy.  She holds a bachelor's degree in biotechnology from the University of Kentucky and master's degrees in food technology from Aarhus University and journalism from New York University.