Human stool is roughly 75% water and 25% solid material. That solid portion is a mix of bacteria (both living and dead), undigested plant fiber, shed intestinal cells, fats, minerals, and various metabolic waste products. What surprises most people is how little of stool actually comes from the food you ate.
Water Makes Up Most of It
The single largest component of stool is water, accounting for about 74 to 75% of total weight by median estimates. The exact percentage varies depending on diet. People who eat more fiber tend to have wetter stool, around 79%, because indigestible fiber absorbs and holds water as it moves through the colon. People who eat more protein and less fiber typically have drier stool, closer to 73%. This is why high-fiber diets produce softer, bulkier bowel movements.
The average person produces about 128 grams of stool per day (a little under 5 ounces), of which only about 29 grams is actual dry material. That means the solid stuff you might think of as “the poop” is only a small fraction of what comes out.
Bacteria: The Biggest Solid Component
A large share of the solid matter in stool is bacteria. Your gut hosts an enormous microbial community, with estimates of 1,000 to over 3,000 distinct bacterial species living in a single person’s intestines at any given time. Across multiple people sampled over time, researchers have identified more than 5,600 species total. These microbes are constantly reproducing and dying, and the dead and living cells get swept along with everything else moving through the colon.
Most of this bacterial diversity is concentrated at the species and strain level, meaning the gut doesn’t contain a huge variety of broad bacterial families. Instead, it contains many closely related but distinct strains within a relatively small number of groups. About 90% of them are from a single major category of bacteria called eubacteria. These organisms play essential roles in breaking down food, producing vitamins, and fermenting fiber, and the byproducts of their activity are responsible for much of what gives stool its other characteristics, including its smell.
Shed Intestinal Cells
Your gut lining replaces itself constantly. The human intestine sheds more than 30 grams of cells every day, roughly the same weight as all the other dry solids in your stool combined. These are cells from the inner surface of the intestinal wall that have completed their short life cycle (the entire lining turns over every three to five days) and slough off into the passing stream of digestive material. Your body replaces them just as quickly, so this shedding is completely normal and not something you’d notice.
Undigested Food and Fiber
Despite what many people assume, leftover food is not the main ingredient in stool. Your digestive system is remarkably efficient. Under normal conditions, nearly all the fat you eat gets absorbed, with less than 5% ending up in feces. Proteins and simple carbohydrates are similarly well absorbed in the small intestine long before reaching the colon.
What does pass through largely intact is insoluble fiber: the structural material in plants that human enzymes cannot break down. This includes the cell walls of vegetables, the husks of grains, and the skins of fruits and seeds. If you’ve ever noticed recognizable fragments in your stool, like corn kernels, leafy greens, or bits of seeds, that’s this fiber-rich plant matter. It’s not a sign of a digestive problem. Your body simply doesn’t have the enzymes to dismantle those particular structures, and gut bacteria can only partially ferment them.
What Gives Stool Its Color
The characteristic brown color of stool comes from a pigment called stercobilin, which is the end product of a chain of chemical transformations that starts with old red blood cells. When your body breaks down aging blood cells, it produces a yellow-green compound called bilirubin, which gets processed by the liver and released into the intestine through bile. Once in the colon, bacteria convert most of that bilirubin into a substance called urobilinogen. The vast majority of urobilinogen then converts into stercobilin, which has a brown pigment. About 80% of this material exits through feces, while 10 to 20% gets reabsorbed and recirculated through the liver.
This is why stool color changes can signal specific things. Very pale or clay-colored stool can mean bile isn’t reaching the intestine. Green stool often means food moved through too quickly for bacteria to fully convert the bilirubin. Dark or black stool can indicate iron supplements or, in some cases, bleeding higher in the digestive tract.
What Causes the Smell
Fecal odor comes from volatile organic compounds, which are gases produced as byproducts of bacterial metabolism in the colon. The dominant odor-causing chemicals include compounds in the indole and phenol families, which are created when gut bacteria break down the amino acid tryptophan from digested proteins. Hydrogen sulfide (the “rotten egg” gas) also contributes, along with various organic acids, alcohols, and aldehydes.
The specific mix of these compounds varies from person to person and meal to meal, which is why stool odor changes with diet. High-protein meals generate more of the sulfur-containing and indole-based compounds, producing a stronger smell. The bacterial species present in your gut also matter: different microbes produce different metabolic byproducts, so two people eating the same meal can produce noticeably different odors.
Minerals and Other Trace Components
Stool also contains small amounts of inorganic minerals, including sodium, potassium, calcium, magnesium, phosphorus, zinc, iron, copper, and manganese. These are minerals your body didn’t absorb during digestion, along with minerals secreted into the intestine through bile and other digestive fluids. The amounts are small but measurable, and they shift with hydration. Wetter stool tends to contain more sodium and less potassium.
Mucus is another minor but normal component. The intestinal lining continuously produces a thin layer of mucus to protect itself and to help material slide through the colon. Small amounts of this mucus mix into stool as part of normal transit. Feces also contain trace amounts of cholesterol, bile salts, and other metabolic waste products that the liver filters from the bloodstream and dumps into the digestive tract for elimination.