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What is poop?

If you're grossed out, resist the conditioning, strip away the stigma, and recognize poop for what it is: water, undigested food, and bacteria.

3 minutes
3 Citations
IntroductionHow is poop made?What is poop made of? Characteristics of poopCitations

Poop is a key biomarker of human health—a window into our inner world that can tell us a great deal about what’s happening inside our gastrointestinal tract. But, in our rush to get rid of this data dump, we don’t pay attention to the insights it could offer. Simple markers like Bristol stool type and regularity, if tracked, could inform our choices and help us better understand how they, in turn, impact our poop. In order to decode what our poop is telling us, we need to understand the poop basics: how it’s made, what it really is, and what characteristics to pay attention to.

Poop is one of the end results of the digestive process. After eating, food makes its way through your digestive system, and is broken down into accessible nutrients your body can absorb along the way. By the time food makes its way through your stomach and small intestine, the bulk of nutrients have been absorbed and any leftover material is shipped to the large intestine (or colon), where trillions of bacteria continue the process of breaking down what your body couldn’t. Any additional nutrients, including water, are absorbed in the large intestine and the remaining waste exits your body as waste. There are many key stakeholders involved in coordinating a bowel movement, including muscles, nerves, signaling molecules like neurotransmitters, and microbes. (Fun fact: resident gut microbes play a role in regulating intestinal motility and stimulating muscle contractions for evacuations. 🤓)

Now, let’s break poop down by zooming in (6,000x).

The composition of human feces is roughly 75% water and 25% solid waste¹, including: cellular lining, plant fibers, fats, proteins, mucus, bile, and a collection of other substances your body couldn’t digest. This 25% also contains an invisible galaxy of microbes—bacteria, viruses, archaea—both living and dead. In fact, it’s estimated that within each gram of stool there are nearly 100 billion bacteria.² (To put that into perspective, the Milky Way galaxy has an estimated 100–400 billion stars within.)

You can’t see these microbes with the naked eye, of course, but you definitely can see one of their byproducts…

Ever wonder why your poop is brown? The answer is mostly bile (and bacteria). Your liver secretes something called bilirubin (found in bile), which makes its way to the large intestine. Certain bacteria work to change this bilirubin to urobilinogen, which is then converted by other bacteria into something called stercobilin. This stercobilin is ultimately excreted through your #2, and gives it its characteristic brown hue.⁠

What should you look out for when you take a #2?

  • Shape and texture. Get acquainted with the Bristol Stool Chart. Developed by Dr. Stephen Lewis and Dr. Ken Heaton at the University of Bristol in 1997, it’s a universal classification tool for human feces. You can think of it as a decoder for your toilet bowl—and for your digestive health.
  • Frequency. Anywhere from three times per day to three times per week can be considered healthy defecation.3 Temporary changes are also normal and can be a result of diet, physical activity, even stress.
  • Color. Yes, it’s usually brown, but color variations are also possible (green, clay-colored, yellow, black, and red) are possible. Variations in color can be a result of dietary inputs (food dyes, certain supplements or medications, vegetables with deep colors) or a variety of medical issues (hemorrhoids, for example).


If you read to the end of this, we hope you find poop nerdy, not scary. If you’re still grossed out, resist the conditioning, strip away the stigma, and recognize poop for what it is—water, undigested food, bacteria, and one of the most visible biomarkers of your gastrointestinal and overall health.

  1. Stephen, A. M., & Cummings, J. H. (1980). The microbial contribution to human faecal mass. Journal of medical microbiology, 13(1), 45–56.
  2. Sender, R., Fuchs, S., & Milo, R. (2016). Revised Estimates for the Number of Human and Bacteria Cells in the Body. PLoS biology, 14(8), e1002533.
  3. Mitsuhashi, S., Ballou, S., Jiang, Z. G., Hirsch, W., Nee, J., Iturrino, J., Cheng, V., & Lembo, A. (2018). Characterizing Normal Bowel Frequency and Consistency in a Representative Sample of Adults in the United States (NHANES). The American journal of gastroenterology, 113(1), 115–123.