Poop is roughly 75% water. The remaining 25% is a mix of dead bacteria, undigested plant fiber, fat, protein, shed intestinal cells, and the bile pigments that give it its brown color. What surprises most people is that bacteria, not leftover food, make up the single largest solid component.
What Stool Is Actually Made Of
A healthy adult produces about 30 grams of dry fecal matter per day. Of that, around 16 grams is bacterial biomass, both living and dead microbes from your gut. That means bacteria account for roughly 25 to 54% of all the solid material in your stool. These are the trillions of microorganisms that live in your large intestine, constantly reproducing and dying. Their sheer volume is what gives stool most of its bulk.
The rest of the solids break down like this:
- Undigested carbohydrates and fiber: about 25% of dry weight, mostly plant material your body can’t break down
- Protein and nitrogen-containing matter: 2 to 25%, depending on diet
- Fat: 2 to 15%, from dietary lipids that escaped absorption
- Inorganic matter: 7 to 16%, including minerals like calcium and phosphates
Your stool also contains shed cells from your intestinal lining, along with mucus that coats the gut wall. The intestinal lining replaces itself every few days, and those old cells get swept into the waste stream. There are also trace amounts of immune cells mixed in.
How Your Body Builds a Bowel Movement
Stool formation starts long before anything reaches your colon. After you eat, your stomach breaks food into a soupy liquid called chyme. The small intestine handles the heavy lifting of digestion, pulling out most of the nutrients, sugars, fats, and amino acids your body needs. What’s left is a watery mixture of indigestible material that passes into the large intestine.
Your colon’s main job is water removal. It absorbs about 400 milliliters of water per day from that liquid waste, gradually turning it from a loose slurry into a formed stool. Bacteria in the colon simultaneously ferment remaining carbohydrates and fiber, producing gases and short-chain fatty acids in the process. They also synthesize vitamin K as a byproduct. The longer material sits in the colon, the more water gets pulled out, which is why constipation produces hard, dry stools.
Muscle contractions called peristalsis push the forming stool through the colon’s roughly five-foot length. The entire journey from mouth to toilet takes 30 to 40 hours on average in Western populations, though it can be faster in other groups. Women tend to have slightly longer transit times than men. The normal upper limit is around 70 hours. Anything beyond that typically signals slow-transit constipation.
Why Poop Is Brown
The characteristic brown color comes from a pigment called stercobilin, and the process that creates it begins with your red blood cells. When old red blood cells break down, they release a compound called heme. Your liver converts heme first into a green pigment (biliverdin), then into a yellow one (bilirubin). Bilirubin gets packaged into bile and secreted into your small intestine to help with fat digestion.
Once bilirubin reaches the large intestine, gut bacteria go to work on it, converting it through several steps into stercobilin, which is brown. This is why stool color can change when something disrupts the process. Antibiotics that kill off gut bacteria can produce lighter or greenish stool. Very pale or clay-colored stool suggests bile isn’t reaching the intestine at all, which can indicate a blockage. Green stool often means food moved through too quickly for bacteria to finish the conversion.
What Creates the Smell
Fecal odor comes from volatile compounds produced when gut bacteria break down proteins. The main culprit is skatole, a chemical produced during the bacterial metabolism of tryptophan, an amino acid found in meat, eggs, cheese, and other protein-rich foods. In high concentrations, skatole has a strong fecal smell. Interestingly, at very low concentrations it smells floral, almost like jasmine, and is actually used in some perfumes.
Skatole isn’t working alone. Hydrogen sulfide (the rotten-egg gas), indole, and various sulfur-containing compounds all contribute to the overall scent. A high-protein diet tends to produce stronger-smelling stool because bacteria have more amino acids to ferment. Foods rich in sulfur, like broccoli, garlic, and eggs, amplify the sulfur gas component specifically.
How Fiber Changes Your Stool
Insoluble fiber, the kind found in whole grains, vegetable skins, and nuts, passes through your digestive system mostly intact. It absorbs water and adds physical bulk to your stool, which stretches the colon wall and triggers stronger peristaltic contractions. The result is faster transit and softer, easier-to-pass bowel movements. This is the mechanical reason high-fiber diets reduce constipation.
Soluble fiber, found in oats, beans, and fruits, works differently. It dissolves in water to form a gel-like substance, which slows digestion in the upper gut but also serves as food for your colon bacteria. When bacteria ferment soluble fiber, they multiply, and since bacterial mass is the largest solid component of stool, this fermentation actually increases stool weight. People who eat high-fiber diets produce significantly heavier, bulkier stools than those on low-fiber diets, sometimes double the weight.
What Stool Consistency Tells You
The Bristol Stool Scale is the standard reference for evaluating what your poop looks like and what it means. It classifies stool into seven types:
- Type 1: Separate hard lumps, like pebbles. Indicates significant constipation.
- Type 2: Lumpy and sausage-shaped. Still constipated, but less severely.
- Type 3: Sausage-shaped with cracks on the surface. Normal.
- Type 4: Smooth, soft, and snake-like. Considered ideal.
- Type 5: Soft blobs with clear edges. Leaning toward loose.
- Type 6: Fluffy, mushy pieces with ragged edges. Mild diarrhea.
- Type 7: Entirely liquid with no solid pieces. Full diarrhea.
The differences between these types come down almost entirely to how long the stool spent in your colon and how much water was absorbed. Types 1 and 2 sat too long, losing too much water. Types 6 and 7 moved through too fast, retaining too much. Diet, hydration, stress, medications, and gut bacteria composition all influence where you land on the scale on any given day. Consistency matters more than frequency as an indicator of digestive health. Anywhere from three times a day to three times a week is considered a normal range, as long as the stool itself falls in the Type 3 to 5 range.