Human feces is roughly 75% water and 25% solid matter. That solid portion is a surprisingly complex mix of bacteria (both living and dead), undigested food fiber, fat, protein remnants, intestinal cell debris, minerals, and bile pigments. The average adult produces about 100 to 106 grams of stool per day, though this varies widely depending on diet, hydration, and gut health.
Bacteria Make Up Most of the Solid Matter
The single largest component of dried stool isn’t leftover food. It’s bacteria. Directly measured, bacteria account for about 55% of fecal dry mass. This includes both living microbes and dead ones that were killed off during digestion. Your large intestine houses trillions of bacteria, and as they multiply, die, and get swept along with waste, they accumulate into the bulk of what you eventually pass.
The vast majority of these bacteria are anaerobic, meaning they thrive in the oxygen-free environment of the colon. They play active roles while alive: fermenting fiber, producing vitamins like K and certain B vitamins, breaking down bile salts, and generating the gases and compounds that give stool its distinctive characteristics. By the time stool exits the body, it carries a dense bacterial load that reflects the unique ecosystem living in your gut.
Undigested Food and Fiber
After bacteria, the next major component is undigested plant fiber and food residue. Humans lack the enzymes to break down cellulose and many other plant fibers, so these pass through the small intestine largely intact. In the colon, gut bacteria ferment some of this fiber into short-chain fatty acids your body can absorb, but a significant portion remains and adds bulk to stool.
This is why high-fiber diets produce larger, softer stools. The fiber holds water, which keeps things moving. Stool also contains small amounts of fat (typically a few grams per day), protein fragments that escaped digestion, and food particles that moved through the system too quickly to be fully broken down. Corn kernels and certain seeds are familiar examples, though most undigested material isn’t visible to the naked eye.
Why Stool Is Brown
The brown color of normal stool comes from a pigment called stercobilin. It starts as bilirubin, a yellow-orange waste product created when your body recycles old red blood cells. The liver packages bilirubin into bile and sends it into the small intestine, where it helps with fat digestion. As bile moves into the large intestine, bacteria strip off chemical groups from bilirubin and reduce it to colorless compounds called urobilinogens. When these colorless compounds are exposed to air (during and after a bowel movement), they oxidize into stercobilin, which is responsible for the characteristic brown shade.
This process explains why stool color changes with certain conditions. Very pale or clay-colored stool can signal that bile isn’t reaching the intestine, possibly due to a blocked bile duct. Green stool often means food moved through the colon too quickly for bacteria to fully convert the bile pigments. Dark or black stool can indicate iron supplements or, more seriously, bleeding in the upper digestive tract.
What Causes the Smell
Fecal odor comes primarily from a compound called skatole, produced when gut bacteria break down the amino acid tryptophan (found in protein-rich foods) through anaerobic fermentation. Skatole is the dominant contributor to what most people recognize as “fecal smell.” A closely related compound, indole, adds a secondary layer to the odor.
Hydrogen sulfide, the compound responsible for a rotten-egg smell, also plays a role, especially when your diet is high in sulfur-containing foods like eggs, meat, and cruciferous vegetables like broccoli. Other volatile compounds including short-chain fatty acids from fiber fermentation contribute to the overall scent. The specific smell of any given bowel movement depends heavily on what you’ve eaten and which bacterial populations are most active in your colon at the time.
Minerals and Inorganic Material
A small but measurable fraction of stool consists of inorganic minerals. The dominant ones are calcium phosphate and iron phosphate. The average person excretes roughly 0.5 to 1 gram of calcium per day through stool, representing dietary calcium that wasn’t absorbed in the small intestine. Iron excretion is much smaller, typically around 30 milligrams per day, though this varies with diet.
Stool also contains shed epithelial cells (the lining of your intestines replaces itself every three to five days, and old cells slough off into the waste stream), dried mucus that originally coated the intestinal walls to protect them, and residual digestive secretions from the pancreas and intestinal glands.
Water Content and Stool Consistency
Water is the largest single ingredient in stool by total weight, and small shifts in water content dramatically change consistency. The Bristol Stool Chart, a widely used clinical tool, classifies stool into seven types based on shape and texture. Types 1 and 2 are hard, dry, and lumpy, indicating constipation from slow transit through the colon, which allows too much water to be reabsorbed. Types 3 and 4 are considered ideal: smooth, soft, and easy to pass. Types 5 through 7 are progressively softer, mushy, or liquid, reflecting rapid transit that doesn’t allow the colon enough time to absorb water.
The colon’s primary job is water recovery. It reabsorbs roughly 1 to 1.5 liters of water per day from the material that enters it. When that process is disrupted by infection, inflammation, or certain foods, the result is diarrhea. When transit slows too much, the result is hard, difficult-to-pass stool.
Hidden Biomarkers in Stool
Beyond its visible components, stool contains trace proteins and compounds that doctors use as diagnostic signals. One of the most clinically useful is calprotectin, a protein released by a type of white blood cell. In a healthy person, fecal calprotectin levels sit below 50 micrograms per gram. Levels above that threshold suggest active inflammation in the intestinal lining and are used to screen for inflammatory bowel diseases like Crohn’s disease and ulcerative colitis. This simple stool test can help determine whether symptoms like chronic diarrhea or abdominal pain warrant further investigation with procedures like colonoscopy.
Stool can also carry detectable traces of blood (used in colorectal cancer screening), parasites, pathogenic bacteria, viral particles, and even DNA shed from intestinal cells. What appears to be simple waste is, from a diagnostic standpoint, a detailed snapshot of gastrointestinal health.