Poop is your body’s solid waste, but it’s far more than just leftover food. By weight, most of what comes out is water, bacteria (both living and dead), shed intestinal cells, and metabolic byproducts. The actual undigested food remnants make up a surprisingly small fraction of the total. Understanding what stool is made of helps explain why it looks, smells, and behaves the way it does.
What Stool Is Made Of
Fresh stool is roughly 75% water. The remaining 25% is solid material, and bacteria account for about 55% of that dry mass. These are gut microbes, some still alive and some dead, that lived in your intestines and helped break down food. The sheer volume of bacteria in every bowel movement reflects the massive microbial ecosystem living in your colon, which houses trillions of organisms.
Beyond bacteria, your gut sheds more than 30 grams of intestinal lining cells every day. These cells wear out quickly because they’re constantly exposed to digestive acids and the mechanical movement of food. They slough off and get mixed into the waste stream. The rest of stool’s dry weight comes from indigestible fiber, small amounts of fats, cholesterol, proteins your body discarded, and various salts and minerals.
So when you look at stool, you’re seeing mostly water held together by dead bacteria, discarded cells from your own body, and whatever your digestive system couldn’t absorb.
How Your Body Makes It
Stool formation begins long before anything reaches the colon. After you eat, your stomach and small intestine spend hours breaking food into nutrients small enough to absorb into the bloodstream. What’s left over, the parts too large or too tough to break down, passes into the large intestine as a watery mixture called chyme.
The large intestine’s primary job is pulling water back out. As this liquid waste moves through the colon, your body reclaims water and electrolytes, gradually transforming the mixture from liquid into the soft, solid form you recognize. The longer material stays in the colon, the more water gets absorbed and the firmer it becomes. This is why constipation produces hard, dry stools and why diarrhea happens when material moves through too quickly for adequate water absorption.
The typical transit time through the colon alone is 30 to 40 hours in someone who isn’t constipated. Total transit time from mouth to toilet can range from one to three days, though up to 72 hours is still normal. Women may have slightly longer transit times, occasionally reaching up to about 100 hours.
Why It’s Brown
The characteristic brown color comes from a pigment called stercobilin. Here’s how it forms: your liver constantly breaks down old red blood cells and produces a yellow-green substance called bilirubin, which gets released into your intestines through bile. Once bilirubin reaches the colon, gut bacteria chemically reduce it through a series of reactions, ultimately producing stercobilin, an orange-brown pigment that gives stool its familiar color.
This is why stool color changes can signal different things. Very pale or clay-colored stool may mean bile isn’t reaching the intestines properly. Black or tarry stool can indicate bleeding higher up in the digestive tract, where blood has been partially digested. Bright red suggests bleeding closer to the exit. Green stool often just means food passed through too quickly for bacteria to fully convert bilirubin, or that you ate a lot of leafy greens.
What Creates the Smell
Fecal odor comes from a cocktail of volatile organic compounds produced mainly by gut bacteria as they ferment undigested material. Several chemical families contribute. Indole and skatole are produced when bacteria break down proteins, and they carry that distinctly “fecal” scent even in tiny concentrations. Sulfur compounds like dimethyl disulfide and dimethyl trisulfide add a rotten-egg quality. Volatile fatty acids, produced when bacteria ferment carbohydrates, contribute a sour note. A compound called 4-methylphenol (a type of phenol) is one of the single biggest contributors to overall odor intensity.
Your diet directly influences which of these compounds dominate. High-protein meals tend to produce more sulfur compounds and indoles, while high-fiber diets shift bacterial fermentation toward less pungent byproducts.
How Much Is Normal
The average adult produces roughly 100 to 106 grams of stool per day, with men producing slightly more than women (about 104 grams versus 99 grams in one large study of healthy adults). That said, individual variation is wide. People eating high-fiber diets in some populations produce two to three times that amount, while low-fiber diets lead to smaller, denser stools.
What Healthy Stool Looks Like
Doctors use the Bristol Stool Scale, a seven-type classification system, to describe stool consistency:
- Type 1: Separate hard lumps, like pebbles
- Type 2: Lumpy and sausage-shaped
- Type 3: Sausage-shaped with cracks on the surface
- Type 4: Smooth, soft, and snakelike
- Type 5: Soft blobs with clear edges
- Type 6: Fluffy, mushy pieces with ragged edges
- Type 7: Entirely liquid with no solid pieces
Types 3 and 4 are the ideal range. These stools hold together but pass easily without straining. Types 1 and 2 indicate constipation: the stool sat in the colon too long and lost too much water. Types 5 through 7 point toward diarrhea, meaning material moved through before the colon could absorb enough water.
How Fiber Shapes Your Stool
Dietary fiber plays a surprisingly large role in determining what your stool looks and feels like. The two types of fiber work differently. Insoluble fiber, found in whole grains, nuts, and vegetable skins, doesn’t dissolve in water. It passes through your system mostly intact, adding physical bulk to stool and helping push material through the colon at a steady pace. Soluble fiber, found in oats, beans, and fruits, dissolves and forms a gel-like substance that slows digestion in the upper gut. In the colon, it absorbs water and helps hold stool together.
This dual action explains why fiber helps with both constipation and loose stools. If your stool is too hard, fiber draws in water and softens it. If it’s too loose, fiber absorbs excess water and adds structure. Most adults benefit from 25 to 30 grams of fiber per day, though the average intake in Western diets falls well short of that.