Human poo is roughly 75% water. The remaining 25% is solid material, and that solid fraction contains a surprisingly complex mix of bacteria, undigested food, dead cells from your intestinal lining, fats, fiber, protein, and the pigments that give stool its characteristic brown color.
Water Makes Up Most of It
Across dozens of studies, healthy stool averages about 75% water by weight, though individual samples can range anywhere from 63% to 86%. That water content is what determines consistency. Stools on the lower end of that range feel hard and dry. Stools closer to 86% are loose or watery. Your hydration, diet, and how quickly food moves through your gut all influence where you land on that spectrum.
Bacteria, Both Living and Dead
The single largest component of the solid portion is bacteria. Your large intestine houses trillions of microorganisms, and a significant share of what you flush is made up of their cells, both alive and dead. These aren’t harmful invaders. Most are the normal gut bacteria that help you break down food, produce vitamins, and maintain the intestinal lining. When researchers weigh dried stool samples, bacterial biomass consistently represents the biggest category of solid matter, outweighing even undigested food.
Undigested Plant Fiber
Not everything you eat gets absorbed. Plant fibers like cellulose, hemicellulose, and lignin pass through your digestive system largely intact because your body lacks the enzymes to break them down. Cellulose is the structural material in plant cell walls. Lignin, found in vegetables, whole grains, and seeds, is especially resistant to digestion. Its complex cross-linked structure makes it nearly impossible for even gut bacteria to break apart, so it passes through almost entirely and exits in your stool.
This is a feature, not a flaw. Indigestible fiber adds bulk to stool, which helps it move through the colon at a healthy pace. The amount of fiber in your poo rises and falls with your diet. A meal heavy in vegetables, legumes, and whole grains produces noticeably more fibrous stool than a diet based on refined carbohydrates and animal products.
Dead Cells From Your Gut Lining
Your intestinal lining replaces itself constantly. Between one-sixth and one-third of your colon’s surface cells are shed every single day, amounting to roughly 10 billion cells in a 24-hour period. These sloughed cells, along with mucus secreted by the gut wall to keep things moving smoothly, make up another meaningful fraction of stool solids. You’re essentially looking at your intestine’s daily renovation project every time you go to the bathroom.
Fats, Protein, and Minerals
Small amounts of dietary fat make it through digestion without being fully absorbed. The fats found in stool include triglycerides, fatty acids, cholesterol, and phospholipids, mostly originating from what you ate. In a healthy gut, the amount of fat excreted is small. When fat absorption is impaired, stools become pale, greasy, and unusually foul-smelling.
Protein also shows up. Your body excretes roughly 1 to 2 grams of nitrogen in stool per day, and fecal nitrogen can account for 30% to 50% of your total daily nitrogen excretion. That nitrogen comes from undigested food proteins, digestive enzymes your body secreted but didn’t reabsorb, and the bacterial cells themselves. Minerals like calcium, phosphorus, and iron also pass through in small quantities.
What Gives It the Brown Color
The brown color comes from a pigment called stercobilin. The journey starts with old red blood cells. When your body breaks them down, it produces a yellow-green compound called bilirubin, which gets dumped into your intestine via bile. Gut bacteria then convert bilirubin into a colorless substance called urobilinogen. As urobilinogen moves through the lower intestine and gets exposed to air, it transforms into stercobilin, the pigment responsible for that familiar brown.
This is why stool color changes when something disrupts the process. 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 complete the conversion. Yellow stool may signal excess fat. The brown you’re used to is actually the end product of a multi-step chemical process involving your liver, your gut bacteria, and oxygen.
What Creates the Smell
The odor of stool is more chemically complex than most people assume. For a long time, two compounds called skatole and indole got the blame. But gas chromatography studies found that skatole and indole actually smell more like mothballs than feces when isolated in pure form. The real culprits are sulfur-containing compounds: methanethiol, dimethyl disulfide, and dimethyl trisulfide, along with small amounts of hydrogen sulfide gas.
These sulfur compounds are produced by gut bacteria as they break down proteins, particularly those rich in sulfur-containing amino acids found in meat, eggs, and cruciferous vegetables like broccoli and cabbage. This is why high-protein meals tend to produce more pungent stool, and why the smell varies so much from day to day depending on what you’ve eaten.